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 2008-2023 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>. 135 All of the rapid development and near-legendary portability 136 that are the hallmarks of an open-source project are applied to libstdc++. 137 </p><p> 138 All of the standard classes and functions from C++98/C++03, C++11 and C++14 139 (such as <code class="classname">string</code>, 140 <code class="classname">vector<></code>, iostreams, algorithms etc.) 141 are freely available and attempt to be fully compliant. 142 Work is ongoing to complete support for the current revision of the 143 ISO C++ Standard. 144 </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> 145 Who's in charge of it? 146 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-who"></a></td><td align="left" valign="top"><p> 147 The libstdc++ project is contributed to by several developers 148 all over the world, in the same way as GCC or the Linux kernel. 149 The current maintainers are listed in the 150 <a class="link" href="https://gcc.gnu.org/cgit/gcc/tree/MAINTAINERS" target="_top"><code class="filename">MAINTAINERS</code></a> 151 file (look for "c++ runtime libs"). 152 </p><p> 153 Development and discussion is held on the libstdc++ mailing 154 list. Subscribing to the list, or searching the list 155 archives, is open to everyone. You can read instructions for 156 doing so on the <a class="link" href="https://gcc.gnu.org/lists.html" target="_top">GCC mailing lists</a> page. 157 If you have questions, ideas, code, or are just curious, sign up! 158 </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> 159 When is libstdc++ going to be finished? 160 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-when"></a></td><td align="left" valign="top"><p> 161 Nathan Myers gave the best of all possible answers, responding to 162 a Usenet article asking this question: <span class="emphasis"><em>Sooner, if you 163 help.</em></span> 164 </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> 165 How do I contribute to the effort? 166 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-how"></a></td><td align="left" valign="top"><p> 167 See the <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">Contributing</a> section in 168 the manual. Subscribing to the mailing list (see above, or 169 the homepage) is a very good idea if you have something to 170 contribute, or if you have spare time and want to 171 help. Contributions don't have to be in the form of source code; 172 anybody who is willing to help write documentation, for example, 173 or has found a bug in code that we all thought was working and is 174 willing to provide details, is more than welcome! 175 </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> 176 What happened to the older libg++? I need that! 177 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-whereis_old"></a></td><td align="left" valign="top"><p> 178 The last libg++ README states 179 <span class="quote"><span class="quote">This package is considered obsolete and is no longer 180 being developed.</span></span> 181 It should not be used for new projects, and won't even compile with 182 recent releases of GCC (or most other C++ compilers). 183 </p><p> 184 More information can be found in the 185 <a class="link" href="manual/backwards.html" title="Backwards Compatibility">Backwards 186 Compatibility</a> section of the libstdc++ manual. 187 </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> 188 What if I have more questions? 189 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-more_questions"></a></td><td align="left" valign="top"><p> 190 If you have read the documentation, and your question remains 191 unanswered, then just ask the mailing list. At present, you do not 192 need to be subscribed to the list to send a message to it. More 193 information is available on the homepage (including how to browse 194 the list archives); to send a message to the list, 195 use <code class="email"><<a class="email" href="mailto:libstdc++@gcc.gnu.org">libstdc++@gcc.gnu.org</a>></code>. 196 </p><p> 197 If you have a question that you think should be included 198 here, or if you have a question <span class="emphasis"><em>about</em></span> a question/answer 199 here, please send email to the libstdc++ mailing list, as above. 200 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>2.1. <a href="faq.html#faq.license.what"> 201 What are the license terms for libstdc++? 202 </a></dt><dt>2.2. <a href="faq.html#faq.license.any_program"> 203 So any program which uses libstdc++ falls under the GPL? 204 </a></dt><dt>2.3. <a href="faq.html#faq.license.lgpl"> 205 How is that different from the GNU {Lesser,Library} GPL? 206 </a></dt><dt>2.4. <a href="faq.html#faq.license.what_restrictions"> 207 I see. So, what restrictions are there on programs that use the library? 208 </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> 209 What are the license terms for libstdc++? 210 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.what"></a></td><td align="left" valign="top"><p> 211 See <a class="link" href="manual/license.html" title="License">our license description</a> 212 for these and related questions. 213 </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> 214 So any program which uses libstdc++ falls under the GPL? 215 </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> 216 No. The special exception permits use of the library in 217 proprietary applications. 218 </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> 219 How is that different from the GNU {Lesser,Library} GPL? 220 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-license.lgpl"></a></td><td align="left" valign="top"><p> 221 The LGPL requires that users be able to replace the LGPL code with a 222 modified version; this is trivial if the library in question is a C 223 shared library. But there's no way to make that work with C++, where 224 much of the library consists of inline functions and templates, which 225 are expanded inside the code that uses the library. So to allow people 226 to replace the library code, someone using the library would have to 227 distribute their own source, rendering the LGPL equivalent to the GPL. 228 </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> 229 I see. So, what restrictions are there on programs that use the library? 230 </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> 231 None. We encourage such programs to be released as free software, 232 but we won't punish you or sue you if you choose otherwise. 233 </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++? 234 </a></dt><dt>3.2. <a href="faq.html#faq.how_to_get_sources">How does one get current libstdc++ sources? 235 </a></dt><dt>3.3. <a href="faq.html#faq.how_to_test">How do I know if it works? 236 </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? 237 </a></dt><dt>3.5. <a href="faq.html#faq.what_is_libsupcxx"> 238 What's libsupc++? 239 </a></dt><dt>3.6. <a href="faq.html#faq.size"> 240 This library is HUGE! 241 </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++? 242 </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> 243 Often libstdc++ comes pre-installed as an integral part of many 244 existing GNU/Linux and Unix systems, as well as many embedded 245 development tools. It may be necessary to install extra 246 development packages to get the headers, or the documentation, or 247 the source: please consult your vendor for details. 248 </p><p> 249 To build and install from the GNU GCC sources, please consult the 250 <a class="link" href="manual/setup.html" title="Chapter2.Setup">setup 251 documentation</a> for detailed 252 instructions. You may wish to browse those files ahead 253 of time to get a feel for what's required. 254 </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? 255 </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> 256 Libstdc++ sources for all official releases can be obtained as 257 part of the GCC sources, available from various sites and 258 mirrors. A full <a class="link" href="https://gcc.gnu.org/mirrors.html" target="_top">list of 259 download sites</a> is provided on the main GCC site. 260 </p><p> 261 Current libstdc++ sources can always be found in the main GCC source 262 repository, available using the appropriate version control tool. 263 At this time, that tool is <span class="application">Git</span>. 264 For more details see the documentation on 265 <a class="link" href="https://gcc.gnu.org/git.html" target="_top">using the Git repository</a>. 266 </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? 267 </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> 268 Libstdc++ comes with its own validation testsuite, which includes 269 conformance testing, regression testing, ABI testing, and 270 performance testing. Please consult the 271 <a class="link" href="http://gcc.gnu.org/install/test.html" target="_top">testing 272 documentation</a> for GCC and 273 <a class="link" href="manual/test.html" title="Testing">Testing</a> in the libstdc++ 274 manual for more details. 275 </p><p> 276 If you find bugs in the testsuite programs themselves, or if you 277 think of a new test program that should be added to the suite, 278 <span class="emphasis"><em>please</em></span> write up your idea and send it to the list! 279 </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? 280 </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> 281 Depending on your platform and library version, the error message might 282 be similar to one of the following: 283 </p><pre class="screen"> 284 ./a.out: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory 285 286 /usr/libexec/ld-elf.so.1: Shared object "libstdc++.so.6" not found 287 </pre><p> 288 This doesn't mean that the shared library isn't installed, only 289 that the dynamic linker can't find it. When a dynamically-linked 290 executable is run the linker finds and loads the required shared 291 libraries by searching a pre-configured list of directories. If 292 the directory where you've installed libstdc++ is not in this list 293 then the libraries won't be found. 294 </p><p> 295 If you already have an older version of libstdc++ installed then the 296 error might look like one of the following instead: 297 </p><pre class="screen"> 298 ./a.out: /usr/lib/libstdc++.so.6: version `GLIBCXX_3.4.20' not found 299 ./a.out: /usr/lib/libstdc++.so.6: version `CXXABI_1.3.8' not found 300 </pre><p> 301 This means the linker found <code class="filename">/usr/lib/libstdc++.so.6</code> 302 but that library belongs to an older version of GCC than was used to 303 compile and link the program <code class="filename">a.out</code> (or some part 304 of it). The program depends on code defined in the newer libstdc++ 305 that belongs to the newer version of GCC, so the linker must be told 306 how to find the newer libstdc++ shared library. 307 </p><p> 308 The simplest way to fix this is 309 to use the <code class="envar">LD_LIBRARY_PATH</code> environment variable, 310 which is a colon-separated list of directories in which the linker 311 will search for shared libraries: 312 </p><pre class="screen"><span class="command"><strong> 313 export LD_LIBRARY_PATH=${prefix}/lib:$LD_LIBRARY_PATH 314 </strong></span></pre><p> 315 Here the shell variable <code class="varname">${prefix}</code> is assumed to contain 316 the directory prefix where GCC was installed to. The directory containing 317 the library might depend on whether you want the 32-bit or 64-bit copy 318 of the library, so for example would be 319 <code class="filename">${prefix}/lib64</code> on some systems. 320 The exact environment variable to use will depend on your 321 platform, e.g. <code class="envar">DYLD_LIBRARY_PATH</code> for Darwin, 322 <code class="envar">LD_LIBRARY_PATH_32</code>/<code class="envar">LD_LIBRARY_PATH_64</code> 323 for Solaris 32-/64-bit, 324 and <code class="envar">SHLIB_PATH</code> for HP-UX. 325 </p><p> 326 See the man pages for <span class="command"><strong>ld</strong></span>, <span class="command"><strong>ldd</strong></span> 327 and <span class="command"><strong>ldconfig</strong></span> for more information. The dynamic 328 linker has different names on different platforms but the man page 329 is usually called something such as <code class="filename">ld.so</code>, 330 <code class="filename">rtld</code> or <code class="filename">dld.so</code>. 331 </p><p> 332 Using <code class="envar">LD_LIBRARY_PATH</code> is not always the best solution, 333 <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 334 Libraries</a> in the manual gives some alternatives. 335 </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> 336 What's libsupc++? 337 </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> 338 If the only functions from <code class="filename">libstdc++.a</code> 339 which you need are language support functions (those listed in 340 <a class="link" href="manual/support.html" title="Chapter4. Support">clause 18</a> of the 341 standard, e.g., <code class="function">new</code> and 342 <code class="function">delete</code>), then try linking against 343 <code class="filename">libsupc++.a</code>, which is a subset of 344 <code class="filename">libstdc++.a</code>. (Using <span class="command"><strong>gcc</strong></span> 345 instead of <span class="command"><strong>g++</strong></span> and explicitly linking in 346 <code class="filename">libsupc++.a</code> via <code class="option">-lsupc++</code> 347 for the final link step will do it). This library contains only 348 those support routines, one per object file. But if you are 349 using anything from the rest of the library, such as IOStreams 350 or vectors, then you'll still need pieces from 351 <code class="filename">libstdc++.a</code>. 352 </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> 353 This library is HUGE! 354 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-size"></a></td><td align="left" valign="top"><p> 355 Usually the size of libraries on disk isn't noticeable. When a 356 link editor (or simply <span class="quote"><span class="quote">linker</span></span>) pulls things from a 357 static archive library, only the necessary object files are copied 358 into your executable, not the entire library. Unfortunately, even 359 if you only need a single function or variable from an object file, 360 the entire object file is extracted. (There's nothing unique to C++ 361 or libstdc++ about this; it's just common behavior, given here 362 for background reasons.) 363 </p><p> 364 Some of the object files which make up 365 <code class="filename">libstdc++.a</code> are rather large. 366 If you create a statically-linked executable with 367 <code class="option">-static</code>, those large object files are suddenly part 368 of your executable. Historically the best way around this was to 369 only place a very few functions (often only a single one) in each 370 source/object file; then extracting a single function is the same 371 as extracting a single <code class="filename">.o</code> file. For libstdc++ this 372 is only possible to a certain extent; the object files in question contain 373 template classes and template functions, pre-instantiated, and 374 splitting those up causes severe maintenance headaches. 375 </p><p> 376 On supported platforms, libstdc++ takes advantage of garbage 377 collection in the GNU linker to get a result similar to separating 378 each symbol into a separate source and object files. On these platforms, 379 GNU ld can place each function and variable into its own 380 section in a <code class="filename">.o</code> file. The GNU linker can then perform 381 garbage collection on unused sections; this reduces the situation to only 382 copying needed functions into the executable, as before, but all 383 happens automatically. 384 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>4.1. <a href="faq.html#faq.other_compilers"> 385 Can libstdc++ be used with non-GNU compilers? 386 </a></dt><dt>4.2. <a href="faq.html#faq.solaris_long_long"> 387 No 'long long' type on Solaris? 388 </a></dt><dt>4.3. <a href="faq.html#faq.predefined"> 389 _XOPEN_SOURCE and _GNU_SOURCE are always defined? 390 </a></dt><dt>4.4. <a href="faq.html#faq.darwin_ctype"> 391 Mac OS X ctype.h is broken! How can I fix it? 392 </a></dt><dt>4.5. <a href="faq.html#faq.threads_i386"> 393 Threading is broken on i386? 394 </a></dt><dt>4.6. <a href="faq.html#faq.atomic_mips"> 395 MIPS atomic operations 396 </a></dt><dt>4.7. <a href="faq.html#faq.linux_glibc"> 397 Recent GNU/Linux glibc required? 398 </a></dt><dt>4.8. <a href="faq.html#faq.freebsd_wchar"> 399 Can't use wchar_t/wstring on FreeBSD 400 </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> 401 Can libstdc++ be used with non-GNU compilers? 402 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-other_compilers"></a></td><td align="left" valign="top"><p> 403 Perhaps. 404 </p><p> 405 Since the goal of ISO Standardization is for all C++ 406 implementations to be able to share code, libstdc++ should be 407 usable under any ISO-compliant compiler, at least in theory. 408 </p><p> 409 However, the reality is that libstdc++ is targeted and optimized 410 for GCC/G++. This means that often libstdc++ uses specific, 411 non-standard features of G++ that are not present in older 412 versions of proprietary compilers. It may take as much as a year or two 413 after an official release of GCC that contains these features for 414 proprietary tools to support these constructs. 415 </p><p> 416 Recent versions of libstdc++ are known to work with the Clang compiler. 417 In the near past, specific released versions of libstdc++ have 418 been known to work with versions of the EDG C++ compiler, and 419 vendor-specific proprietary C++ compilers such as the Intel ICC 420 C++ compiler. 421 </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> 422 No '<span class="type">long long</span>' type on Solaris? 423 </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> 424 By default we try to support the C99 <span class="type">long long</span> type. 425 This requires that certain functions from your C library be present. 426 </p><p> 427 Up through release 3.0.2 the platform-specific tests performed by 428 libstdc++ were too general, resulting in a conservative approach 429 to enabling the <span class="type">long long</span> code paths. The most 430 commonly reported platform affected was Solaris. 431 </p><p> 432 This has been fixed for libstdc++ releases greater than 3.0.3. 433 </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> 434 <code class="constant">_XOPEN_SOURCE</code> and <code class="constant">_GNU_SOURCE</code> are always defined? 435 </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>) 436 always defines the preprocessor macro 437 <code class="constant">_XOPEN_SOURCE</code>. On GNU/Linux, the same happens 438 with <code class="constant">_GNU_SOURCE</code>. (This is not an exhaustive list; 439 other macros and other platforms are also affected.) 440 </p><p>These macros are typically used in C library headers, guarding new 441 versions of functions from their older versions. The C++98 standard 442 library includes the C standard library, but it requires the C90 443 version, which for backwards-compatibility reasons is often not the 444 default for many vendors. 445 </p><p>More to the point, the C++ standard requires behavior which is only 446 available on certain platforms after certain symbols are defined. 447 Usually the issue involves I/O-related typedefs. In order to 448 ensure correctness, the compiler simply predefines those symbols. 449 </p><p>Note that it's not enough to <code class="literal">#define</code> them only when the library is 450 being built (during installation). Since we don't have an 'export' 451 keyword, much of the library exists as headers, which means that 452 the symbols must also be defined as your programs are parsed and 453 compiled. 454 </p><p>To see which symbols are defined, look for 455 <code class="varname">CPLUSPLUS_CPP_SPEC</code> in 456 the gcc config headers for your target (and try changing them to 457 see what happens when building complicated code). You can also run 458 <span class="command"><strong>g++ -E -dM -x c++ /dev/null</strong></span> to display 459 a list of predefined macros for any particular installation. 460 </p><p>This has been discussed on the mailing lists 461 <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>. 462 </p><p>This method is something of a wart. We'd like to find a cleaner 463 solution, but nobody yet has contributed the time. 464 </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> 465 Mac OS X <code class="filename">ctype.h</code> is broken! How can I fix it? 466 </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> 467 This was a long-standing bug in the OS X support. Fortunately, the 468 <a class="link" href="http://gcc.gnu.org/ml/gcc/2002-03/msg00817.html" target="_top">patch</a> 469 was quite simple, and well-known. 470 </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> 471 Threading is broken on i386? 472 </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 473 platforms. The assembly code accidentally used opcodes that are 474 only available on the i486 and later. So if you configured GCC 475 to target, for example, i386-linux, but actually used the programs 476 on an i686, then you would encounter no problems. Only when 477 actually running the code on a i386 will the problem appear. 478 </p><p>This is fixed in 3.2.2. 479 </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> 480 MIPS atomic operations 481 </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> 482 The atomic locking routines for MIPS targets requires MIPS II 483 and later. A patch went in just after the 3.3 release to 484 make mips* use the generic implementation instead. You can also 485 configure for mipsel-elf as a workaround. 486 </p><p> 487 The mips*-*-linux* port continues to use the MIPS II routines, and more 488 work in this area is expected. 489 </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> 490 Recent GNU/Linux glibc required? 491 </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 492 5.0.1) and later uses localization and formatting code from the system 493 C library (glibc) version 2.2.5 which contains necessary bugfixes. 494 All GNU/Linux distros make more recent versions available now. 495 libstdc++ 4.6.0 and later require glibc 2.3 or later for this 496 localization and formatting code. 497 </p><p>The guideline is simple: the more recent the C++ library, the 498 more recent the C library. (This is also documented in the main 499 GCC installation instructions.) 500 </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> 501 Can't use <span class="type">wchar_t</span>/<code class="classname">wstring</code> on FreeBSD 502 </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> 503 Older versions of FreeBSD's C library do not have sufficient 504 support for wide character functions, and as a result the 505 libstdc++ configury decides that <span class="type">wchar_t</span> support should be 506 disabled. In addition, the libstdc++ platform checks that 507 enabled <span class="type">wchar_t</span> were quite strict, and not granular 508 enough to detect when the minimal support to 509 enable <span class="type">wchar_t</span> and C++ library structures 510 like <code class="classname">wstring</code> were present. This impacted Solaris, 511 Darwin, and BSD variants, and is fixed in libstdc++ versions post 4.1.0. 512 </p><p> 513 </p></td></tr><tr class="toc"><td align="left" valign="top" colspan="2"><dl><dt>5.1. <a href="faq.html#faq.what_works"> 514 What works already? 515 </a></dt><dt>5.2. <a href="faq.html#faq.standard_bugs"> 516 Bugs in the ISO C++ language or library specification 517 </a></dt><dt>5.3. <a href="faq.html#faq.compiler_bugs"> 518 Bugs in the compiler (gcc/g++) and not libstdc++ 519 </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> 520 What works already? 521 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-what_works"></a></td><td align="left" valign="top"><p> 522 Short answer: Pretty much everything <span class="emphasis"><em>works</em></span> 523 except for some corner cases. Support for localization 524 in <code class="classname">locale</code> may be incomplete on some non-GNU 525 platforms. Also dependent on the underlying platform is support 526 for <span class="type">wchar_t</span> and <span class="type">long long</span> specializations, 527 and details of thread support. 528 </p><p> 529 Long answer: See the implementation status pages for 530 <a class="link" href="manual/status.html#status.iso.1998" title="C++ 1998/2003">C++98</a>, 531 <a class="link" href="manual/status.html#status.iso.tr1" title="C++ TR1">TR1</a>, 532 <a class="link" href="manual/status.html#status.iso.2011" title="C++ 2011">C++11</a>, 533 <a class="link" href="manual/status.html#status.iso.2014" title="C++ 2014">C++14</a>, and 534 <a class="link" href="manual/status.html#status.iso.2017" title="C++ 2017">C++17</a>. 535 </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> 536 Bugs in the ISO C++ language or library specification 537 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-standard_bugs"></a></td><td align="left" valign="top"><p> 538 Unfortunately, there are some. 539 </p><p> 540 For those people who are not part of the ISO Library Group 541 (i.e., nearly all of us needing to read this page in the first 542 place), a public list of the library defects is occasionally 543 published on <a class="link" href="https://www.open-std.org/jtc1/sc22/wg21/" target="_top">the WG21 544 website</a>. 545 Many of these issues have resulted in 546 <a class="link" href="manual/bugs.html#manual.intro.status.bugs.iso" title="Standard Bugs">code changes in libstdc++</a>. 547 </p><p> 548 If you think you've discovered a new bug that is not listed, 549 please post a message describing your problem to the author of 550 the library issues list. 551 </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> 552 Bugs in the compiler (gcc/g++) and not libstdc++ 553 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-compiler_bugs"></a></td><td align="left" valign="top"><p> 554 On occasion, the compiler is wrong. Please be advised that this 555 happens much less often than one would think, and avoid jumping to 556 conclusions. 557 </p><p> 558 First, examine the ISO C++ standard. Second, try another compiler 559 or an older version of the GNU compilers. Third, you can find more 560 information on the libstdc++ and the GCC mailing lists: search 561 these lists with terms describing your issue. 562 </p><p> 563 Before reporting a bug, please examine the 564 <a class="link" href="https://gcc.gnu.org/bugs/" target="_top">bugs database</a>, with the 565 component set to <span class="quote"><span class="quote">c++</span></span>. 566 </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"> 567 Reopening a stream fails 568 </a></dt><dt>6.2. <a href="faq.html#faq.wefcxx_verbose"> 569 -Weffc++ complains too much 570 </a></dt><dt>6.3. <a href="faq.html#faq.ambiguous_overloads"> 571 Ambiguous overloads after including an old-style header 572 </a></dt><dt>6.4. <a href="faq.html#faq.v2_headers"> 573 The g++-3 headers are not ours 574 </a></dt><dt>6.5. <a href="faq.html#faq.boost_concept_checks"> 575 Errors about *Concept and 576 constraints in the STL 577 </a></dt><dt>6.6. <a href="faq.html#faq.dlopen_crash"> 578 Program crashes when using library code in a 579 dynamically-loaded library 580 </a></dt><dt>6.7. <a href="faq.html#faq.memory_leaks"> 581 Memory leaks in libstdc++ 582 </a></dt><dt>6.8. <a href="faq.html#faq.list_size_on"> 583 list::size() is O(n)! 584 </a></dt><dt>6.9. <a href="faq.html#faq.easy_to_fix"> 585 Aw, that's easy to fix! 586 </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> 587 Reopening a stream fails 588 </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> 589 Prior to GCC 4.0 this was one of the most-reported non-bug reports. 590 Executing a sequence like this would fail: 591 </p><pre class="programlisting"> 592 #include <fstream> 593 ... 594 std::fstream fs("a_file"); 595 // . 596 // . do things with fs... 597 // . 598 fs.close(); 599 fs.open("a_new_file"); 600 </pre><p> 601 All operations on the re-opened <code class="varname">fs</code> would fail, or at 602 least act very strangely, especially if <code class="varname">fs</code> reached the 603 EOF state on the previous file. 604 The original C++98 standard did not specify behavior in this case, and 605 the <a class="link" href="manual/bugs.html#manual.bugs.dr22">resolution of DR #22</a> was to 606 leave the state flags unchanged on a successful call to 607 <code class="function">open()</code>. 608 You had to insert a call to <code class="function">fs.clear()</code> between the 609 calls to <code class="function">close()</code> and <code class="function">open()</code>, 610 and then everything will work as expected. 611 <span class="emphasis"><em>Update:</em></span> For GCC 4.0 we implemented the resolution 612 of <a class="link" href="manual/bugs.html#manual.bugs.dr409">DR #409</a> and 613 <code class="function">open()</code> 614 now calls <code class="function">clear()</code> on success. 615 </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> 616 -Weffc++ complains too much 617 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-wefcxx_verbose"></a></td><td align="left" valign="top"><p> 618 Many warnings are emitted when <code class="option">-Weffc++</code> is used. Making 619 libstdc++ <code class="option">-Weffc++</code>-clean is not a goal of the project, 620 for a few reasons. Mainly, that option tries to enforce 621 object-oriented programming, while the Standard Library isn't 622 necessarily trying to be OO. The option also enforces outdated guidelines 623 from old editions of the books, and the advice isn't all relevant to 624 modern C++ (especially C++11 and later). 625 </p><p> 626 We do, however, try to have libstdc++ sources as clean as possible. If 627 you see some simple changes that pacify <code class="option">-Weffc++</code> 628 without other drawbacks, send us a patch. 629 </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> 630 Ambiguous overloads after including an old-style header 631 </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> 632 Another problem is the <code class="literal">rel_ops</code> namespace and the template 633 comparison operator functions contained therein. If they become 634 visible in the same namespace as other comparison functions 635 (e.g., <span class="quote"><span class="quote">using</span></span> them and the 636 <code class="filename"><iterator></code> header), 637 then you will suddenly be faced with huge numbers of ambiguity 638 errors. This was discussed on the mailing list; Nathan Myers 639 <a class="link" href="http://gcc.gnu.org/ml/libstdc++/2001-01/msg00247.html" target="_top">sums 640 things up here</a>. The collisions with vector/string iterator 641 types have been fixed for 3.1. 642 </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> 643 The g++-3 headers are <span class="emphasis"><em>not ours</em></span> 644 </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> 645 If you are using headers in 646 <code class="filename">${prefix}/include/g++-3</code>, or if 647 the installed library's name looks like 648 <code class="filename">libstdc++-2.10.a</code> or 649 <code class="filename">libstdc++-libc6-2.10.so</code>, then 650 you are using the old libstdc++-v2 library, which is non-standard and 651 unmaintained. Do not report problems with -v2 to the -v3 652 mailing list. 653 </p><p> 654 For GCC versions 3.0 and 3.1 the libstdc++ header files are installed in 655 <code class="filename">${prefix}/include/g++-v3</code> 656 (see the 'v'?). Starting with version 3.2 the headers are installed in 657 <code class="filename">${prefix}/include/c++/${version}</code> 658 as this prevents headers from previous versions being found by mistake. 659 </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> 660 Errors about <span class="emphasis"><em>*Concept</em></span> and 661 <span class="emphasis"><em>constraints</em></span> in the STL 662 </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> 663 If you see compilation errors containing messages about 664 <span class="errortext">foo Concept</span> and something to do with a 665 <span class="errortext">constraints</span> member function, then most 666 likely you have violated one of the requirements for types used 667 during instantiation of template containers and functions. For 668 example, EqualityComparableConcept appears if your types must be 669 comparable with == and you have not provided this capability (a 670 typo, or wrong visibility, or you just plain forgot, etc). 671 </p><p> 672 More information, including how to optionally enable/disable the 673 checks, is available in the 674 <a class="link" href="manual/concept_checking.html" title="Concept Checking">Diagnostics</a>. 675 chapter of the manual. 676 </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> 677 Program crashes when using library code in a 678 dynamically-loaded library 679 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-dlopen_crash"></a></td><td align="left" valign="top"><p> 680 If you are using the C++ library across dynamically-loaded 681 objects, make certain that you are passing the correct options 682 when compiling and linking: 683 </p><div class="literallayout"><p><br /> 684 Compileyourlibrarycomponents:<br /> 685 <span class="command"><strong>g++-fPIC-ca.cc</strong></span><br /> 686 <span class="command"><strong>g++-fPIC-cb.cc</strong></span><br /> 687 ...<br /> 688 <span class="command"><strong>g++-fPIC-cz.cc</strong></span><br /> 689 <br /> 690 Createyourlibrary:<br /> 691 <span class="command"><strong>g++-fPIC-shared-rdynamic-olibfoo.soa.ob.o...z.o</strong></span><br /> 692 <br /> 693 Linktheexecutable:<br /> 694 <span class="command"><strong>g++-fPIC-rdynamic-ofoo...-L.-lfoo-ldl</strong></span><br /> 695 </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> 696 <span class="quote"><span class="quote">Memory leaks</span></span> in libstdc++ 697 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-memory_leaks"></a></td><td align="left" valign="top"><p> 698 Since GCC 5.1.0, libstdc++ automatically allocates a pool 699 of a few dozen kilobytes on startup. This pool is used to ensure it's 700 possible to throw exceptions (such as <code class="classname">bad_alloc</code>) 701 even when <code class="code">malloc</code> is unable to allocate any more memory. 702 With some versions of <a class="link" href="https://valgrind.org" target="_top"><span class="command"><strong>valgrind</strong></span></a> 703 this pool will be shown as "still reachable" when the process exits, e.g. 704 <code class="code">still reachable: 72,704 bytes in 1 blocks</code>. 705 This memory is not a leak, because it's still in use by libstdc++, 706 and the memory will be returned to the OS when the process exits. 707 Later versions of <span class="command"><strong>valgrind</strong></span> know how to free this 708 pool as the process exits, and so won't show any "still reachable" memory. 709 </p><p> 710 In the past, a few people reported that the standard containers appear 711 to leak memory when tested with memory checkers such as 712 <span class="command"><strong>valgrind</strong></span>. 713 Under some (non-default) configurations the library's allocators keep 714 free memory in a 715 pool for later reuse, rather than deallocating it with <code class="code">delete</code> 716 Although this memory is always reachable by the library and is never 717 lost, memory debugging tools can report it as a leak. If you 718 want to test the library for memory leaks please read 719 <a class="link" href="manual/debug.html#debug.memory" title="Memory Leak Hunting">Tips for memory leak hunting</a> 720 first. 721 </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> 722 <code class="code">list::size()</code> is O(n)! 723 </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> 724 See 725 the <a class="link" href="manual/containers.html" title="Chapter9. Containers">Containers</a> 726 chapter. 727 </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> 728 Aw, that's easy to fix! 729 </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> 730 If you have found a bug in the library and you think you have 731 a working fix, then send it in! The main GCC site has a page 732 on <a class="link" href="http://gcc.gnu.org/contribute.html" target="_top">submitting 733 patches</a> that covers the procedure, but for libstdc++ you 734 should also send the patch to our mailing list in addition to 735 the GCC patches mailing list. The libstdc++ 736 <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">contributors' page</a> 737 also talks about how to submit patches. 738 </p><p> 739 In addition to the description, the patch, and the ChangeLog 740 entry, it is a Good Thing if you can additionally create a small 741 test program to test for the presence of the bug that your patch 742 fixes. Bugs have a way of being reintroduced; if an old bug 743 creeps back in, it will be caught immediately by the testsuite - 744 but only if such a test exists. 745 </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"> 746 string::iterator is not char*; 747 vector<T>::iterator is not T* 748 </a></dt><dt>7.2. <a href="faq.html#faq.what_is_next"> 749 What's next after libstdc++? 750 </a></dt><dt>7.3. <a href="faq.html#faq.sgi_stl"> 751 What about the STL from SGI? 752 </a></dt><dt>7.4. <a href="faq.html#faq.extensions_and_backwards_compat"> 753 Extensions and Backward Compatibility 754 </a></dt><dt>7.5. <a href="faq.html#faq.tr1_support"> 755 Does libstdc++ support TR1? 756 </a></dt><dt>7.6. <a href="faq.html#faq.get_iso_cxx">How do I get a copy of the ISO C++ Standard? 757 </a></dt><dt>7.7. <a href="faq.html#faq.what_is_abi"> 758 What's an ABI and why is it so messy? 759 </a></dt><dt>7.8. <a href="faq.html#faq.size_equals_capacity"> 760 How do I make std::vector<T>::capacity() == std::vector<T>::size? 761 </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> 762 <code class="classname">string::iterator</code> is not <code class="code">char*</code>; 763 <code class="classname">vector<T>::iterator</code> is not <code class="code">T*</code> 764 </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> 765 If you have code that depends on container<T> iterators 766 being implemented as pointer-to-T, your code is broken. It's 767 considered a feature, not a bug, that libstdc++ points this out. 768 </p><p> 769 While there are arguments for iterators to be implemented in 770 that manner, A) they aren't very good ones in the long term, 771 and B) they were never guaranteed by the Standard anyway. The 772 type-safety achieved by making iterators a real class rather 773 than a typedef for <span class="type">T*</span> outweighs nearly all opposing 774 arguments. 775 </p><p> 776 Code which does assume that a vector/string iterator <code class="varname">i</code> 777 is a pointer can often be fixed by changing <code class="varname">i</code> in 778 certain expressions to <code class="varname">&*i</code>. 779 </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> 780 What's next after libstdc++? 781 </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> 782 The goal of libstdc++ is to produce a 783 fully-compliant, fully-portable Standard Library. 784 While the C++ Standard continues to evolve the libstdc++ will 785 continue to track it. 786 </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> 787 What about the STL from SGI? 788 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-sgi_stl"></a></td><td align="left" valign="top"><p> 789 The STL (Standard Template Library) was the inspiration for large chunks 790 of the C++ Standard Library, but the terms are not interchangeable and 791 they don't mean the same thing. The C++ Standard Library includes lots of 792 things that didn't come from the STL, and some of them aren't even 793 templates, such as <code class="classname">std::locale</code> and 794 <code class="classname">std::thread</code>. 795 </p><p> 796 Libstdc++-v3 incorporates a lot of code from 797 <a class="link" href="https://web.archive.org/web/20171225062613/http://www.sgi.com/tech/stl/" target="_top">the SGI STL</a> 798 (the final merge was from 799 <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>). 800 The code in libstdc++ contains many fixes and changes compared to the 801 original SGI code. 802 </p><p> 803 In particular, <code class="classname">string</code> is not from SGI and makes no 804 use of their "rope" class (although that is included as an optional 805 extension), neither is <code class="classname">valarray</code> nor some others. 806 Classes like <code class="classname">vector<></code> were from SGI, but have 807 been extensively modified. 808 </p><p> 809 More information on the evolution of libstdc++ can be found at the 810 <a class="link" href="manual/api.html" title="API Evolution and Deprecation History">API 811 evolution</a> 812 and <a class="link" href="manual/backwards.html" title="Backwards Compatibility">backwards 813 compatibility</a> documentation. 814 </p><p> 815 The <a class="link" href="https://web.archive.org/web/20171104092813/http://www.sgi.com/tech/stl/FAQ.html" target="_top">FAQ</a> 816 for SGI's STL is still recommended reading. 817 </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> 818 Extensions and Backward Compatibility 819 </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> 820 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. 821 </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> 822 Does libstdc++ support TR1? 823 </p></td></tr><tr class="answer"><td align="left" valign="top"><a id="a-tr1_support"></a></td><td align="left" valign="top"><p> 824 Yes. 825 </p><p> 826 The C++ Standard Library 827 <a class="link" href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1836.pdf" target="_top"> 828 Technical Report 1</a> added many new features to the library. 829 </p><p> 830 The implementation status of TR1 in libstdc++ can be tracked 831 <a class="link" href="manual/status.html#status.iso.tr1" title="C++ TR1">on the TR1 status page</a>. 832 </p><p> 833 New code should probably not use TR1, because almost everything in it has 834 been added to the main C++ Standard Library (usually with significant 835 improvements). 836 The TR1 implementation in libstdc++ is no longer actively maintained. 837 </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? 838 </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> 839 Please refer to the <a class="link" href="manual/appendix_contributing.html" title="AppendixA. Contributing">Contributing</a> 840 section in our manual. 841 </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> 842 What's an ABI and why is it so messy? 843 </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> 844 <acronym class="acronym">ABI</acronym> stands for <span class="quote"><span class="quote">Application Binary 845 Interface</span></span>. Conventionally, it refers to a great 846 mass of details about how arguments are arranged on the call 847 stack and/or in registers, and how various types are arranged 848 and padded in structs. A single CPU design may suffer 849 multiple ABIs designed by different development tool vendors 850 who made different choices, or even by the same vendor for 851 different target applications or compiler versions. In ideal 852 circumstances the CPU designer presents one ABI and all the 853 OSes and compilers use it. In practice every ABI omits 854 details that compiler implementers (consciously or 855 accidentally) must choose for themselves. 856 </p><p> 857 That ABI definition suffices for compilers to generate code so a 858 program can interact safely with an OS and its lowest-level libraries. 859 Users usually want an ABI to encompass more detail, allowing libraries 860 built with different compilers (or different releases of the same 861 compiler!) to be linked together. For C++, this includes many more 862 details than for C, and most CPU designers (for good reasons elaborated 863 below) have not stepped up to publish C++ ABIs. Such an ABI has been 864 defined for the Itanium architecture (see 865 <a class="link" href="https://itanium-cxx-abi.github.io/cxx-abi/" target="_top">C++ 866 ABI for Itanium</a>) and that is used by G++ and other compilers 867 as the de facto standard ABI on many common architectures (including x86). 868 G++ can also use the ARM architecture's EABI, for embedded 869 systems relying only on a <span class="quote"><span class="quote">free-standing implementation</span></span> that 870 doesn't include (much of) the standard library, and the GNU EABI for 871 hosted implementations on ARM. Those ABIs cover low-level details 872 such as virtual function implementation, struct inheritance layout, 873 name mangling, and exception handling. 874 </p><p> 875 A useful C++ ABI must also incorporate many details of the standard 876 library implementation. For a C ABI, the layouts of a few structs 877 (such as <span class="type">FILE</span>, <span class="type">stat</span>, <span class="type">jmpbuf</span>, 878 and the like) and a few macros suffice. 879 For C++, the details include the complete set of names of functions 880 and types used, the offsets of class members and virtual functions, 881 and the actual definitions of all inlines. C++ exposes many more 882 library details to the caller than C does. It makes defining 883 a complete ABI a much bigger undertaking, and requires not just 884 documenting library implementation details, but carefully designing 885 those details so that future bug fixes and optimizations don't 886 force breaking the ABI. 887 </p><p> 888 There are ways to help isolate library implementation details from the 889 ABI, but they trade off against speed. Library details used in inner 890 loops (e.g., <code class="function">getchar</code>) must be exposed and frozen for 891 all time, but many others may reasonably be kept hidden from user code, 892 so they may later be changed. Deciding which, and implementing 893 the decisions, must happen before you can reasonably document a 894 candidate C++ ABI that encompasses the standard library. 895 </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> 896 How do I make <code class="code">std::vector<T>::capacity() == std::vector<T>::size</code>? 897 </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> 898 Since C++11 just call the <code class="function">shrink_to_fit()</code> member 899 function. 900 </p><p> 901 Before C++11, the standard idiom for deallocating a 902 <code class="classname">vector<T></code>'s 903 unused memory was to create a temporary copy of the vector and swap their 904 contents, e.g. for <code class="classname">vector<T> v</code> 905 </p><div class="literallayout"><p><br /> 906 std::vector<T>(v).swap(v);<br /> 907 </p></div><p> 908 The copy will take O(n) time and the swap is constant time. 909 </p><p> 910 See <a class="link" href="manual/strings.html#strings.string.shrink" title="Shrink to Fit">Shrink-to-fit 911 strings</a> for a similar solution for strings. 912 </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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