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1 <?xml version="1.0" encoding="UTF-8" standalone="no"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Frequently Asked Questions</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="index.html" title="The GNU C++ Library" /><link rel="up" href="bk03.html" title="" /><link rel="prev" href="bk03.html" title="" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Frequently Asked Questions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk03.html">Prev</a></td><th width="60%" align="center"></th><td width="20%" align="right"></td></tr></table><hr /></div><div class="article"><div class="titlepage"><div><div><h1 class="title"><a id="faq"></a>Frequently Asked Questions</h1></div><div><p class="copyright">Copyright 3 2008-2018 4 5 <a class="link" href="https://www.fsf.org" target="_top">FSF</a> 6 </p></div></div><hr /></div><div class="qandaset"><a id="faq.faq"></a><dl><dt></dt><dd><dl><dt>1.1. <a href="faq.html#faq.what"> 7 What is libstdc++? 8 </a></dt><dt>1.2. <a href="faq.html#faq.why"> 9 Why should I use libstdc++? 10 </a></dt><dt>1.3. <a href="faq.html#faq.who"> 11 Who's in charge of it? 12 </a></dt><dt>1.4. <a href="faq.html#faq.when"> 13 When is libstdc++ going to be finished? 14 </a></dt><dt>1.5. <a href="faq.html#faq.how"> 15 How do I contribute to the effort? 16 </a></dt><dt>1.6. <a href="faq.html#faq.whereis_old"> 17 What happened to the older libg++? I need that! 18 </a></dt><dt>1.7. <a href="faq.html#faq.more_questions"> 19 What if I have more questions? 20 </a></dt></dl></dd><dt></dt><dd><dl><dt>2.1. <a href="faq.html#faq.license.what"> 21 What are the license terms for libstdc++? 22 </a></dt><dt>2.2. <a href="faq.html#faq.license.any_program"> 23 So any program which uses libstdc++ falls under the GPL? 24 </a></dt><dt>2.3. <a href="faq.html#faq.license.lgpl"> 25 How is that different from the GNU {Lesser,Library} GPL? 26 </a></dt><dt>2.4. <a href="faq.html#faq.license.what_restrictions"> 27 I see. So, what restrictions are there on programs that use the library? 28 </a></dt></dl></dd><dt></dt><dd><dl><dt>3.1. <a href="faq.html#faq.how_to_install">How do I install libstdc++? 29 </a></dt><dt>3.2. <a href="faq.html#faq.how_to_get_sources">How does one get current libstdc++ sources? 30 </a></dt><dt>3.3. <a href="faq.html#faq.how_to_test">How do I know if it works? 31 </a></dt><dt>3.4. <a href="faq.html#faq.how_to_set_paths">How do I insure that the dynamically linked library will be found? 32 </a></dt><dt>3.5. <a href="faq.html#faq.what_is_libsupcxx"> 33 What's libsupc++? 34 </a></dt><dt>3.6. <a href="faq.html#faq.size"> 35 This library is HUGE! 36 </a></dt></dl></dd><dt></dt><dd><dl><dt>4.1. <a href="faq.html#faq.other_compilers"> 37 Can libstdc++ be used with non-GNU compilers? 38 </a></dt><dt>4.2. <a href="faq.html#faq.solaris_long_long"> 39 No 'long long' type on Solaris? 40 </a></dt><dt>4.3. <a href="faq.html#faq.predefined"> 41 _XOPEN_SOURCE and _GNU_SOURCE are always defined? 42 </a></dt><dt>4.4. <a href="faq.html#faq.darwin_ctype"> 43 Mac OS X ctype.h is broken! How can I fix it? 44 </a></dt><dt>4.5. <a href="faq.html#faq.threads_i386"> 45 Threading is broken on i386? 46 </a></dt><dt>4.6. <a href="faq.html#faq.atomic_mips"> 47 MIPS atomic operations 48 </a></dt><dt>4.7. <a href="faq.html#faq.linux_glibc"> 49 Recent GNU/Linux glibc required? 50 </a></dt><dt>4.8. <a href="faq.html#faq.freebsd_wchar"> 51 Can't use wchar_t/wstring on FreeBSD 52 </a></dt></dl></dd><dt></dt><dd><dl><dt>5.1. <a href="faq.html#faq.what_works"> 53 What works already? 54 </a></dt><dt>5.2. <a href="faq.html#faq.standard_bugs"> 55 Bugs in the ISO C++ language or library specification 56 </a></dt><dt>5.3. <a href="faq.html#faq.compiler_bugs"> 57 Bugs in the compiler (gcc/g++) and not libstdc++ 58 </a></dt></dl></dd><dt></dt><dd><dl><dt>6.1. <a href="faq.html#faq.stream_reopening_fails"> 59 Reopening a stream fails 60 </a></dt><dt>6.2. <a href="faq.html#faq.wefcxx_verbose"> 61 -Weffc++ complains too much 62 </a></dt><dt>6.3. <a href="faq.html#faq.ambiguous_overloads"> 63 Ambiguous overloads after including an old-style header 64 </a></dt><dt>6.4. <a href="faq.html#faq.v2_headers"> 65 The g++-3 headers are not ours 66 </a></dt><dt>6.5. <a href="faq.html#faq.boost_concept_checks"> 67 Errors about *Concept and 68 constraints in the STL 69 </a></dt><dt>6.6. <a href="faq.html#faq.dlopen_crash"> 70 Program crashes when using library code in a 71 dynamically-loaded library 72 </a></dt><dt>6.7. <a href="faq.html#faq.memory_leaks"> 73 Memory leaks in libstdc++ 74 </a></dt><dt>6.8. <a href="faq.html#faq.list_size_on"> 75 list::size() is O(n)! 76 </a></dt><dt>6.9. <a href="faq.html#faq.easy_to_fix"> 77 Aw, that's easy to fix! 78 </a></dt></dl></dd><dt></dt><dd><dl><dt>7.1. <a href="faq.html#faq.iterator_as_pod"> 79 string::iterator is not char*; 80 vector<T>::iterator is not T* 81 </a></dt><dt>7.2. <a href="faq.html#faq.what_is_next"> 82 What's next after libstdc++? 83 </a></dt><dt>7.3. <a href="faq.html#faq.sgi_stl"> 84 What about the STL from SGI? 85 </a></dt><dt>7.4. <a href="faq.html#faq.extensions_and_backwards_compat"> 86 Extensions and Backward Compatibility 87 </a></dt><dt>7.5. <a href="faq.html#faq.tr1_support"> 88 Does libstdc++ support TR1? 89 </a></dt><dt>7.6. <a href="faq.html#faq.get_iso_cxx">How do I get a copy of the ISO C++ Standard? 90 </a></dt><dt>7.7. <a href="faq.html#faq.what_is_abi"> 91 What's an ABI and why is it so messy? 92 </a></dt><dt>7.8. <a href="faq.html#faq.size_equals_capacity"> 93 How do I make std::vector<T>::capacity() == std::vector<T>::size? 94 </a></dt></dl></dd></dl><table border="0" style="width: 100%;"><colgroup><col align="left" width="1%" /><col /></colgroup><tbody><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>1.1. <a href="faq.html#faq.what"> 95 What is libstdc++? 96 </a></dt><dt>1.2. <a href="faq.html#faq.why"> 97 Why should I use libstdc++? 98 </a></dt><dt>1.3. <a href="faq.html#faq.who"> 99 Who's in charge of it? 100 </a></dt><dt>1.4. <a href="faq.html#faq.when"> 101 When is libstdc++ going to be finished? 102 </a></dt><dt>1.5. <a href="faq.html#faq.how"> 103 How do I contribute to the effort? 104 </a></dt><dt>1.6. <a href="faq.html#faq.whereis_old"> 105 What happened to the older libg++? I need that! 106 </a></dt><dt>1.7. <a href="faq.html#faq.more_questions"> 107 What if I have more questions? 108 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.what"></a><a id="faq.what.q"></a><p><strong>1.1.</strong></p></td><td align="left" valign="top"><p> 109 What is libstdc++? 110 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="faq.what.a"></a></td><td align="left" valign="top"><p> 111 The GNU Standard C++ Library v3 is an ongoing project to 112 implement the ISO 14882 C++ Standard Library as described in 113 clauses 20 through 33 and annex D (prior to the 2017 standard 114 the library clauses started with 17). For those who want to see 115 exactly how far the project has come, or just want the latest 116 bleeding-edge code, the up-to-date source can be cloned via 117 <a class="link" href="https://gcc.gnu.org/git.html" target="_top">Git</a>. 118 </p><p> 119 N.B. The library is called libstdc++ <span class="emphasis"><em>not</em></span> stdlibc++. 120 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.why"></a><a id="q-why"></a><p><strong>1.2.</strong></p></td><td align="left" valign="top"><p> 121 Why should I use libstdc++? 122 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-why"></a></td><td align="left" valign="top"><p> 123 The completion of the initial ISO C++ standardization effort gave the C++ 124 community a powerful set of reuseable tools in the form of the C++ 125 Standard Library. However, for several years C++ implementations were 126 (as the Draft Standard used to say) <span class="quote"><span class="quote">incomplet and 127 incorrekt</span></span>, and many suffered from limitations of the compilers 128 that used them. 129 </p><p> 130 The GNU compiler collection 131 (<span class="command"><strong>gcc</strong></span>, <span class="command"><strong>g++</strong></span>, etc) is widely 132 considered to be one of the leading compilers in the world. Its 133 development is overseen by the 134 <a class="link" href="https://gcc.gnu.org/" target="_top">GCC team</a>. All of 135 the rapid development and near-legendary 136 <a class="link" href="https://gcc.gnu.org/buildstat.html" target="_top">portability</a> 137 that are the hallmarks of an open-source project are applied to libstdc++. 138 </p><p> 139 All of the standard classes and functions from C++98/C++03, C++11 and C++14 140 (such as <code class="classname">string</code>, 141 <code class="classname">vector<></code>, iostreams, algorithms etc.) 142 are freely available and attempt to be fully compliant. 143 Work is ongoing to complete support for the current revision of the 144 ISO C++ Standard. 145 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.who"></a><a id="q-who"></a><p><strong>1.3.</strong></p></td><td align="left" valign="top"><p> 146 Who's in charge of it? 147 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-who"></a></td><td align="left" valign="top"><p> 148 The libstdc++ project is contributed to by several developers 149 all over the world, in the same way as GCC or the Linux kernel. 150 The current maintainers are listed in the 151 <a class="link" href="https://gcc.gnu.org/viewcvs/gcc/trunk/MAINTAINERS?view=co" target="_top"><code class="filename">MAINTAINERS</code></a> 152 file (look for "c++ runtime libs"). 153 </p><p> 154 Development and discussion is held on the libstdc++ mailing 155 list. Subscribing to the list, or searching the list 156 archives, is open to everyone. You can read instructions for 157 doing so on the <a class="link" href="https://gcc.gnu.org/lists.html" target="_top">GCC mailing lists</a> page. 158 If you have questions, ideas, code, or are just curious, sign up! 159 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.when"></a><a id="q-when"></a><p><strong>1.4.</strong></p></td><td align="left" valign="top"><p> 160 When is libstdc++ going to be finished? 161 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-when"></a></td><td align="left" valign="top"><p> 162 Nathan Myers gave the best of all possible answers, responding to 163 a Usenet article asking this question: <span class="emphasis"><em>Sooner, if you 164 help.</em></span> 165 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.how"></a><a id="q-how"></a><p><strong>1.5.</strong></p></td><td align="left" valign="top"><p> 166 How do I contribute to the effort? 167 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how"></a></td><td align="left" valign="top"><p> 168 See the <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">Contributing</a> section in 169 the manual. Subscribing to the mailing list (see above, or 170 the homepage) is a very good idea if you have something to 171 contribute, or if you have spare time and want to 172 help. Contributions don't have to be in the form of source code; 173 anybody who is willing to help write documentation, for example, 174 or has found a bug in code that we all thought was working and is 175 willing to provide details, is more than welcome! 176 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.whereis_old"></a><a id="q-whereis_old"></a><p><strong>1.6.</strong></p></td><td align="left" valign="top"><p> 177 What happened to the older libg++? I need that! 178 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-whereis_old"></a></td><td align="left" valign="top"><p> 179 The last libg++ README states 180 <span class="quote"><span class="quote">This package is considered obsolete and is no longer 181 being developed.</span></span> 182 It should not be used for new projects, and won't even compile with 183 recent releases of GCC (or most other C++ compilers). 184 </p><p> 185 More information can be found in the 186 <a class="link" href="manual/backwards.html" title="Backwards Compatibility">Backwards 187 Compatibility</a> section of the libstdc++ manual. 188 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.more_questions"></a><a id="q-more_questions"></a><p><strong>1.7.</strong></p></td><td align="left" valign="top"><p> 189 What if I have more questions? 190 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-more_questions"></a></td><td align="left" valign="top"><p> 191 If you have read the documentation, and your question remains 192 unanswered, then just ask the mailing list. At present, you do not 193 need to be subscribed to the list to send a message to it. More 194 information is available on the homepage (including how to browse 195 the list archives); to send a message to the list, 196 use <code class="email"><<a class="email" href="mailto:libstdc++@gcc.gnu.org">libstdc++@gcc.gnu.org</a>></code>. 197 </p><p> 198 If you have a question that you think should be included 199 here, or if you have a question <span class="emphasis"><em>about</em></span> a question/answer 200 here, please send email to the libstdc++ mailing list, as above. 201 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>2.1. <a href="faq.html#faq.license.what"> 202 What are the license terms for libstdc++? 203 </a></dt><dt>2.2. <a href="faq.html#faq.license.any_program"> 204 So any program which uses libstdc++ falls under the GPL? 205 </a></dt><dt>2.3. <a href="faq.html#faq.license.lgpl"> 206 How is that different from the GNU {Lesser,Library} GPL? 207 </a></dt><dt>2.4. <a href="faq.html#faq.license.what_restrictions"> 208 I see. So, what restrictions are there on programs that use the library? 209 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.license.what"></a><a id="q-license.what"></a><p><strong>2.1.</strong></p></td><td align="left" valign="top"><p> 210 What are the license terms for libstdc++? 211 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.what"></a></td><td align="left" valign="top"><p> 212 See <a class="link" href="manual/license.html" title="License">our license description</a> 213 for these and related questions. 214 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.license.any_program"></a><a id="q-license.any_program"></a><p><strong>2.2.</strong></p></td><td align="left" valign="top"><p> 215 So any program which uses libstdc++ falls under the GPL? 216 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.any_program"></a></td><td align="left" valign="top"><p> 217 No. The special exception permits use of the library in 218 proprietary applications. 219 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.license.lgpl"></a><a id="q-license.lgpl"></a><p><strong>2.3.</strong></p></td><td align="left" valign="top"><p> 220 How is that different from the GNU {Lesser,Library} GPL? 221 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.lgpl"></a></td><td align="left" valign="top"><p> 222 The LGPL requires that users be able to replace the LGPL code with a 223 modified version; this is trivial if the library in question is a C 224 shared library. But there's no way to make that work with C++, where 225 much of the library consists of inline functions and templates, which 226 are expanded inside the code that uses the library. So to allow people 227 to replace the library code, someone using the library would have to 228 distribute their own source, rendering the LGPL equivalent to the GPL. 229 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.license.what_restrictions"></a><a id="q-license.what_restrictions"></a><p><strong>2.4.</strong></p></td><td align="left" valign="top"><p> 230 I see. So, what restrictions are there on programs that use the library? 231 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.what_restrictions"></a></td><td align="left" valign="top"><p> 232 None. We encourage such programs to be released as free software, 233 but we won't punish you or sue you if you choose otherwise. 234 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>3.1. <a href="faq.html#faq.how_to_install">How do I install libstdc++? 235 </a></dt><dt>3.2. <a href="faq.html#faq.how_to_get_sources">How does one get current libstdc++ sources? 236 </a></dt><dt>3.3. <a href="faq.html#faq.how_to_test">How do I know if it works? 237 </a></dt><dt>3.4. <a href="faq.html#faq.how_to_set_paths">How do I insure that the dynamically linked library will be found? 238 </a></dt><dt>3.5. <a href="faq.html#faq.what_is_libsupcxx"> 239 What's libsupc++? 240 </a></dt><dt>3.6. <a href="faq.html#faq.size"> 241 This library is HUGE! 242 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.how_to_install"></a><a id="q-how_to_install"></a><p><strong>3.1.</strong></p></td><td align="left" valign="top"><p>How do I install libstdc++? 243 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how_to_install"></a></td><td align="left" valign="top"><p> 244 Often libstdc++ comes pre-installed as an integral part of many 245 existing GNU/Linux and Unix systems, as well as many embedded 246 development tools. It may be necessary to install extra 247 development packages to get the headers, or the documentation, or 248 the source: please consult your vendor for details. 249 </p><p> 250 To build and install from the GNU GCC sources, please consult the 251 <a class="link" href="manual/setup.html" title="Chapter2.Setup">setup 252 documentation</a> for detailed 253 instructions. You may wish to browse those files ahead 254 of time to get a feel for what's required. 255 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.how_to_get_sources"></a><a id="q-how_to_get_sources"></a><p><strong>3.2.</strong></p></td><td align="left" valign="top"><p>How does one get current libstdc++ sources? 256 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how_to_get_sources"></a></td><td align="left" valign="top"><p> 257 Libstdc++ sources for all official releases can be obtained as 258 part of the GCC sources, available from various sites and 259 mirrors. A full <a class="link" href="https://gcc.gnu.org/mirrors.html" target="_top">list of 260 download sites</a> is provided on the main GCC site. 261 </p><p> 262 Current libstdc++ sources can always be found in the main GCC source 263 repository, available using the appropriate version control tool. 264 At this time, that tool is <span class="application">Git</span>. 265 For more details see the documentation on 266 <a class="link" href="https://gcc.gnu.org/git.html" target="_top">using the Git repository</a>. 267 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.how_to_test"></a><a id="q-how_to_test"></a><p><strong>3.3.</strong></p></td><td align="left" valign="top"><p>How do I know if it works? 268 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how_to_test"></a></td><td align="left" valign="top"><p> 269 Libstdc++ comes with its own validation testsuite, which includes 270 conformance testing, regression testing, ABI testing, and 271 performance testing. Please consult the 272 <a class="link" href="http://gcc.gnu.org/install/test.html" target="_top">testing 273 documentation</a> for GCC and 274 <a class="link" href="manual/test.html" title="Testing">Testing</a> in the libstdc++ 275 manual for more details. 276 </p><p> 277 If you find bugs in the testsuite programs themselves, or if you 278 think of a new test program that should be added to the suite, 279 <span class="emphasis"><em>please</em></span> write up your idea and send it to the list! 280 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.how_to_set_paths"></a><a id="q-how_to_set_paths"></a><p><strong>3.4.</strong></p></td><td align="left" valign="top"><p>How do I insure that the dynamically linked library will be found? 281 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how_to_set_paths"></a></td><td align="left" valign="top"><p> 282 Depending on your platform and library version, the error message might 283 be similar to one of the following: 284 </p><pre class="screen"> 285 ./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory 286 287 /usr/libexec/ld-elf.so.1: Shared object "libstdc++.so.6" not found 288 </pre><p> 289 This doesn't mean that the shared library isn't installed, only 290 that the dynamic linker can't find it. When a dynamically-linked 291 executable is run the linker finds and loads the required shared 292 libraries by searching a pre-configured list of directories. If 293 the directory where you've installed libstdc++ is not in this list 294 then the libraries won't be found. 295 </p><p> 296 If you already have an older version of libstdc++ installed then the 297 error might look like one of the following instead: 298 </p><pre class="screen"> 299 ./a.out: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.20' not found 300 ./a.out: /usr/lib/libstdc++.so.6: version `CXXABI_1.3.8' not found 301 </pre><p> 302 This means the linker found <code class="filename">/usr/lib/libstdc++.so.6</code> 303 but that library belongs to an older version of GCC than was used to 304 compile and link the program <code class="filename">a.out</code> (or some part 305 of it). The program depends on code defined in the newer libstdc++ 306 that belongs to the newer version of GCC, so the linker must be told 307 how to find the newer libstdc++ shared library. 308 </p><p> 309 The simplest way to fix this is 310 to use the <code class="envar">LD_LIBRARY_PATH</code> environment variable, 311 which is a colon-separated list of directories in which the linker 312 will search for shared libraries: 313 </p><pre class="screen"><span class="command"><strong> 314 export LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH 315 </strong></span></pre><p> 316 Here the shell variable <code class="varname">${prefix}</code> is assumed to contain 317 the directory prefix where GCC was installed to. The directory containing 318 the library might depend on whether you want the 32-bit or 64-bit copy 319 of the library, so for example would be 320 <code class="filename">${prefix}/lib64</code> on some systems. 321 The exact environment variable to use will depend on your 322 platform, e.g. <code class="envar">DYLD_LIBRARY_PATH</code> for Darwin, 323 <code class="envar">LD_LIBRARY_PATH_32</code>/<code class="envar">LD_LIBRARY_PATH_64</code> 324 for Solaris 32-/64-bit, 325 and <code class="envar">SHLIB_PATH</code> for HP-UX. 326 </p><p> 327 See the man pages for <span class="command"><strong>ld</strong></span>, <span class="command"><strong>ldd</strong></span> 328 and <span class="command"><strong>ldconfig</strong></span> for more information. The dynamic 329 linker has different names on different platforms but the man page 330 is usually called something such as <code class="filename">ld.so</code>, 331 <code class="filename">rtld</code> or <code class="filename">dld.so</code>. 332 </p><p> 333 Using <code class="envar">LD_LIBRARY_PATH</code> is not always the best solution, 334 <a class="link" href="manual/using_dynamic_or_shared.html#manual.intro.using.linkage.dynamic" title="Finding Dynamic or Shared Libraries">Finding Dynamic or Shared 335 Libraries</a> in the manual gives some alternatives. 336 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.what_is_libsupcxx"></a><a id="q-what_is_libsupcxx"></a><p><strong>3.5.</strong></p></td><td align="left" valign="top"><p> 337 What's libsupc++? 338 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-what_is_libsupcxx"></a></td><td align="left" valign="top"><p> 339 If the only functions from <code class="filename">libstdc++.a</code> 340 which you need are language support functions (those listed in 341 <a class="link" href="manual/support.html" title="Chapter4. Support">clause 18</a> of the 342 standard, e.g., <code class="function">new</code> and 343 <code class="function">delete</code>), then try linking against 344 <code class="filename">libsupc++.a</code>, which is a subset of 345 <code class="filename">libstdc++.a</code>. (Using <span class="command"><strong>gcc</strong></span> 346 instead of <span class="command"><strong>g++</strong></span> and explicitly linking in 347 <code class="filename">libsupc++.a</code> via <code class="option">-lsupc++</code> 348 for the final link step will do it). This library contains only 349 those support routines, one per object file. But if you are 350 using anything from the rest of the library, such as IOStreams 351 or vectors, then you'll still need pieces from 352 <code class="filename">libstdc++.a</code>. 353 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.size"></a><a id="q-size"></a><p><strong>3.6.</strong></p></td><td align="left" valign="top"><p> 354 This library is HUGE! 355 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-size"></a></td><td align="left" valign="top"><p> 356 Usually the size of libraries on disk isn't noticeable. When a 357 link editor (or simply <span class="quote"><span class="quote">linker</span></span>) pulls things from a 358 static archive library, only the necessary object files are copied 359 into your executable, not the entire library. Unfortunately, even 360 if you only need a single function or variable from an object file, 361 the entire object file is extracted. (There's nothing unique to C++ 362 or libstdc++ about this; it's just common behavior, given here 363 for background reasons.) 364 </p><p> 365 Some of the object files which make up 366 <code class="filename">libstdc++.a</code> are rather large. 367 If you create a statically-linked executable with 368 <code class="option">-static</code>, those large object files are suddenly part 369 of your executable. Historically the best way around this was to 370 only place a very few functions (often only a single one) in each 371 source/object file; then extracting a single function is the same 372 as extracting a single <code class="filename">.o</code> file. For libstdc++ this 373 is only possible to a certain extent; the object files in question contain 374 template classes and template functions, pre-instantiated, and 375 splitting those up causes severe maintenance headaches. 376 </p><p> 377 On supported platforms, libstdc++ takes advantage of garbage 378 collection in the GNU linker to get a result similar to separating 379 each symbol into a separate source and object files. On these platforms, 380 GNU ld can place each function and variable into its own 381 section in a <code class="filename">.o</code> file. The GNU linker can then perform 382 garbage collection on unused sections; this reduces the situation to only 383 copying needed functions into the executable, as before, but all 384 happens automatically. 385 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>4.1. <a href="faq.html#faq.other_compilers"> 386 Can libstdc++ be used with non-GNU compilers? 387 </a></dt><dt>4.2. <a href="faq.html#faq.solaris_long_long"> 388 No 'long long' type on Solaris? 389 </a></dt><dt>4.3. <a href="faq.html#faq.predefined"> 390 _XOPEN_SOURCE and _GNU_SOURCE are always defined? 391 </a></dt><dt>4.4. <a href="faq.html#faq.darwin_ctype"> 392 Mac OS X ctype.h is broken! How can I fix it? 393 </a></dt><dt>4.5. <a href="faq.html#faq.threads_i386"> 394 Threading is broken on i386? 395 </a></dt><dt>4.6. <a href="faq.html#faq.atomic_mips"> 396 MIPS atomic operations 397 </a></dt><dt>4.7. <a href="faq.html#faq.linux_glibc"> 398 Recent GNU/Linux glibc required? 399 </a></dt><dt>4.8. <a href="faq.html#faq.freebsd_wchar"> 400 Can't use wchar_t/wstring on FreeBSD 401 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.other_compilers"></a><a id="q-other_compilers"></a><p><strong>4.1.</strong></p></td><td align="left" valign="top"><p> 402 Can libstdc++ be used with non-GNU compilers? 403 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-other_compilers"></a></td><td align="left" valign="top"><p> 404 Perhaps. 405 </p><p> 406 Since the goal of ISO Standardization is for all C++ 407 implementations to be able to share code, libstdc++ should be 408 usable under any ISO-compliant compiler, at least in theory. 409 </p><p> 410 However, the reality is that libstdc++ is targeted and optimized 411 for GCC/G++. This means that often libstdc++ uses specific, 412 non-standard features of G++ that are not present in older 413 versions of proprietary compilers. It may take as much as a year or two 414 after an official release of GCC that contains these features for 415 proprietary tools to support these constructs. 416 </p><p> 417 Recent versions of libstdc++ are known to work with the Clang compiler. 418 In the near past, specific released versions of libstdc++ have 419 been known to work with versions of the EDG C++ compiler, and 420 vendor-specific proprietary C++ compilers such as the Intel ICC 421 C++ compiler. 422 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.solaris_long_long"></a><a id="q-solaris_long_long"></a><p><strong>4.2.</strong></p></td><td align="left" valign="top"><p> 423 No '<span class="type">long long</span>' type on Solaris? 424 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-solaris_long_long"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 425 By default we try to support the C99 <span class="type">long long</span> type. 426 This requires that certain functions from your C library be present. 427 </p><p> 428 Up through release 3.0.2 the platform-specific tests performed by 429 libstdc++ were too general, resulting in a conservative approach 430 to enabling the <span class="type">long long</span> code paths. The most 431 commonly reported platform affected was Solaris. 432 </p><p> 433 This has been fixed for libstdc++ releases greater than 3.0.3. 434 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.predefined"></a><a id="q-predefined"></a><p><strong>4.3.</strong></p></td><td align="left" valign="top"><p> 435 <code class="constant">_XOPEN_SOURCE</code> and <code class="constant">_GNU_SOURCE</code> are always defined? 436 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-predefined"></a></td><td align="left" valign="top"><p>On Solaris, <span class="command"><strong>g++</strong></span> (but not <span class="command"><strong>gcc</strong></span>) 437 always defines the preprocessor macro 438 <code class="constant">_XOPEN_SOURCE</code>. On GNU/Linux, the same happens 439 with <code class="constant">_GNU_SOURCE</code>. (This is not an exhaustive list; 440 other macros and other platforms are also affected.) 441 </p><p>These macros are typically used in C library headers, guarding new 442 versions of functions from their older versions. The C++98 standard 443 library includes the C standard library, but it requires the C90 444 version, which for backwards-compatibility reasons is often not the 445 default for many vendors. 446 </p><p>More to the point, the C++ standard requires behavior which is only 447 available on certain platforms after certain symbols are defined. 448 Usually the issue involves I/O-related typedefs. In order to 449 ensure correctness, the compiler simply predefines those symbols. 450 </p><p>Note that it's not enough to <code class="literal">#define</code> them only when the library is 451 being built (during installation). Since we don't have an 'export' 452 keyword, much of the library exists as headers, which means that 453 the symbols must also be defined as your programs are parsed and 454 compiled. 455 </p><p>To see which symbols are defined, look for 456 <code class="varname">CPLUSPLUS_CPP_SPEC</code> in 457 the gcc config headers for your target (and try changing them to 458 see what happens when building complicated code). You can also run 459 <span class="command"><strong>g++ -E -dM - < /dev/null"</strong></span> to display 460 a list of predefined macros for any particular installation. 461 </p><p>This has been discussed on the mailing lists 462 <a class="link" href="http://gcc.gnu.org/cgi-bin/htsearch?method=and&format=builtin-long&sort=score&words=_XOPEN_SOURCE+Solaris" target="_top">quite a bit</a>. 463 </p><p>This method is something of a wart. We'd like to find a cleaner 464 solution, but nobody yet has contributed the time. 465 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.darwin_ctype"></a><a id="q-darwin_ctype"></a><p><strong>4.4.</strong></p></td><td align="left" valign="top"><p> 466 Mac OS X <code class="filename">ctype.h</code> is broken! How can I fix it? 467 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-darwin_ctype"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 468 This was a long-standing bug in the OS X support. Fortunately, the 469 <a class="link" href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html" target="_top">patch</a> 470 was quite simple, and well-known. 471 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.threads_i386"></a><a id="q-threads_i386"></a><p><strong>4.5.</strong></p></td><td align="left" valign="top"><p> 472 Threading is broken on i386? 473 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-threads_i386"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p>Support for atomic integer operations was broken on i386 474 platforms. The assembly code accidentally used opcodes that are 475 only available on the i486 and later. So if you configured GCC 476 to target, for example, i386-linux, but actually used the programs 477 on an i686, then you would encounter no problems. Only when 478 actually running the code on a i386 will the problem appear. 479 </p><p>This is fixed in 3.2.2. 480 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.atomic_mips"></a><a id="q-atomic_mips"></a><p><strong>4.6.</strong></p></td><td align="left" valign="top"><p> 481 MIPS atomic operations 482 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-atomic_mips"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 483 The atomic locking routines for MIPS targets requires MIPS II 484 and later. A patch went in just after the 3.3 release to 485 make mips* use the generic implementation instead. You can also 486 configure for mipsel-elf as a workaround. 487 </p><p> 488 The mips*-*-linux* port continues to use the MIPS II routines, and more 489 work in this area is expected. 490 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.linux_glibc"></a><a id="q-linux_glibc"></a><p><strong>4.7.</strong></p></td><td align="left" valign="top"><p> 491 Recent GNU/Linux glibc required? 492 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-linux_glibc"></a></td><td align="left" valign="top"><p>When running on GNU/Linux, libstdc++ 3.2.1 (shared library version 493 5.0.1) and later uses localization and formatting code from the system 494 C library (glibc) version 2.2.5 which contains necessary bugfixes. 495 All GNU/Linux distros make more recent versions available now. 496 libstdc++ 4.6.0 and later require glibc 2.3 or later for this 497 localization and formatting code. 498 </p><p>The guideline is simple: the more recent the C++ library, the 499 more recent the C library. (This is also documented in the main 500 GCC installation instructions.) 501 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.freebsd_wchar"></a><a id="q-freebsd_wchar"></a><p><strong>4.8.</strong></p></td><td align="left" valign="top"><p> 502 Can't use <span class="type">wchar_t</span>/<code class="classname">wstring</code> on FreeBSD 503 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-freebsd_wchar"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 504 Older versions of FreeBSD's C library do not have sufficient 505 support for wide character functions, and as a result the 506 libstdc++ configury decides that <span class="type">wchar_t</span> support should be 507 disabled. In addition, the libstdc++ platform checks that 508 enabled <span class="type">wchar_t</span> were quite strict, and not granular 509 enough to detect when the minimal support to 510 enable <span class="type">wchar_t</span> and C++ library structures 511 like <code class="classname">wstring</code> were present. This impacted Solaris, 512 Darwin, and BSD variants, and is fixed in libstdc++ versions post 4.1.0. 513 </p><p> 514 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>5.1. <a href="faq.html#faq.what_works"> 515 What works already? 516 </a></dt><dt>5.2. <a href="faq.html#faq.standard_bugs"> 517 Bugs in the ISO C++ language or library specification 518 </a></dt><dt>5.3. <a href="faq.html#faq.compiler_bugs"> 519 Bugs in the compiler (gcc/g++) and not libstdc++ 520 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.what_works"></a><a id="q-what_works"></a><p><strong>5.1.</strong></p></td><td align="left" valign="top"><p> 521 What works already? 522 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-what_works"></a></td><td align="left" valign="top"><p> 523 Short answer: Pretty much everything <span class="emphasis"><em>works</em></span> 524 except for some corner cases. Support for localization 525 in <code class="classname">locale</code> may be incomplete on some non-GNU 526 platforms. Also dependent on the underlying platform is support 527 for <span class="type">wchar_t</span> and <span class="type">long long</span> specializations, 528 and details of thread support. 529 </p><p> 530 Long answer: See the implementation status pages for 531 <a class="link" href="manual/status.html#status.iso.1998" title="C++ 1998/2003">C++98</a>, 532 <a class="link" href="manual/status.html#status.iso.tr1" title="C++ TR1">TR1</a>, 533 <a class="link" href="manual/status.html#status.iso.2011" title="C++ 2011">C++11</a>, 534 <a class="link" href="manual/status.html#status.iso.2014" title="C++ 2014">C++14</a>, and 535 <a class="link" href="manual/status.html#status.iso.2017" title="C++ 2017">C++17</a>. 536 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.standard_bugs"></a><a id="q-standard_bugs"></a><p><strong>5.2.</strong></p></td><td align="left" valign="top"><p> 537 Bugs in the ISO C++ language or library specification 538 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-standard_bugs"></a></td><td align="left" valign="top"><p> 539 Unfortunately, there are some. 540 </p><p> 541 For those people who are not part of the ISO Library Group 542 (i.e., nearly all of us needing to read this page in the first 543 place), a public list of the library defects is occasionally 544 published on <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/" target="_top">the WG21 545 website</a>. 546 Many of these issues have resulted in 547 <a class="link" href="manual/bugs.html#manual.intro.status.bugs.iso" title="Standard Bugs">code changes in libstdc++</a>. 548 </p><p> 549 If you think you've discovered a new bug that is not listed, 550 please post a message describing your problem to the author of 551 the library issues list. 552 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.compiler_bugs"></a><a id="q-compiler_bugs"></a><p><strong>5.3.</strong></p></td><td align="left" valign="top"><p> 553 Bugs in the compiler (gcc/g++) and not libstdc++ 554 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-compiler_bugs"></a></td><td align="left" valign="top"><p> 555 On occasion, the compiler is wrong. Please be advised that this 556 happens much less often than one would think, and avoid jumping to 557 conclusions. 558 </p><p> 559 First, examine the ISO C++ standard. Second, try another compiler 560 or an older version of the GNU compilers. Third, you can find more 561 information on the libstdc++ and the GCC mailing lists: search 562 these lists with terms describing your issue. 563 </p><p> 564 Before reporting a bug, please examine the 565 <a class="link" href="https://gcc.gnu.org/bugs/" target="_top">bugs database</a>, with the 566 component set to <span class="quote"><span class="quote">c++</span></span>. 567 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>6.1. <a href="faq.html#faq.stream_reopening_fails"> 568 Reopening a stream fails 569 </a></dt><dt>6.2. <a href="faq.html#faq.wefcxx_verbose"> 570 -Weffc++ complains too much 571 </a></dt><dt>6.3. <a href="faq.html#faq.ambiguous_overloads"> 572 Ambiguous overloads after including an old-style header 573 </a></dt><dt>6.4. <a href="faq.html#faq.v2_headers"> 574 The g++-3 headers are not ours 575 </a></dt><dt>6.5. <a href="faq.html#faq.boost_concept_checks"> 576 Errors about *Concept and 577 constraints in the STL 578 </a></dt><dt>6.6. <a href="faq.html#faq.dlopen_crash"> 579 Program crashes when using library code in a 580 dynamically-loaded library 581 </a></dt><dt>6.7. <a href="faq.html#faq.memory_leaks"> 582 Memory leaks in libstdc++ 583 </a></dt><dt>6.8. <a href="faq.html#faq.list_size_on"> 584 list::size() is O(n)! 585 </a></dt><dt>6.9. <a href="faq.html#faq.easy_to_fix"> 586 Aw, that's easy to fix! 587 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.stream_reopening_fails"></a><a id="q-stream_reopening_fails"></a><p><strong>6.1.</strong></p></td><td align="left" valign="top"><p> 588 Reopening a stream fails 589 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-stream_reopening_fails"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 590 Prior to GCC 4.0 this was one of the most-reported non-bug reports. 591 Executing a sequence like this would fail: 592 </p><pre class="programlisting"> 593 #include <fstream> 594 ... 595 std::fstream fs("a_file"); 596 // . 597 // . do things with fs... 598 // . 599 fs.close(); 600 fs.open("a_new_file"); 601 </pre><p> 602 All operations on the re-opened <code class="varname">fs</code> would fail, or at 603 least act very strangely, especially if <code class="varname">fs</code> reached the 604 EOF state on the previous file. 605 The original C++98 standard did not specify behavior in this case, and 606 the <a class="link" href="manual/bugs.html#manual.bugs.dr22">resolution of DR #22</a> was to 607 leave the state flags unchanged on a successful call to 608 <code class="function">open()</code>. 609 You had to insert a call to <code class="function">fs.clear()</code> between the 610 calls to <code class="function">close()</code> and <code class="function">open()</code>, 611 and then everything will work as expected. 612 <span class="emphasis"><em>Update:</em></span> For GCC 4.0 we implemented the resolution 613 of <a class="link" href="manual/bugs.html#manual.bugs.dr409">DR #409</a> and 614 <code class="function">open()</code> 615 now calls <code class="function">clear()</code> on success. 616 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.wefcxx_verbose"></a><a id="q-wefcxx_verbose"></a><p><strong>6.2.</strong></p></td><td align="left" valign="top"><p> 617 -Weffc++ complains too much 618 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-wefcxx_verbose"></a></td><td align="left" valign="top"><p> 619 Many warnings are emitted when <code class="option">-Weffc++</code> is used. Making 620 libstdc++ <code class="option">-Weffc++</code>-clean is not a goal of the project, 621 for a few reasons. Mainly, that option tries to enforce 622 object-oriented programming, while the Standard Library isn't 623 necessarily trying to be OO. The option also enforces outdated guidelines 624 from old editions of the books, and the advice isn't all relevant to 625 modern C++ (especially C++11 and later). 626 </p><p> 627 We do, however, try to have libstdc++ sources as clean as possible. If 628 you see some simple changes that pacify <code class="option">-Weffc++</code> 629 without other drawbacks, send us a patch. 630 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.ambiguous_overloads"></a><a id="q-ambiguous_overloads"></a><p><strong>6.3.</strong></p></td><td align="left" valign="top"><p> 631 Ambiguous overloads after including an old-style header 632 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-ambiguous_overloads"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 633 Another problem is the <code class="literal">rel_ops</code> namespace and the template 634 comparison operator functions contained therein. If they become 635 visible in the same namespace as other comparison functions 636 (e.g., <span class="quote"><span class="quote">using</span></span> them and the 637 <code class="filename"><iterator></code> header), 638 then you will suddenly be faced with huge numbers of ambiguity 639 errors. This was discussed on the mailing list; Nathan Myers 640 <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html" target="_top">sums 641 things up here</a>. The collisions with vector/string iterator 642 types have been fixed for 3.1. 643 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.v2_headers"></a><a id="q-v2_headers"></a><p><strong>6.4.</strong></p></td><td align="left" valign="top"><p> 644 The g++-3 headers are <span class="emphasis"><em>not ours</em></span> 645 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-v2_headers"></a></td><td align="left" valign="top"><div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3><p>This answer is old and probably no longer be relevant.</p></div><p> 646 If you are using headers in 647 <code class="filename">${prefix}/include/g++-3</code>, or if 648 the installed library's name looks like 649 <code class="filename">libstdc++-2.10.a</code> or 650 <code class="filename">libstdc++-libc6-2.10.so</code>, then 651 you are using the old libstdc++-v2 library, which is non-standard and 652 unmaintained. Do not report problems with -v2 to the -v3 653 mailing list. 654 </p><p> 655 For GCC versions 3.0 and 3.1 the libstdc++ header files are installed in 656 <code class="filename">${prefix}/include/g++-v3</code> 657 (see the 'v'?). Starting with version 3.2 the headers are installed in 658 <code class="filename">${prefix}/include/c++/${version}</code> 659 as this prevents headers from previous versions being found by mistake. 660 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.boost_concept_checks"></a><a id="q-boost_concept_checks"></a><p><strong>6.5.</strong></p></td><td align="left" valign="top"><p> 661 Errors about <span class="emphasis"><em>*Concept</em></span> and 662 <span class="emphasis"><em>constraints</em></span> in the STL 663 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-boost_concept_checks"></a></td><td align="left" valign="top"><p> 664 If you see compilation errors containing messages about 665 <span class="errortext">foo Concept</span> and something to do with a 666 <span class="errortext">constraints</span> member function, then most 667 likely you have violated one of the requirements for types used 668 during instantiation of template containers and functions. For 669 example, EqualityComparableConcept appears if your types must be 670 comparable with == and you have not provided this capability (a 671 typo, or wrong visibility, or you just plain forgot, etc). 672 </p><p> 673 More information, including how to optionally enable/disable the 674 checks, is available in the 675 <a class="link" href="manual/concept_checking.html" title="Concept Checking">Diagnostics</a>. 676 chapter of the manual. 677 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.dlopen_crash"></a><a id="q-dlopen_crash"></a><p><strong>6.6.</strong></p></td><td align="left" valign="top"><p> 678 Program crashes when using library code in a 679 dynamically-loaded library 680 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-dlopen_crash"></a></td><td align="left" valign="top"><p> 681 If you are using the C++ library across dynamically-loaded 682 objects, make certain that you are passing the correct options 683 when compiling and linking: 684 </p><div class="literallayout"><p><br /> 685 Compileyourlibrarycomponents:<br /> 686 <span class="command"><strong>g++-fPIC-ca.cc</strong></span><br /> 687 <span class="command"><strong>g++-fPIC-cb.cc</strong></span><br /> 688 ...<br /> 689 <span class="command"><strong>g++-fPIC-cz.cc</strong></span><br /> 690 <br /> 691 Createyourlibrary:<br /> 692 <span class="command"><strong>g++-fPIC-shared-rdynamic-olibfoo.soa.ob.o...z.o</strong></span><br /> 693 <br /> 694 Linktheexecutable:<br /> 695 <span class="command"><strong>g++-fPIC-rdynamic-ofoo...-L.-lfoo-ldl</strong></span><br /> 696 </p></div></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.memory_leaks"></a><a id="q-memory_leaks"></a><p><strong>6.7.</strong></p></td><td align="left" valign="top"><p> 697 <span class="quote"><span class="quote">Memory leaks</span></span> in libstdc++ 698 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-memory_leaks"></a></td><td align="left" valign="top"><p> 699 Since GCC 5.1.0, libstdc++ automatically allocates a pool 700 of a few dozen kilobytes on startup. This pool is used to ensure it's 701 possible to throw exceptions (such as <code class="classname">bad_alloc</code>) 702 even when <code class="code">malloc</code> is unable to allocate any more memory. 703 With some versions of <a class="link" href="http://valgrind.org/" target="_top"><span class="command"><strong>valgrind</strong></span></a> 704 this pool will be shown as "still reachable" when the process exits, e.g. 705 <code class="code">still reachable: 72,704 bytes in 1 blocks</code>. 706 This memory is not a leak, because it's still in use by libstdc++, 707 and the memory will be returned to the OS when the process exits. 708 Later versions of <span class="command"><strong>valgrind</strong></span> know how to free this 709 pool as the process exits, and so won't show any "still reachable" memory. 710 </p><p> 711 In the past, a few people reported that the standard containers appear 712 to leak memory when tested with memory checkers such as 713 <a class="link" href="http://valgrind.org/" target="_top"><span class="command"><strong>valgrind</strong></span></a>. 714 Under some (non-default) configurations the library's allocators keep 715 free memory in a 716 pool for later reuse, rather than deallocating it with <code class="code">delete</code> 717 Although this memory is always reachable by the library and is never 718 lost, memory debugging tools can report it as a leak. If you 719 want to test the library for memory leaks please read 720 <a class="link" href="manual/debug.html#debug.memory" title="Memory Leak Hunting">Tips for memory leak hunting</a> 721 first. 722 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.list_size_on"></a><a id="q-list_size_on"></a><p><strong>6.8.</strong></p></td><td align="left" valign="top"><p> 723 <code class="code">list::size()</code> is O(n)! 724 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-list_size_on"></a></td><td align="left" valign="top"><p> 725 See 726 the <a class="link" href="manual/containers.html" title="Chapter9. Containers">Containers</a> 727 chapter. 728 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.easy_to_fix"></a><a id="q-easy_to_fix"></a><p><strong>6.9.</strong></p></td><td align="left" valign="top"><p> 729 Aw, that's easy to fix! 730 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-easy_to_fix"></a></td><td align="left" valign="top"><p> 731 If you have found a bug in the library and you think you have 732 a working fix, then send it in! The main GCC site has a page 733 on <a class="link" href="http://gcc.gnu.org/contribute.html" target="_top">submitting 734 patches</a> that covers the procedure, but for libstdc++ you 735 should also send the patch to our mailing list in addition to 736 the GCC patches mailing list. The libstdc++ 737 <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">contributors' page</a> 738 also talks about how to submit patches. 739 </p><p> 740 In addition to the description, the patch, and the ChangeLog 741 entry, it is a Good Thing if you can additionally create a small 742 test program to test for the presence of the bug that your patch 743 fixes. Bugs have a way of being reintroduced; if an old bug 744 creeps back in, it will be caught immediately by the testsuite - 745 but only if such a test exists. 746 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>7.1. <a href="faq.html#faq.iterator_as_pod"> 747 string::iterator is not char*; 748 vector<T>::iterator is not T* 749 </a></dt><dt>7.2. <a href="faq.html#faq.what_is_next"> 750 What's next after libstdc++? 751 </a></dt><dt>7.3. <a href="faq.html#faq.sgi_stl"> 752 What about the STL from SGI? 753 </a></dt><dt>7.4. <a href="faq.html#faq.extensions_and_backwards_compat"> 754 Extensions and Backward Compatibility 755 </a></dt><dt>7.5. <a href="faq.html#faq.tr1_support"> 756 Does libstdc++ support TR1? 757 </a></dt><dt>7.6. <a href="faq.html#faq.get_iso_cxx">How do I get a copy of the ISO C++ Standard? 758 </a></dt><dt>7.7. <a href="faq.html#faq.what_is_abi"> 759 What's an ABI and why is it so messy? 760 </a></dt><dt>7.8. <a href="faq.html#faq.size_equals_capacity"> 761 How do I make std::vector<T>::capacity() == std::vector<T>::size? 762 </a></dt></dl></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.iterator_as_pod"></a><a id="faq.iterator_as_pod_q"></a><p><strong>7.1.</strong></p></td><td align="left" valign="top"><p> 763 <code class="classname">string::iterator</code> is not <code class="code">char*</code>; 764 <code class="classname">vector<T>::iterator</code> is not <code class="code">T*</code> 765 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="faq.iterator_as_pod_a"></a></td><td align="left" valign="top"><p> 766 If you have code that depends on container<T> iterators 767 being implemented as pointer-to-T, your code is broken. It's 768 considered a feature, not a bug, that libstdc++ points this out. 769 </p><p> 770 While there are arguments for iterators to be implemented in 771 that manner, A) they aren't very good ones in the long term, 772 and B) they were never guaranteed by the Standard anyway. The 773 type-safety achieved by making iterators a real class rather 774 than a typedef for <span class="type">T*</span> outweighs nearly all opposing 775 arguments. 776 </p><p> 777 Code which does assume that a vector/string iterator <code class="varname">i</code> 778 is a pointer can often be fixed by changing <code class="varname">i</code> in 779 certain expressions to <code class="varname">&*i</code>. 780 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.what_is_next"></a><a id="q-what_is_next"></a><p><strong>7.2.</strong></p></td><td align="left" valign="top"><p> 781 What's next after libstdc++? 782 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-what_is_next"></a></td><td align="left" valign="top"><p> 783 The goal of libstdc++ is to produce a 784 fully-compliant, fully-portable Standard Library. 785 While the C++ Standard continues to evolve the libstdc++ will 786 continue to track it. 787 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.sgi_stl"></a><a id="q-sgi_stl"></a><p><strong>7.3.</strong></p></td><td align="left" valign="top"><p> 788 What about the STL from SGI? 789 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-sgi_stl"></a></td><td align="left" valign="top"><p> 790 The STL (Standard Template Library) was the inspiration for large chunks 791 of the C++ Standard Library, but the terms are not interchangeable and 792 they don't mean the same thing. The C++ Standard Library includes lots of 793 things that didn't come from the STL, and some of them aren't even 794 templates, such as <code class="classname">std::locale</code> and 795 <code class="classname">std::thread</code>. 796 </p><p> 797 Libstdc++-v3 incorporates a lot of code from 798 <a class="link" href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/" target="_top">the SGI STL</a> 799 (the final merge was from 800 <a class="link" href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/whats_new.html" target="_top">release 3.3</a>). 801 The code in libstdc++ contains many fixes and changes compared to the 802 original SGI code. 803 </p><p> 804 In particular, <code class="classname">string</code> is not from SGI and makes no 805 use of their "rope" class (although that is included as an optional 806 extension), neither is <code class="classname">valarray</code> nor some others. 807 Classes like <code class="classname">vector<></code> were from SGI, but have 808 been extensively modified. 809 </p><p> 810 More information on the evolution of libstdc++ can be found at the 811 <a class="link" href="manual/api.html" title="API Evolution and Deprecation History">API 812 evolution</a> 813 and <a class="link" href="manual/backwards.html" title="Backwards Compatibility">backwards 814 compatibility</a> documentation. 815 </p><p> 816 The <a class="link" href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/FAQ.html" target="_top">FAQ</a> 817 for SGI's STL is still recommended reading. 818 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.extensions_and_backwards_compat"></a><a id="q-extensions_and_backwards_compat"></a><p><strong>7.4.</strong></p></td><td align="left" valign="top"><p> 819 Extensions and Backward Compatibility 820 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-extensions_and_backwards_compat"></a></td><td align="left" valign="top"><p> 821 See the <a class="link" href="manual/backwards.html" title="Backwards Compatibility">link</a> on backwards compatibility and <a class="link" href="manual/api.html" title="API Evolution and Deprecation History">link</a> on evolution. 822 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.tr1_support"></a><a id="q-tr1_support"></a><p><strong>7.5.</strong></p></td><td align="left" valign="top"><p> 823 Does libstdc++ support TR1? 824 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-tr1_support"></a></td><td align="left" valign="top"><p> 825 Yes. 826 </p><p> 827 The C++ Standard Library 828 <a class="link" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf" target="_top"> 829 Technical Report 1</a> added many new features to the library. 830 </p><p> 831 The implementation status of TR1 in libstdc++ can be tracked 832 <a class="link" href="manual/status.html#status.iso.tr1" title="C++ TR1">on the TR1 status page</a>. 833 </p><p> 834 New code should probably not use TR1, because almost everything in it has 835 been added to the main C++ Standard Library (usually with significant 836 improvements). 837 The TR1 implementation in libstdc++ is no longer actively maintained. 838 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.get_iso_cxx"></a><a id="q-get_iso_cxx"></a><p><strong>7.6.</strong></p></td><td align="left" valign="top"><p>How do I get a copy of the ISO C++ Standard? 839 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-get_iso_cxx"></a></td><td align="left" valign="top"><p> 840 Please refer to the <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">Contributing</a> 841 section in our manual. 842 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.what_is_abi"></a><a id="q-what_is_abi"></a><p><strong>7.7.</strong></p></td><td align="left" valign="top"><p> 843 What's an ABI and why is it so messy? 844 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-what_is_abi"></a></td><td align="left" valign="top"><p> 845 <acronym class="acronym">ABI</acronym> stands for <span class="quote"><span class="quote">Application Binary 846 Interface</span></span>. Conventionally, it refers to a great 847 mass of details about how arguments are arranged on the call 848 stack and/or in registers, and how various types are arranged 849 and padded in structs. A single CPU design may suffer 850 multiple ABIs designed by different development tool vendors 851 who made different choices, or even by the same vendor for 852 different target applications or compiler versions. In ideal 853 circumstances the CPU designer presents one ABI and all the 854 OSes and compilers use it. In practice every ABI omits 855 details that compiler implementers (consciously or 856 accidentally) must choose for themselves. 857 </p><p> 858 That ABI definition suffices for compilers to generate code so a 859 program can interact safely with an OS and its lowest-level libraries. 860 Users usually want an ABI to encompass more detail, allowing libraries 861 built with different compilers (or different releases of the same 862 compiler!) to be linked together. For C++, this includes many more 863 details than for C, and most CPU designers (for good reasons elaborated 864 below) have not stepped up to publish C++ ABIs. Such an ABI has been 865 defined for the Itanium architecture (see 866 <a class="link" href="https://itanium-cxx-abi.github.io/cxx-abi/" target="_top">C++ 867 ABI for Itanium</a>) and that is used by G++ and other compilers 868 as the de facto standard ABI on many common architectures (including x86). 869 G++ can also use the ARM architecture's EABI, for embedded 870 systems relying only on a <span class="quote"><span class="quote">free-standing implementation</span></span> that 871 doesn't include (much of) the standard library, and the GNU EABI for 872 hosted implementations on ARM. Those ABIs cover low-level details 873 such as virtual function implementation, struct inheritance layout, 874 name mangling, and exception handling. 875 </p><p> 876 A useful C++ ABI must also incorporate many details of the standard 877 library implementation. For a C ABI, the layouts of a few structs 878 (such as <span class="type">FILE</span>, <span class="type">stat</span>, <span class="type">jmpbuf</span>, 879 and the like) and a few macros suffice. 880 For C++, the details include the complete set of names of functions 881 and types used, the offsets of class members and virtual functions, 882 and the actual definitions of all inlines. C++ exposes many more 883 library details to the caller than C does. It makes defining 884 a complete ABI a much bigger undertaking, and requires not just 885 documenting library implementation details, but carefully designing 886 those details so that future bug fixes and optimizations don't 887 force breaking the ABI. 888 </p><p> 889 There are ways to help isolate library implementation details from the 890 ABI, but they trade off against speed. Library details used in inner 891 loops (e.g., <code class="function">getchar</code>) must be exposed and frozen for 892 all time, but many others may reasonably be kept hidden from user code, 893 so they may later be changed. Deciding which, and implementing 894 the decisions, must happen before you can reasonably document a 895 candidate C++ ABI that encompasses the standard library. 896 </p></td></tr><tr class="question"><td align="left" valign="top"><a id="faq.size_equals_capacity"></a><a id="q-size_equals_capacity"></a><p><strong>7.8.</strong></p></td><td align="left" valign="top"><p> 897 How do I make <code class="code">std::vector<T>::capacity() == std::vector<T>::size</code>? 898 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-size_equals_capacity"></a></td><td align="left" valign="top"><p> 899 Since C++11 just call the <code class="function">shrink_to_fit()</code> member 900 function. 901 </p><p> 902 Before C++11, the standard idiom for deallocating a 903 <code class="classname">vector<T></code>'s 904 unused memory was to create a temporary copy of the vector and swap their 905 contents, e.g. for <code class="classname">vector<T> v</code> 906 </p><div class="literallayout"><p><br /> 907 std::vector<T>(v).swap(v);<br /> 908 </p></div><p> 909 The copy will take O(n) time and the swap is constant time. 910 </p><p> 911 See <a class="link" href="manual/strings.html#strings.string.shrink" title="Shrink to Fit">Shrink-to-fit 912 strings</a> for a similar solution for strings. 913 </p></td></tr></tbody></table></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk03.html">Prev</a></td><td width="20%" align="center"><a accesskey="u" href="bk03.html">Up</a></td><td width="40%" align="right"></td></tr><tr><td width="40%" align="left" valign="top"></td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"></td></tr></table></div></body></html>
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