1<html> 2 3<head> 4<title>GCC Bugs</title> 5</head> 6 7<body> 8<h1>GCC Bugs</h1> 9 10<p>The latest version of this document is always available at 11<a href="http://gcc.gnu.org/bugs.html">http://gcc.gnu.org/bugs.html</a>.</p> 12 13<hr /> 14 15<h2>Table of Contents</h2> 16<ul> 17<li><a href="#report">Reporting Bugs</a> 18 <ul> 19 <li><a href="#need">What we need</a></li> 20 <li><a href="#dontwant">What we DON'T want</a></li> 21 <li><a href="#where">Where to post it</a></li> 22 <li><a href="#detailed">Detailed bug reporting instructions</a></li> 23 <li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li> 24 <li><a href="#pch">Detailed bug reporting instructions when using a precompiled header</a></li> 25 </ul> 26</li> 27<li><a href="#known">Frequently Reported Bugs in GCC</a> 28 <ul> 29 <li><a href="#cxx">C++</a> 30 <ul> 31 <li><a href="#missing">Missing features</a></li> 32 <li><a href="#fixed34">Bugs fixed in the 3.4 series</a></li> 33 </ul> 34 </li> 35 <li><a href="#fortran">Fortran</a></li> 36 </ul> 37</li> 38<li><a href="#nonbugs">Non-bugs</a> 39 <ul> 40 <li><a href="#nonbugs_general">General</a></li> 41 <li><a href="#nonbugs_c">C</a></li> 42 <li><a href="#nonbugs_cxx">C++</a> 43 <ul> 44 <li><a href="#upgrading">Common problems when upgrading the compiler</a></li> 45 </ul> 46 </li> 47 </ul> 48</li> 49</ul> 50 51<hr /> 52 53<h1><a name="report">Reporting Bugs</a></h1> 54 55<p>The main purpose of a bug report is to enable us to fix the bug. The 56most important prerequisite for this is that the report must be complete and 57self-contained, which we explain in detail below.</p> 58 59<p>Before you report a bug, please check the 60<a href="#known">list of well-known bugs</a> and, <strong>if possible 61in any way, try a current development snapshot</strong>. 62If you want to report a bug with versions of GCC before 3.1 we strongly 63recommend upgrading to the current release first.</p> 64 65<p>Before reporting that GCC compiles your code incorrectly, please 66compile it with <code>gcc -Wall</code> and see whether this shows 67anything wrong with your code that could be the cause instead of a bug 68in GCC.</p> 69 70<h2>Summarized bug reporting instructions</h2> 71 72<p>After this summary, you'll find detailed bug reporting 73instructions, that explain how to obtain some of the information 74requested in this summary.</p> 75 76<h3><a name="need">What we need</a></h3> 77 78<p>Please include in your bug report all of the following items, the first 79three of which can be obtained from the output of <code>gcc -v</code>:</p> 80 81<ul> 82 <li>the exact version of GCC;</li> 83 <li>the system type;</li> 84 <li>the options given when GCC was configured/built;</li> 85 <li>the complete command line that triggers the bug;</li> 86 <li>the compiler output (error messages, warnings, etc.); and</li> 87 <li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the 88 bug, generated by adding <code>-save-temps</code> to the complete 89 compilation command, or, in the case of a bug report for the GNAT front end, 90 a complete set of source files (see below).</li> 91</ul> 92 93<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3> 94 95<ul> 96 <li>A source file that <code>#include</code>s header files that are left 97 out of the bug report (see above)</li> 98 99 <li>That source file and a collection of header files.</li> 100 101 <li>An attached archive (tar, zip, shar, whatever) containing all 102 (or some :-) of the above.</li> 103 104 <li>A code snippet that won't cause the compiler to produce the 105 exact output mentioned in the bug report (e.g., a snippet with just 106 a few lines around the one that <b>apparently</b> triggers the bug, 107 with some pieces replaced with ellipses or comments for extra 108 obfuscation :-)</li> 109 110 <li>The location (URL) of the package that failed to build (we won't 111 download it, anyway, since you've already given us what we need to 112 duplicate the bug, haven't you? :-)</li> 113 114 <li>An error that occurs only some of the times a certain file is 115 compiled, such that retrying a sufficient number of times results in 116 a successful compilation; this is a symptom of a hardware problem, 117 not of a compiler bug (sorry)</li> 118 119 <li>E-mail messages that complement previous, incomplete bug 120 reports. Post a new, self-contained, full bug report instead, if 121 possible as a follow-up to the original bug report</li> 122 123 <li>Assembly files (<code>*.s</code>) produced by the compiler, or any 124 binary files, such as object files, executables, core files, or 125 precompiled header files</li> 126 127 <li>Duplicate bug reports, or reports of bugs already fixed in the 128 development tree, especially those that have already been reported 129 as fixed last week :-)</li> 130 131 <li>Bugs in the assembler, the linker or the C library. These are 132 separate projects, with separate mailing lists and different bug 133 reporting procedures</li> 134 135 <li>Bugs in releases or snapshots of GCC not issued by the GNU 136 Project. Report them to whoever provided you with the release</li> 137 138 <li>Questions about the correctness or the expected behavior of 139 certain constructs that are not GCC extensions. Ask them in forums 140 dedicated to the discussion of the programming language</li> 141</ul> 142 143<h3><a name="where">Where to post it</a></h3> 144 145<p>Please submit your bug report directly to the 146<a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>. 147Alternatively, you can use the <code>gccbug</code> script that mails your bug 148report to the bug database. 149<br /> 150Only if all this is absolutely impossible, mail all information to 151<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p> 152 153<h2><a name="detailed">Detailed bug reporting instructions</a></h2> 154 155<p>Please refer to the <a href="#gnat">next section</a> when reporting 156bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after 157that</a> when reporting bugs that appear when using a precompiled header.</p> 158 159<p>In general, all the information we need can be obtained by 160collecting the command line below, as well as its output and the 161preprocessed file it generates.</p> 162 163<blockquote><p><code>gcc -v -save-temps <i>all-your-options 164source-file</i></code></p></blockquote> 165 166<p>Typically the preprocessed file (extension <code>.i</code> for C or 167<code>.ii</code> for C++, and <code>.f</code> if the preprocessor is used on 168Fortran files) will be large, so please compress the 169resulting file with one of the popular compression programs such as 170bzip2, gzip, zip or compress (in 171decreasing order of preference). Use maximum compression 172(<code>-9</code>) if available. Please include the compressed 173preprocessor output in your bug report, even if the source code is 174freely available elsewhere; it makes the job of our volunteer testers 175much easier.</p> 176 177<p>The <b>only</b> excuses to not send us the preprocessed sources are 178(i) if you've found a bug in the preprocessor, (ii) if you've reduced 179the testcase to a small file that doesn't include any other file or 180(iii) if the bug appears only when using precompiled headers. If you 181can't post the preprocessed sources because they're proprietary code, 182then try to create a small file that triggers the same problem.</p> 183 184<p>Since we're supposed to be able to re-create the assembly output 185(extension <code>.s</code>), you usually should not include 186it in the bug report, although you may want to post parts of it to 187point out assembly code you consider to be wrong.</p> 188 189<p>Whether to use MIME attachments or <code>uuencode</code> is up to 190you. In any case, make sure the compiler command line, version and 191error output are in plain text, so that we don't have to decode the 192bug report in order to tell who should take care of it. A meaningful 193subject indicating language and platform also helps.</p> 194 195<p>Please avoid posting an archive (.tar, .shar or .zip); we generally 196need just a single file to reproduce the bug (the .i/.ii/.f preprocessed 197file), and, by storing it in an archive, you're just making our 198volunteers' jobs harder. Only when your bug report requires multiple 199source files to be reproduced should you use an archive. This is, for example, 200the case if you are using <code>INCLUDE</code> directives in Fortran code, 201which are not processed by the preprocessor, but the compiler. In that case, 202we need the main file and all <code>INCLUDE</code>d files. In any case, 203make sure the compiler version, error message, etc, are included in 204the body of your bug report as plain text, even if needlessly 205duplicated as part of an archive.</p> 206 207<p>If you fail to supply enough information for a bug report to be 208reproduced, someone will probably ask you to post additional 209information (or just ignore your bug report, if they're in a bad day, 210so try to get it right on the first posting :-). In this case, please 211post the additional information to the bug reporting mailing list, not 212just to the person who requested it, unless explicitly told so. If 213possible, please include in this follow-up all the information you had 214supplied in the incomplete bug report (including the preprocessor 215output), so that the new bug report is self-contained.</p> 216 217<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2> 218 219<p>See the <a href="#detailed">previous section</a> for bug reporting 220instructions for GCC language implementations other than Ada.</p> 221 222<p>Bug reports have to contain at least the following information in 223order to be useful:</p> 224 225<ul> 226<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li> 227<li>the system type;</li> 228<li>the options when GCC was configured/built;</li> 229<li>the exact command line passed to the <code>gcc</code> program 230triggering the bug 231(not just the flags passed to <code>gnatmake</code>, but 232<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li> 233<li>a collection of source files for reproducing the bug, 234preferably a minimal set (see below);</li> 235<li>a description of the expected behavior;</li> 236<li>a description of actual behavior.</li> 237</ul> 238 239<p>If your code depends on additional source files (usually package 240specifications), submit the source code for these compilation units in 241a single file that is acceptable input to <code>gnatchop</code>, 242i.e. contains no non-Ada text. If the compilation terminated 243normally, you can usually obtain a list of dependencies using the 244"<code>gnatls -d <i>main_unit</i></code>" command, where 245<code><i>main_unit</i></code> is the file name of the main compilation 246unit (which is also passed to <code>gcc</code>).</p> 247 248<p>If you report a bug which causes the compiler to print a bug box, 249include that bug box in your report, and do not forget to send all the 250source files listed after the bug box along with your report.</p> 251 252<p>If you use <code>gnatprep</code>, be sure to send in preprocessed 253sources (unless you have to report a bug in <code>gnatprep</code>).</p> 254 255<p>When you have checked that your report meets these criteria, please 256submit it according to our <a href="#where">generic instructions</a>. 257(If you use a mailing list for reporting, please include an 258"<code>[Ada]</code>" tag in the subject.)</p> 259 260<h2><a name="pch">Detailed bug reporting instructions when using a 261precompiled header</a></h2> 262 263<p>If you're encountering a bug when using a precompiled header, the 264first thing to do is to delete the precompiled header, and try running 265the same GCC command again. If the bug happens again, the bug doesn't 266really involve precompiled headers, please report it without using 267them by following the instructions <a href="#detailed">above</a>.</p> 268 269<p>If you've found a bug while <i>building</i> a precompiled header 270(for instance, the compiler crashes), follow the usual instructions 271<a href="#detailed">above</a>.</p> 272 273<p>If you've found a real precompiled header bug, what we'll need to 274reproduce it is the sources to build the precompiled header (as a 275single <code>.i</code> file), the source file that uses the 276precompiled header, any other headers that source file includes, and 277the command lines that you used to build the precompiled header and to 278use it.</p> 279 280<p>Please <strong>don't</strong> send us the actual precompiled 281header. It is likely to be very large and we can't use it to 282reproduce the problem.</p> 283 284<hr /> 285 286<h1><a name="known">Frequently Reported Bugs in GCC</a></h1> 287 288<p>This is a list of bugs in GCC that are reported very often, but not 289yet fixed. While it is certainly better to fix bugs instead of documenting 290them, this document might save people the effort of writing a bug report 291when the bug is already well-known.</p> 292 293<p>There are many reasons why a reported bug doesn't get fixed. 294It might be difficult to fix, or fixing it might break compatibility. 295Often, reports get a low priority when there is a simple work-around. 296In particular, bugs caused by invalid code have a simple work-around: 297<em>fix the code</em>.</p> 298 299<hr /> 300 301<h2><a name="cxx">C++</a></h2> 302 303<h3><a name="missing">Missing features</a></h3> 304 305<dl> 306 307<dt>The <code>export</code> keyword is not implemented.</dt> 308<dd><p>Most C++ compilers (G++ included) do not yet implement 309<code>export</code>, which is necessary for separate compilation of 310template declarations and definitions. Without <code>export</code>, a 311template definition must be in scope to be used. The obvious 312workaround is simply to place all definitions in the header 313itself. Alternatively, the compilation unit containing template 314definitions may be included from the header.</p></dd> 315 316</dl> 317 318<h3><a name="fixed34">Bugs fixed in the 3.4 series</a></h3> 319 320<p>The following bugs are present up to (and including) GCC 3.3.x. 321They have been fixed in 3.4.0.</p> 322 323<dl> 324 325<dt>Two-stage name-lookup.</dt> 326 327<dd><p>GCC did not implement two-stage name-lookup (also see 328<a href="#new34">below</a>).</p></dd> 329 330<dt>Covariant return types.</dt> 331 332<dd><p>GCC did not implement non-trivial covariant returns.</p></dd> 333 334<dt>Parse errors for "simple" code.</dt> 335 336<dd><p>GCC gave parse errors for seemingly simple code, such as</p> 337 338<blockquote><pre> 339struct A 340{ 341 A(); 342 A(int); 343}; 344 345struct B 346{ 347 B(A); 348 B(A,A); 349 void foo(); 350}; 351 352A bar() 353{ 354 B b(A(),A(1)); // Variable b, initialized with two temporaries 355 B(A(2)).foo(); // B temporary, initialized with A temporary 356 return (A()); // return A temporary 357} 358</pre></blockquote> 359 360<p>Although being valid code, each of the three lines with a comment was 361rejected by GCC. The work-arounds for older compiler versions proposed 362below do not change the semantics of the programs at all.</p> 363 364<p>The problem in the first case was that GCC started to parse the 365declaration of <code>b</code> as a function called <code>b</code> returning 366<code>B</code>, taking a function returning <code>A</code> as an argument. 367When it encountered the <code>1</code>, it was too late. To show the 368compiler that this should be really an expression, a comma operator with 369a dummy argument could be used:</p> 370 371<blockquote><pre> 372B b((0,A()),A(1)); 373</pre></blockquote> 374 375<p>The work-around for simpler cases like the second one was to add 376additional parentheses around the expressions that were mistaken as 377declarations:</p> 378 379<blockquote><pre> 380(B(A(2))).foo(); 381</pre></blockquote> 382 383<p>In the third case, however, additional parentheses were causing 384the problems: The compiler interpreted <code>A()</code> as a function 385(taking no arguments, returning <code>A</code>), and <code>(A())</code> 386as a cast lacking an expression to be casted, hence the parse error. 387The work-around was to omit the parentheses:</p> 388 389<blockquote><pre> 390return A(); 391</pre></blockquote> 392 393<p>This problem occurred in a number of variants; in <code>throw</code> 394statements, people also frequently put the object in parentheses.</p></dd> 395 396</dl> 397 398<hr /> 399 400<h2><a name="fortran">Fortran</a></h2> 401 402<p>Fortran bugs are documented in the G77 manual rather than 403explicitly listed here. Please see 404<a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of 405Trouble with GNU Fortran</a> in the G77 manual.</p> 406 407<hr /> 408 409<h1><a name="nonbugs">Non-bugs</a></h1> 410 411<p>The following are not actually bugs, but are reported often 412enough to warrant a mention here.</p> 413 414<p>It is not always a bug in the compiler, if code which "worked" in a 415previous version, is now rejected. Earlier versions of GCC sometimes were 416less picky about standard conformance and accepted invalid source code. 417In addition, programming languages themselves change, rendering code 418invalid that used to be conforming (this holds especially for C++). 419In either case, you should update your code to match recent language 420standards.</p> 421 422<hr /> 423 424<h2><a name="nonbugs_general">General</a></h2> 425 426<dl> 427<dt>Problems with floating point numbers - the 428<a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt> 429<dd><p>In a number of cases, GCC appears to perform floating point 430computations incorrectly. For example, the C++ program</p> 431<blockquote><pre> 432#include <iostream> 433 434int main() 435{ 436 double a = 0.5; 437 double b = 0.01; 438 std::cout << (int)(a / b) << std::endl; 439 return 0; 440} 441</pre></blockquote> 442<p>might print 50 on some systems and optimization levels, and 49 on 443others.</p> 444 445<p>This is the result of <em>rounding</em>: The computer cannot 446represent all real numbers exactly, so it has to use 447approximations. When computing with approximation, the computer needs 448to round to the nearest representable number.</p> 449 450<p>This is not a bug in the compiler, but an inherent limitation of 451the floating point types. Please study 452<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a> 453for more information.</p></dd> 454</dl> 455 456<hr /> 457 458<h2><a name="nonbugs_c">C</a></h2> 459 460<dl> 461<dt>Increment/decrement operator (<code>++</code>/<code>--</code>) not 462working as expected - a <a href="http://gcc.gnu.org/PR11751">problem with 463many variations</a>.</dt> 464 465<dd><p>The following expressions have unpredictable results:</p> 466<blockquote><pre> 467x[i]=++i 468foo(i,++i) 469i*(++i) /* special case with foo=="operator*" */ 470std::cout << i << ++i /* foo(foo(std::cout,i),++i) */ 471</pre></blockquote> 472<p>since the <code>i</code> without increment can be evaluated before or 473after <code>++i</code>.</p> 474 475<p>The C and C++ standards have the notion of "sequence points". Everything 476that happens between two sequence points happens in an unspecified order, 477but it has to happen after the first and before the second sequence point. 478The end of a statement and a function call are examples for sequence points, 479whereas assignments and the comma between function arguments are not.</p> 480 481<p>Modifying a value twice between two sequence points as shown in the 482following examples is even worse:</p> 483<blockquote><pre> 484i=++i 485foo(++i,++i) 486(++i)*(++i) /* special case with foo=="operator*" */ 487std::cout << ++i << ++i /* foo(foo(std::cout,++i),++i) */ 488</pre></blockquote> 489<p>This leads to undefined behavior (i.e. the compiler can do 490anything).</p></dd> 491 492 493<dt>Casting does not work as expected when optimization is turned on.</dt> 494 495<dd><p>This is often caused by a violation of aliasing rules, which are part 496of the ISO C standard. These rules say that a program is invalid if you try 497to access a variable through a pointer of an incompatible type. This is 498happening in the following example where a short is accessed through a 499pointer to integer (the code assumes 16-bit <code>short</code>s and 32-bit 500<code>int</code>s):</p> 501<blockquote><pre> 502#include <stdio.h> 503 504int main() 505{ 506 short a[2]; 507 508 a[0]=0x1111; 509 a[1]=0x1111; 510 511 *(int *)a = 0x22222222; /* violation of aliasing rules */ 512 513 printf("%x %x\n", a[0], a[1]); 514 return 0; 515} 516</pre></blockquote> 517<p>The aliasing rules were designed to allow compilers more aggressive 518optimization. Basically, a compiler can assume that all changes to variables 519happen through pointers or references to variables of a type compatible to 520the accessed variable. Dereferencing a pointer that violates the aliasing 521rules results in undefined behavior.</p> 522 523<p>In the case above, the compiler may assume that no access through an 524integer pointer can change the array <code>a</code>, consisting of shorts. 525Thus, <code>printf</code> may be called with the original values of 526<code>a[0]</code> and <code>a[1]</code>. What really happens is up to 527the compiler and may change with architecture and optimization level.</p> 528 529<p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code> 530(which allows alias-based optimizations) by default with <code>-O2</code>. 531And some architectures then really print "1111 1111" as result. Without 532optimization the executable will generate the "expected" output 533"2222 2222".</p> 534 535<p>To disable optimizations based on alias-analysis for faulty legacy code, 536the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p> 537 538<p>The option <code>-Wstrict-aliasing</code> (which is included in 539<code>-Wall</code>) warns about some - but not all - cases of violation 540of aliasing rules when <code>-fstrict-aliasing</code> is active.</p> 541 542<p>To fix the code above, you can use a <code>union</code> instead of a 543cast (note that this is a GCC extension which might not work with other 544compilers):</p> 545<blockquote><pre> 546#include <stdio.h> 547 548int main() 549{ 550 union 551 { 552 short a[2]; 553 int i; 554 } u; 555 556 u.a[0]=0x1111; 557 u.a[1]=0x1111; 558 559 u.i = 0x22222222; 560 561 printf("%x %x\n", u.a[0], u.a[1]); 562 return 0; 563} 564</pre></blockquote> 565<p>Now the result will always be "2222 2222".</p> 566 567<p>For some more insight into the subject, please have a look at 568<a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this 569article</a>.</p></dd> 570 571 572<dt>Cannot use preprocessor directive in macro arguments.</dt> 573<dd><p>Let me guess... you used an older version of GCC to compile code 574that looks something like this:</p> 575<blockquote><pre> 576 memcpy(dest, src, 577#ifdef PLATFORM1 578 12 579#else 580 24 581#endif 582 ); 583</pre></blockquote> 584<p>and you got a whole pile of error messages:</p> 585<blockquote><pre> 586test.c:11: warning: preprocessing directive not recognized within macro arg 587test.c:11: warning: preprocessing directive not recognized within macro arg 588test.c:11: warning: preprocessing directive not recognized within macro arg 589test.c: In function `foo': 590test.c:6: undefined or invalid # directive 591test.c:8: undefined or invalid # directive 592test.c:9: parse error before `24' 593test.c:10: undefined or invalid # directive 594</pre></blockquote> 595 596<p>This is because your C library's <code><string.h></code> happens 597to define <code>memcpy</code> as a macro - which is perfectly legitimate. 598In recent versions of glibc, for example, <code>printf</code> is among those 599functions which are implemented as macros.</p> 600 601<p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code> 602(or any other preprocessor directive) inside the arguments of a macro. The 603code therefore would not compile.</p> 604 605<p>As of GCC 3.3 this kind of construct is always accepted and the 606preprocessor will probably do what you expect, but see the manual for 607detailed semantics.</p> 608 609<p>However, this kind of code is not portable. It is "undefined behavior" 610according to the C standard; that means different compilers may do 611different things with it. It is always possible to rewrite code which 612uses conditionals inside macros so that it doesn't. You could write 613the above example</p> 614<blockquote><pre> 615#ifdef PLATFORM1 616 memcpy(dest, src, 12); 617#else 618 memcpy(dest, src, 24); 619#endif 620</pre></blockquote> 621<p>This is a bit more typing, but I personally think it's better style 622in addition to being more portable.</p></dd> 623 624 625<dt>Cannot initialize a static variable with <code>stdin</code>.</dt> 626<dd><p>This has nothing to do with GCC, but people ask us about it a 627lot. Code like this:</p> 628 629<blockquote><pre> 630#include <stdio.h> 631 632FILE *yyin = stdin; 633</pre></blockquote> 634 635<p>will not compile with GNU libc, because <code>stdin</code> is not a 636constant. This was done deliberately, to make it easier to maintain 637binary compatibility when the type <code>FILE</code> needs to be changed. 638It is surprising for people used to traditional Unix C libraries, but it 639is permitted by the C standard.</p> 640 641<p>This construct commonly occurs in code generated by old versions of 642lex or yacc. We suggest you try regenerating the parser with a 643current version of flex or bison, respectively. In your own code, the 644appropriate fix is to move the initialization to the beginning of 645main.</p> 646 647<p>There is a common misconception that the GCC developers are 648responsible for GNU libc. These are in fact two entirely separate 649projects; please check the 650<a href="http://www.gnu.org/software/libc/">GNU libc web pages</a> 651for details. 652</p></dd> 653</dl> 654 655<hr /> 656 657<h2><a name="nonbugs_cxx">C++</a></h2> 658 659<dl> 660<dt>Nested classes can access private members and types of the containing 661class.</dt> 662 663<dd><p>Defect report 45 clarifies that nested classes are members of the 664class they are nested in, and so are granted access to private members of 665that class.</p></dd> 666 667<dt>G++ emits two copies of constructors and destructors.</dt> 668 669<dd><p>In general there are <em>three</em> types of constructors (and 670destructors).</p> 671<ol> 672<li>The complete object constructor/destructor.</li> 673<li>The base object constructor/destructor.</li> 674<li>The allocating constructor/deallocating destructor.</li> 675</ol> 676<p>The first two are different, when virtual base classes are involved. 677</p></dd> 678 679<dt>Global destructors are not run in the correct order.</dt> 680 681<dd><p>Global destructors should be run in the reverse order of their 682constructors <em>completing</em>. In most cases this is the same as 683the reverse order of constructors <em>starting</em>, but sometimes it 684is different, and that is important. You need to compile and link your 685programs with <code>--use-cxa-atexit</code>. We have not turned this 686switch on by default, as it requires a <code>cxa</code> aware runtime 687library (<code>libc</code>, <code>glibc</code>, or equivalent).</p></dd> 688 689<dt>Classes in exception specifiers must be complete types.</dt> 690 691<dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or 692pointer to incomplete (other than <code><i>cv</i> void *</code>) in 693an exception specification.</p></dd> 694 695<dt>Exceptions don't work in multithreaded applications.</dt> 696 697<dd><p>You need to rebuild g++ and libstdc++ with 698<code>--enable-threads</code>. Remember, C++ exceptions are not like 699hardware interrupts. You cannot throw an exception in one thread and 700catch it in another. You cannot throw an exception from a signal 701handler and catch it in the main thread.</p></dd> 702 703<dt>Templates, scoping, and digraphs.</dt> 704 705<dd><p>If you have a class in the global namespace, say named <code>X</code>, 706and want to give it as a template argument to some other class, say 707<code>std::vector</code>, then <code>std::vector<::X></code> 708fails with a parser error.</p> 709 710<p>The reason is that the standard mandates that the sequence 711<code><:</code> is treated as if it were the token <code>[</code>. 712(There are several such combinations of characters - they are called 713<em>digraphs</em>.) Depending on the version, the compiler then reports 714a parse error before the character <code>:</code> (the colon before 715<code>X</code>) or a missing closing bracket <code>]</code>.</p> 716 717<p>The simplest way to avoid this is to write <code>std::vector< 718::X></code>, i.e. place a space between the opening angle bracket 719and the scope operator.</p></dd> 720 721 722<dt><a name="cxx_rvalbind">Copy constructor access check while 723initializing a reference.</a></dt> 724 725<dd><p>Consider this code:</p> 726 727<blockquote><pre> 728class A 729{ 730public: 731 A(); 732 733private: 734 A(const A&); // private copy ctor 735}; 736 737A makeA(void); 738void foo(const A&); 739 740void bar(void) 741{ 742 foo(A()); // error, copy ctor is not accessible 743 foo(makeA()); // error, copy ctor is not accessible 744 745 A a1; 746 foo(a1); // OK, a1 is a lvalue 747}</pre></blockquote> 748 749<p>Starting with GCC 3.4.0, binding an rvalue to a const reference requires 750an accessible copy constructor. This might be surprising at first sight, 751especially since most popular compilers do not correctly implement this 752rule.</p> 753 754<p>The C++ Standard says that a temporary object should be created in 755this context and its contents filled with a copy of the object we are 756trying to bind to the reference; it also says that the temporary copy 757can be elided, but the semantic constraints (eg. accessibility) of the 758copy constructor still have to be checked.</p> 759 760<p>For further information, you can consult the following paragraphs of 761the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and 762[class.temporary]/2.</p></dd> 763</dl> 764 765<h3><a name="upgrading">Common problems when upgrading the compiler</a></h3> 766 767<h4>ABI changes</h4> 768 769<p>The C++ application binary interface (ABI) consists of two 770components: the first defines how the elements of classes are laid 771out, how functions are called, how function names are mangled, etc; 772the second part deals with the internals of the objects in libstdc++. 773Although we strive for a non-changing ABI, so far we have had to 774modify it with each major release. If you change your compiler to a 775different major release <em>you must recompile all libraries that 776contain C++ code</em>. If you fail to do so you risk getting linker 777errors or malfunctioning programs. Some of our Java support libraries 778also contain C++ code, so you might want to recompile all libraries to 779be safe. It should not be necessary to recompile if you have changed 780to a bug-fix release of the same version of the compiler; bug-fix 781releases are careful to avoid ABI changes. See also the 782<a href="http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html">compatibility 783section</a> of the GCC manual.</p> 784 785<p>Remark: A major release is designated by a change to the first or second 786component of the two- or three-part version number. A minor (bug-fix) 787release is designated by a change to the third component only. Thus GCC 7883.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases 789for GCC 3.3. With the 3.4 series we are introducing a new naming scheme; 790the first release of this series is 3.4.0 instead of just 3.4.</p> 791 792<h4>Standard conformance</h4> 793 794<p>With each release, we try to make G++ conform closer to the ISO C++ standard 795(available at 796<a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>). 797We have also implemented some of the core and library defect reports 798(available at 799<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html</a> 800& 801<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html">http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html</a> 802respectively).</p> 803 804<p>Non-conforming legacy code that worked with older versions of GCC may be 805rejected by more recent compilers. There is no command-line switch to ensure 806compatibility in general, because trying to parse standard-conforming and 807old-style code at the same time would render the C++ frontend unmaintainable. 808However, some non-conforming constructs are allowed when the command-line 809option <code>-fpermissive</code> is used.</p> 810 811<p>Two milestones in standard conformance are GCC 3.0 (including a major 812overhaul of the standard library) and the 3.4.0 version (with its new C++ 813parser).</p> 814 815<h4>New in GCC 3.0</h4> 816 817<ul> 818 819<li>The standard library is much more conformant, and uses the 820<code>std::</code> namespace (which is now a real namespace, not an 821alias for <code>::</code>).</li> 822 823<li>The standard header files for the c library don't end with 824<code>.h</code>, but begin with <code>c</code> (i.e. 825<code><cstdlib></code> rather than <code><stdlib.h></code>). 826The <code>.h</code> names are still available, but are deprecated.</li> 827 828<li><code><strstream></code> is deprecated, use 829<code><sstream></code> instead.</li> 830 831<li><code>streambuf::seekoff</code> & 832<code>streambuf::seekpos</code> are private, instead use 833<code>streambuf::pubseekoff</code> & 834<code>streambuf::pubseekpos</code> respectively.</li> 835 836<li>If <code>std::operator << (std::ostream &, long long)</code> 837doesn't exist, you need to recompile libstdc++ with 838<code>--enable-long-long</code>.</li> 839 840</ul> 841 842<p>If you get lots of errors about things like <code>cout</code> not being 843found, you've most likely forgotten to tell the compiler to look in the 844<code>std::</code> namespace. There are several ways to do this:</p> 845 846<ul> 847 848<li>Say <code>std::cout</code> at the call. This is the most explicit 849way of saying what you mean.</li> 850 851<li>Say <code>using std::cout;</code> somewhere before the call. You 852will need to do this for each function or type you wish to use from the 853standard library.</li> 854 855<li>Say <code>using namespace std;</code> somewhere before the call. 856This is the quick-but-dirty fix. This brings the <em>whole</em> of the 857<code>std::</code> namespace into scope. <em>Never</em> do this in a 858header file, as every user of your header file will be affected by this 859decision.</li> 860 861</ul> 862 863<h4><a name="new34">New in GCC 3.4.0</a></h4> 864 865<p>The new parser brings a lot of improvements, especially concerning 866name-lookup.</p> 867 868<ul> 869 870<li>The "implicit typename" extension got removed (it was already deprecated 871since GCC 3.1), so that the following code is now rejected, see [14.6]: 872<blockquote><pre> 873template <typename> struct A 874{ 875 typedef int X; 876}; 877 878template <typename T> struct B 879{ 880 A<T>::X x; // error 881 typename A<T>::X y; // OK 882}; 883 884B<void> b; 885</pre></blockquote></li> 886 887<li>For similar reasons, the following code now requires the 888<code>template</code> keyword, see [14.2]: 889<blockquote><pre> 890template <typename> struct A 891{ 892 template <int> struct X {}; 893}; 894 895template <typename T> struct B 896{ 897 typename A<T>::X<0> x; // error 898 typename A<T>::template X<0> y; // OK 899}; 900 901B<void> b; 902</pre></blockquote></li> 903 904<li>We now have two-stage name-lookup, so that the following code is 905rejected, see [14.6]/9: 906<blockquote><pre> 907template <typename T> int foo() 908{ 909 return i; // error 910} 911</pre></blockquote></li> 912 913<li>This also affects members of base classes, see [14.6.2]: 914<blockquote><pre> 915template <typename> struct A 916{ 917 int i, j; 918}; 919 920template <typename T> struct B : A<T> 921{ 922 int foo1() { return i; } // error 923 int foo2() { return this->i; } // OK 924 int foo3() { return B<T>::i; } // OK 925 int foo4() { return A<T>::i; } // OK 926 927 using A<T>::j; 928 int foo5() { return j; } // OK 929}; 930</pre></blockquote></li> 931 932</ul> 933 934<p>In addition to the problems listed above, the manual contains a section on 935<a href="http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Misunderstandings.html"> 936Common Misunderstandings with GNU C++</a>.</p> 937 938</body> 939</html> 940