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|
<html>
<head>
<title>GCC Bugs</title>
</head>
<body>
<h1>GCC Bugs</h1>
<p>The latest version of this document is always available at
<a href="http://gcc.gnu.org/bugs.html">http://gcc.gnu.org/bugs.html</a>.</p>
<hr />
<h2>Table of Contents</h2>
<ul>
<li><a href="#report">Reporting Bugs</a>
<ul>
<li><a href="#need">What we need</a></li>
<li><a href="#dontwant">What we DON'T want</a></li>
<li><a href="#where">Where to post it</a></li>
<li><a href="#detailed">Detailed bug reporting instructions</a></li>
<li><a href="#gnat">Detailed bug reporting instructions for GNAT</a></li>
<li><a href="#pch">Detailed bug reporting instructions when using a precompiled header</a></li>
</ul>
</li>
<li><a href="#known">Frequently Reported Bugs in GCC</a>
<ul>
<li><a href="#cxx">C++</a>
<ul>
<li><a href="#missing">Missing features</a></li>
<li><a href="#fixed34">Bugs fixed in the 3.4 series</a></li>
</ul>
</li>
<li><a href="#fortran">Fortran</a></li>
</ul>
</li>
<li><a href="#nonbugs">Non-bugs</a>
<ul>
<li><a href="#nonbugs_general">General</a></li>
<li><a href="#nonbugs_c">C</a></li>
<li><a href="#nonbugs_cxx">C++</a>
<ul>
<li><a href="#upgrading">Common problems when upgrading the compiler</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<hr />
<h1><a name="report">Reporting Bugs</a></h1>
<p>The main purpose of a bug report is to enable us to fix the bug. The
most important prerequisite for this is that the report must be complete and
self-contained, which we explain in detail below.</p>
<p>Before you report a bug, please check the
<a href="#known">list of well-known bugs</a> and, <strong>if possible
in any way, try a current development snapshot</strong>.
If you want to report a bug with versions of GCC before 3.1 we strongly
recommend upgrading to the current release first.</p>
<p>Before reporting that GCC compiles your code incorrectly, please
compile it with <code>gcc -Wall</code> and see whether this shows
anything wrong with your code that could be the cause instead of a bug
in GCC.</p>
<h2>Summarized bug reporting instructions</h2>
<p>After this summary, you'll find detailed bug reporting
instructions, that explain how to obtain some of the information
requested in this summary.</p>
<h3><a name="need">What we need</a></h3>
<p>Please include in your bug report all of the following items, the first
three of which can be obtained from the output of <code>gcc -v</code>:</p>
<ul>
<li>the exact version of GCC;</li>
<li>the system type;</li>
<li>the options given when GCC was configured/built;</li>
<li>the complete command line that triggers the bug;</li>
<li>the compiler output (error messages, warnings, etc.); and</li>
<li>the <em>preprocessed</em> file (<code>*.i*</code>) that triggers the
bug, generated by adding <code>-save-temps</code> to the complete
compilation command, or, in the case of a bug report for the GNAT front end,
a complete set of source files (see below).</li>
</ul>
<h3><a name="dontwant">What we do <strong>not</strong> want</a></h3>
<ul>
<li>A source file that <code>#include</code>s header files that are left
out of the bug report (see above)</li>
<li>That source file and a collection of header files.</li>
<li>An attached archive (tar, zip, shar, whatever) containing all
(or some :-) of the above.</li>
<li>A code snippet that won't cause the compiler to produce the
exact output mentioned in the bug report (e.g., a snippet with just
a few lines around the one that <b>apparently</b> triggers the bug,
with some pieces replaced with ellipses or comments for extra
obfuscation :-)</li>
<li>The location (URL) of the package that failed to build (we won't
download it, anyway, since you've already given us what we need to
duplicate the bug, haven't you? :-)</li>
<li>An error that occurs only some of the times a certain file is
compiled, such that retrying a sufficient number of times results in
a successful compilation; this is a symptom of a hardware problem,
not of a compiler bug (sorry)</li>
<li>E-mail messages that complement previous, incomplete bug
reports. Post a new, self-contained, full bug report instead, if
possible as a follow-up to the original bug report</li>
<li>Assembly files (<code>*.s</code>) produced by the compiler, or any
binary files, such as object files, executables, core files, or
precompiled header files</li>
<li>Duplicate bug reports, or reports of bugs already fixed in the
development tree, especially those that have already been reported
as fixed last week :-)</li>
<li>Bugs in the assembler, the linker or the C library. These are
separate projects, with separate mailing lists and different bug
reporting procedures</li>
<li>Bugs in releases or snapshots of GCC not issued by the GNU
Project. Report them to whoever provided you with the release</li>
<li>Questions about the correctness or the expected behavior of
certain constructs that are not GCC extensions. Ask them in forums
dedicated to the discussion of the programming language</li>
</ul>
<h3><a name="where">Where to post it</a></h3>
<p>Please submit your bug report directly to the
<a href="http://gcc.gnu.org/bugzilla/">GCC bug database</a>.
Alternatively, you can use the <code>gccbug</code> script that mails your bug
report to the bug database.
<br />
Only if all this is absolutely impossible, mail all information to
<a href="mailto:gcc-bugs@gcc.gnu.org">gcc-bugs@gcc.gnu.org</a>.</p>
<h2><a name="detailed">Detailed bug reporting instructions</a></h2>
<p>Please refer to the <a href="#gnat">next section</a> when reporting
bugs in GNAT, the Ada compiler, or to the <a href="#pch">one after
that</a> when reporting bugs that appear when using a precompiled header.</p>
<p>In general, all the information we need can be obtained by
collecting the command line below, as well as its output and the
preprocessed file it generates.</p>
<blockquote><p><code>gcc -v -save-temps <i>all-your-options
source-file</i></code></p></blockquote>
<p>Typically the preprocessed file (extension <code>.i</code> for C or
<code>.ii</code> for C++, and <code>.f</code> if the preprocessor is used on
Fortran files) will be large, so please compress the
resulting file with one of the popular compression programs such as
bzip2, gzip, zip or compress (in
decreasing order of preference). Use maximum compression
(<code>-9</code>) if available. Please include the compressed
preprocessor output in your bug report, even if the source code is
freely available elsewhere; it makes the job of our volunteer testers
much easier.</p>
<p>The <b>only</b> excuses to not send us the preprocessed sources are
(i) if you've found a bug in the preprocessor, (ii) if you've reduced
the testcase to a small file that doesn't include any other file or
(iii) if the bug appears only when using precompiled headers. If you
can't post the preprocessed sources because they're proprietary code,
then try to create a small file that triggers the same problem.</p>
<p>Since we're supposed to be able to re-create the assembly output
(extension <code>.s</code>), you usually should not include
it in the bug report, although you may want to post parts of it to
point out assembly code you consider to be wrong.</p>
<p>Whether to use MIME attachments or <code>uuencode</code> is up to
you. In any case, make sure the compiler command line, version and
error output are in plain text, so that we don't have to decode the
bug report in order to tell who should take care of it. A meaningful
subject indicating language and platform also helps.</p>
<p>Please avoid posting an archive (.tar, .shar or .zip); we generally
need just a single file to reproduce the bug (the .i/.ii/.f preprocessed
file), and, by storing it in an archive, you're just making our
volunteers' jobs harder. Only when your bug report requires multiple
source files to be reproduced should you use an archive. This is, for example,
the case if you are using <code>INCLUDE</code> directives in Fortran code,
which are not processed by the preprocessor, but the compiler. In that case,
we need the main file and all <code>INCLUDE</code>d files. In any case,
make sure the compiler version, error message, etc, are included in
the body of your bug report as plain text, even if needlessly
duplicated as part of an archive.</p>
<p>If you fail to supply enough information for a bug report to be
reproduced, someone will probably ask you to post additional
information (or just ignore your bug report, if they're in a bad day,
so try to get it right on the first posting :-). In this case, please
post the additional information to the bug reporting mailing list, not
just to the person who requested it, unless explicitly told so. If
possible, please include in this follow-up all the information you had
supplied in the incomplete bug report (including the preprocessor
output), so that the new bug report is self-contained.</p>
<h2><a name="gnat">Detailed bug reporting instructions for GNAT</a></h2>
<p>See the <a href="#detailed">previous section</a> for bug reporting
instructions for GCC language implementations other than Ada.</p>
<p>Bug reports have to contain at least the following information in
order to be useful:</p>
<ul>
<li>the exact version of GCC, as shown by "<code>gcc -v</code>";</li>
<li>the system type;</li>
<li>the options when GCC was configured/built;</li>
<li>the exact command line passed to the <code>gcc</code> program
triggering the bug
(not just the flags passed to <code>gnatmake</code>, but
<code>gnatmake</code> prints the parameters it passed to <code>gcc</code>)</li>
<li>a collection of source files for reproducing the bug,
preferably a minimal set (see below);</li>
<li>a description of the expected behavior;</li>
<li>a description of actual behavior.</li>
</ul>
<p>If your code depends on additional source files (usually package
specifications), submit the source code for these compilation units in
a single file that is acceptable input to <code>gnatchop</code>,
i.e. contains no non-Ada text. If the compilation terminated
normally, you can usually obtain a list of dependencies using the
"<code>gnatls -d <i>main_unit</i></code>" command, where
<code><i>main_unit</i></code> is the file name of the main compilation
unit (which is also passed to <code>gcc</code>).</p>
<p>If you report a bug which causes the compiler to print a bug box,
include that bug box in your report, and do not forget to send all the
source files listed after the bug box along with your report.</p>
<p>If you use <code>gnatprep</code>, be sure to send in preprocessed
sources (unless you have to report a bug in <code>gnatprep</code>).</p>
<p>When you have checked that your report meets these criteria, please
submit it according to our <a href="#where">generic instructions</a>.
(If you use a mailing list for reporting, please include an
"<code>[Ada]</code>" tag in the subject.)</p>
<h2><a name="pch">Detailed bug reporting instructions when using a
precompiled header</a></h2>
<p>If you're encountering a bug when using a precompiled header, the
first thing to do is to delete the precompiled header, and try running
the same GCC command again. If the bug happens again, the bug doesn't
really involve precompiled headers, please report it without using
them by following the instructions <a href="#detailed">above</a>.</p>
<p>If you've found a bug while <i>building</i> a precompiled header
(for instance, the compiler crashes), follow the usual instructions
<a href="#detailed">above</a>.</p>
<p>If you've found a real precompiled header bug, what we'll need to
reproduce it is the sources to build the precompiled header (as a
single <code>.i</code> file), the source file that uses the
precompiled header, any other headers that source file includes, and
the command lines that you used to build the precompiled header and to
use it.</p>
<p>Please <strong>don't</strong> send us the actual precompiled
header. It is likely to be very large and we can't use it to
reproduce the problem.</p>
<hr />
<h1><a name="known">Frequently Reported Bugs in GCC</a></h1>
<p>This is a list of bugs in GCC that are reported very often, but not
yet fixed. While it is certainly better to fix bugs instead of documenting
them, this document might save people the effort of writing a bug report
when the bug is already well-known.</p>
<p>There are many reasons why a reported bug doesn't get fixed.
It might be difficult to fix, or fixing it might break compatibility.
Often, reports get a low priority when there is a simple work-around.
In particular, bugs caused by invalid code have a simple work-around:
<em>fix the code</em>.</p>
<hr />
<h2><a name="cxx">C++</a></h2>
<h3><a name="missing">Missing features</a></h3>
<dl>
<dt>The <code>export</code> keyword is not implemented.</dt>
<dd><p>Most C++ compilers (G++ included) do not yet implement
<code>export</code>, which is necessary for separate compilation of
template declarations and definitions. Without <code>export</code>, a
template definition must be in scope to be used. The obvious
workaround is simply to place all definitions in the header
itself. Alternatively, the compilation unit containing template
definitions may be included from the header.</p></dd>
</dl>
<h3><a name="fixed34">Bugs fixed in the 3.4 series</a></h3>
<p>The following bugs are present up to (and including) GCC 3.3.x.
They have been fixed in 3.4.0.</p>
<dl>
<dt>Two-stage name-lookup.</dt>
<dd><p>GCC did not implement two-stage name-lookup (also see
<a href="#new34">below</a>).</p></dd>
<dt>Covariant return types.</dt>
<dd><p>GCC did not implement non-trivial covariant returns.</p></dd>
<dt>Parse errors for "simple" code.</dt>
<dd><p>GCC gave parse errors for seemingly simple code, such as</p>
<blockquote><pre>
struct A
{
A();
A(int);
};
struct B
{
B(A);
B(A,A);
void foo();
};
A bar()
{
B b(A(),A(1)); // Variable b, initialized with two temporaries
B(A(2)).foo(); // B temporary, initialized with A temporary
return (A()); // return A temporary
}
</pre></blockquote>
<p>Although being valid code, each of the three lines with a comment was
rejected by GCC. The work-arounds for older compiler versions proposed
below do not change the semantics of the programs at all.</p>
<p>The problem in the first case was that GCC started to parse the
declaration of <code>b</code> as a function called <code>b</code> returning
<code>B</code>, taking a function returning <code>A</code> as an argument.
When it encountered the <code>1</code>, it was too late. To show the
compiler that this should be really an expression, a comma operator with
a dummy argument could be used:</p>
<blockquote><pre>
B b((0,A()),A(1));
</pre></blockquote>
<p>The work-around for simpler cases like the second one was to add
additional parentheses around the expressions that were mistaken as
declarations:</p>
<blockquote><pre>
(B(A(2))).foo();
</pre></blockquote>
<p>In the third case, however, additional parentheses were causing
the problems: The compiler interpreted <code>A()</code> as a function
(taking no arguments, returning <code>A</code>), and <code>(A())</code>
as a cast lacking an expression to be casted, hence the parse error.
The work-around was to omit the parentheses:</p>
<blockquote><pre>
return A();
</pre></blockquote>
<p>This problem occurred in a number of variants; in <code>throw</code>
statements, people also frequently put the object in parentheses.</p></dd>
</dl>
<hr />
<h2><a name="fortran">Fortran</a></h2>
<p>Fortran bugs are documented in the G77 manual rather than
explicitly listed here. Please see
<a href="http://gcc.gnu.org/onlinedocs/g77/Trouble.html">Known Causes of
Trouble with GNU Fortran</a> in the G77 manual.</p>
<hr />
<h1><a name="nonbugs">Non-bugs</a></h1>
<p>The following are not actually bugs, but are reported often
enough to warrant a mention here.</p>
<p>It is not always a bug in the compiler, if code which "worked" in a
previous version, is now rejected. Earlier versions of GCC sometimes were
less picky about standard conformance and accepted invalid source code.
In addition, programming languages themselves change, rendering code
invalid that used to be conforming (this holds especially for C++).
In either case, you should update your code to match recent language
standards.</p>
<hr />
<h2><a name="nonbugs_general">General</a></h2>
<dl>
<dt>Problems with floating point numbers - the
<a href="http://gcc.gnu.org/PR323">most often reported non-bug</a>.</dt>
<dd><p>In a number of cases, GCC appears to perform floating point
computations incorrectly. For example, the C++ program</p>
<blockquote><pre>
#include <iostream>
int main()
{
double a = 0.5;
double b = 0.01;
std::cout << (int)(a / b) << std::endl;
return 0;
}
</pre></blockquote>
<p>might print 50 on some systems and optimization levels, and 49 on
others.</p>
<p>This is the result of <em>rounding</em>: The computer cannot
represent all real numbers exactly, so it has to use
approximations. When computing with approximation, the computer needs
to round to the nearest representable number.</p>
<p>This is not a bug in the compiler, but an inherent limitation of
the floating point types. Please study
<a href="http://www.validlab.com/goldberg/paper.ps">this paper</a>
for more information.</p></dd>
</dl>
<hr />
<h2><a name="nonbugs_c">C</a></h2>
<dl>
<dt>Increment/decrement operator (<code>++</code>/<code>--</code>) not
working as expected - a <a href="http://gcc.gnu.org/PR11751">problem with
many variations</a>.</dt>
<dd><p>The following expressions have unpredictable results:</p>
<blockquote><pre>
x[i]=++i
foo(i,++i)
i*(++i) /* special case with foo=="operator*" */
std::cout << i << ++i /* foo(foo(std::cout,i),++i) */
</pre></blockquote>
<p>since the <code>i</code> without increment can be evaluated before or
after <code>++i</code>.</p>
<p>The C and C++ standards have the notion of "sequence points". Everything
that happens between two sequence points happens in an unspecified order,
but it has to happen after the first and before the second sequence point.
The end of a statement and a function call are examples for sequence points,
whereas assignments and the comma between function arguments are not.</p>
<p>Modifying a value twice between two sequence points as shown in the
following examples is even worse:</p>
<blockquote><pre>
i=++i
foo(++i,++i)
(++i)*(++i) /* special case with foo=="operator*" */
std::cout << ++i << ++i /* foo(foo(std::cout,++i),++i) */
</pre></blockquote>
<p>This leads to undefined behavior (i.e. the compiler can do
anything).</p></dd>
<dt>Casting does not work as expected when optimization is turned on.</dt>
<dd><p>This is often caused by a violation of aliasing rules, which are part
of the ISO C standard. These rules say that a program is invalid if you try
to access a variable through a pointer of an incompatible type. This is
happening in the following example where a short is accessed through a
pointer to integer (the code assumes 16-bit <code>short</code>s and 32-bit
<code>int</code>s):</p>
<blockquote><pre>
#include <stdio.h>
int main()
{
short a[2];
a[0]=0x1111;
a[1]=0x1111;
*(int *)a = 0x22222222; /* violation of aliasing rules */
printf("%x %x\n", a[0], a[1]);
return 0;
}
</pre></blockquote>
<p>The aliasing rules were designed to allow compilers more aggressive
optimization. Basically, a compiler can assume that all changes to variables
happen through pointers or references to variables of a type compatible to
the accessed variable. Dereferencing a pointer that violates the aliasing
rules results in undefined behavior.</p>
<p>In the case above, the compiler may assume that no access through an
integer pointer can change the array <code>a</code>, consisting of shorts.
Thus, <code>printf</code> may be called with the original values of
<code>a[0]</code> and <code>a[1]</code>. What really happens is up to
the compiler and may change with architecture and optimization level.</p>
<p>Recent versions of GCC turn on the option <code>-fstrict-aliasing</code>
(which allows alias-based optimizations) by default with <code>-O2</code>.
And some architectures then really print "1111 1111" as result. Without
optimization the executable will generate the "expected" output
"2222 2222".</p>
<p>To disable optimizations based on alias-analysis for faulty legacy code,
the option <code>-fno-strict-aliasing</code> can be used as a work-around.</p>
<p>The option <code>-Wstrict-aliasing</code> (which is included in
<code>-Wall</code>) warns about some - but not all - cases of violation
of aliasing rules when <code>-fstrict-aliasing</code> is active.</p>
<p>To fix the code above, you can use a <code>union</code> instead of a
cast (note that this is a GCC extension which might not work with other
compilers):</p>
<blockquote><pre>
#include <stdio.h>
int main()
{
union
{
short a[2];
int i;
} u;
u.a[0]=0x1111;
u.a[1]=0x1111;
u.i = 0x22222222;
printf("%x %x\n", u.a[0], u.a[1]);
return 0;
}
</pre></blockquote>
<p>Now the result will always be "2222 2222".</p>
<p>For some more insight into the subject, please have a look at
<a href="http://mail-index.NetBSD.org/tech-kern/2003/08/11/0001.html">this
article</a>.</p></dd>
<dt>Cannot use preprocessor directive in macro arguments.</dt>
<dd><p>Let me guess... you used an older version of GCC to compile code
that looks something like this:</p>
<blockquote><pre>
memcpy(dest, src,
#ifdef PLATFORM1
12
#else
24
#endif
);
</pre></blockquote>
<p>and you got a whole pile of error messages:</p>
<blockquote><pre>
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c:11: warning: preprocessing directive not recognized within macro arg
test.c: In function `foo':
test.c:6: undefined or invalid # directive
test.c:8: undefined or invalid # directive
test.c:9: parse error before `24'
test.c:10: undefined or invalid # directive
</pre></blockquote>
<p>This is because your C library's <code><string.h></code> happens
to define <code>memcpy</code> as a macro - which is perfectly legitimate.
In recent versions of glibc, for example, <code>printf</code> is among those
functions which are implemented as macros.</p>
<p>Versions of GCC prior to 3.3 did not allow you to put <code>#ifdef</code>
(or any other preprocessor directive) inside the arguments of a macro. The
code therefore would not compile.</p>
<p>As of GCC 3.3 this kind of construct is always accepted and the
preprocessor will probably do what you expect, but see the manual for
detailed semantics.</p>
<p>However, this kind of code is not portable. It is "undefined behavior"
according to the C standard; that means different compilers may do
different things with it. It is always possible to rewrite code which
uses conditionals inside macros so that it doesn't. You could write
the above example</p>
<blockquote><pre>
#ifdef PLATFORM1
memcpy(dest, src, 12);
#else
memcpy(dest, src, 24);
#endif
</pre></blockquote>
<p>This is a bit more typing, but I personally think it's better style
in addition to being more portable.</p></dd>
<dt>Cannot initialize a static variable with <code>stdin</code>.</dt>
<dd><p>This has nothing to do with GCC, but people ask us about it a
lot. Code like this:</p>
<blockquote><pre>
#include <stdio.h>
FILE *yyin = stdin;
</pre></blockquote>
<p>will not compile with GNU libc, because <code>stdin</code> is not a
constant. This was done deliberately, to make it easier to maintain
binary compatibility when the type <code>FILE</code> needs to be changed.
It is surprising for people used to traditional Unix C libraries, but it
is permitted by the C standard.</p>
<p>This construct commonly occurs in code generated by old versions of
lex or yacc. We suggest you try regenerating the parser with a
current version of flex or bison, respectively. In your own code, the
appropriate fix is to move the initialization to the beginning of
main.</p>
<p>There is a common misconception that the GCC developers are
responsible for GNU libc. These are in fact two entirely separate
projects; please check the
<a href="http://www.gnu.org/software/libc/">GNU libc web pages</a>
for details.
</p></dd>
</dl>
<hr />
<h2><a name="nonbugs_cxx">C++</a></h2>
<dl>
<dt>Nested classes can access private members and types of the containing
class.</dt>
<dd><p>Defect report 45 clarifies that nested classes are members of the
class they are nested in, and so are granted access to private members of
that class.</p></dd>
<dt>G++ emits two copies of constructors and destructors.</dt>
<dd><p>In general there are <em>three</em> types of constructors (and
destructors).</p>
<ol>
<li>The complete object constructor/destructor.</li>
<li>The base object constructor/destructor.</li>
<li>The allocating constructor/deallocating destructor.</li>
</ol>
<p>The first two are different, when virtual base classes are involved.
</p></dd>
<dt>Global destructors are not run in the correct order.</dt>
<dd><p>Global destructors should be run in the reverse order of their
constructors <em>completing</em>. In most cases this is the same as
the reverse order of constructors <em>starting</em>, but sometimes it
is different, and that is important. You need to compile and link your
programs with <code>--use-cxa-atexit</code>. We have not turned this
switch on by default, as it requires a <code>cxa</code> aware runtime
library (<code>libc</code>, <code>glibc</code>, or equivalent).</p></dd>
<dt>Classes in exception specifiers must be complete types.</dt>
<dd><p>[15.4]/1 tells you that you cannot have an incomplete type, or
pointer to incomplete (other than <code><i>cv</i> void *</code>) in
an exception specification.</p></dd>
<dt>Exceptions don't work in multithreaded applications.</dt>
<dd><p>You need to rebuild g++ and libstdc++ with
<code>--enable-threads</code>. Remember, C++ exceptions are not like
hardware interrupts. You cannot throw an exception in one thread and
catch it in another. You cannot throw an exception from a signal
handler and catch it in the main thread.</p></dd>
<dt>Templates, scoping, and digraphs.</dt>
<dd><p>If you have a class in the global namespace, say named <code>X</code>,
and want to give it as a template argument to some other class, say
<code>std::vector</code>, then <code>std::vector<::X></code>
fails with a parser error.</p>
<p>The reason is that the standard mandates that the sequence
<code><:</code> is treated as if it were the token <code>[</code>.
(There are several such combinations of characters - they are called
<em>digraphs</em>.) Depending on the version, the compiler then reports
a parse error before the character <code>:</code> (the colon before
<code>X</code>) or a missing closing bracket <code>]</code>.</p>
<p>The simplest way to avoid this is to write <code>std::vector<
::X></code>, i.e. place a space between the opening angle bracket
and the scope operator.</p></dd>
<dt><a name="cxx_rvalbind">Copy constructor access check while
initializing a reference.</a></dt>
<dd><p>Consider this code:</p>
<blockquote><pre>
class A
{
public:
A();
private:
A(const A&); // private copy ctor
};
A makeA(void);
void foo(const A&);
void bar(void)
{
foo(A()); // error, copy ctor is not accessible
foo(makeA()); // error, copy ctor is not accessible
A a1;
foo(a1); // OK, a1 is a lvalue
}</pre></blockquote>
<p>Starting with GCC 3.4.0, binding an rvalue to a const reference requires
an accessible copy constructor. This might be surprising at first sight,
especially since most popular compilers do not correctly implement this
rule.</p>
<p>The C++ Standard says that a temporary object should be created in
this context and its contents filled with a copy of the object we are
trying to bind to the reference; it also says that the temporary copy
can be elided, but the semantic constraints (eg. accessibility) of the
copy constructor still have to be checked.</p>
<p>For further information, you can consult the following paragraphs of
the C++ standard: [dcl.init.ref]/5, bullet 2, sub-bullet 1, and
[class.temporary]/2.</p></dd>
</dl>
<h3><a name="upgrading">Common problems when upgrading the compiler</a></h3>
<h4>ABI changes</h4>
<p>The C++ application binary interface (ABI) consists of two
components: the first defines how the elements of classes are laid
out, how functions are called, how function names are mangled, etc;
the second part deals with the internals of the objects in libstdc++.
Although we strive for a non-changing ABI, so far we have had to
modify it with each major release. If you change your compiler to a
different major release <em>you must recompile all libraries that
contain C++ code</em>. If you fail to do so you risk getting linker
errors or malfunctioning programs. Some of our Java support libraries
also contain C++ code, so you might want to recompile all libraries to
be safe. It should not be necessary to recompile if you have changed
to a bug-fix release of the same version of the compiler; bug-fix
releases are careful to avoid ABI changes. See also the
<a href="http://gcc.gnu.org/onlinedocs/gcc/Compatibility.html">compatibility
section</a> of the GCC manual.</p>
<p>Remark: A major release is designated by a change to the first or second
component of the two- or three-part version number. A minor (bug-fix)
release is designated by a change to the third component only. Thus GCC
3.2 and 3.3 are major releases, while 3.3.1 and 3.3.2 are bug-fix releases
for GCC 3.3. With the 3.4 series we are introducing a new naming scheme;
the first release of this series is 3.4.0 instead of just 3.4.</p>
<h4>Standard conformance</h4>
<p>With each release, we try to make G++ conform closer to the ISO C++ standard
(available at
<a href="http://www.ncits.org/cplusplus.htm">http://www.ncits.org/cplusplus.htm</a>).
We have also implemented some of the core and library defect reports
(available at
<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>
&
<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>
respectively).</p>
<p>Non-conforming legacy code that worked with older versions of GCC may be
rejected by more recent compilers. There is no command-line switch to ensure
compatibility in general, because trying to parse standard-conforming and
old-style code at the same time would render the C++ frontend unmaintainable.
However, some non-conforming constructs are allowed when the command-line
option <code>-fpermissive</code> is used.</p>
<p>Two milestones in standard conformance are GCC 3.0 (including a major
overhaul of the standard library) and the 3.4.0 version (with its new C++
parser).</p>
<h4>New in GCC 3.0</h4>
<ul>
<li>The standard library is much more conformant, and uses the
<code>std::</code> namespace (which is now a real namespace, not an
alias for <code>::</code>).</li>
<li>The standard header files for the c library don't end with
<code>.h</code>, but begin with <code>c</code> (i.e.
<code><cstdlib></code> rather than <code><stdlib.h></code>).
The <code>.h</code> names are still available, but are deprecated.</li>
<li><code><strstream></code> is deprecated, use
<code><sstream></code> instead.</li>
<li><code>streambuf::seekoff</code> &
<code>streambuf::seekpos</code> are private, instead use
<code>streambuf::pubseekoff</code> &
<code>streambuf::pubseekpos</code> respectively.</li>
<li>If <code>std::operator << (std::ostream &, long long)</code>
doesn't exist, you need to recompile libstdc++ with
<code>--enable-long-long</code>.</li>
</ul>
<p>If you get lots of errors about things like <code>cout</code> not being
found, you've most likely forgotten to tell the compiler to look in the
<code>std::</code> namespace. There are several ways to do this:</p>
<ul>
<li>Say <code>std::cout</code> at the call. This is the most explicit
way of saying what you mean.</li>
<li>Say <code>using std::cout;</code> somewhere before the call. You
will need to do this for each function or type you wish to use from the
standard library.</li>
<li>Say <code>using namespace std;</code> somewhere before the call.
This is the quick-but-dirty fix. This brings the <em>whole</em> of the
<code>std::</code> namespace into scope. <em>Never</em> do this in a
header file, as every user of your header file will be affected by this
decision.</li>
</ul>
<h4><a name="new34">New in GCC 3.4.0</a></h4>
<p>The new parser brings a lot of improvements, especially concerning
name-lookup.</p>
<ul>
<li>The "implicit typename" extension got removed (it was already deprecated
since GCC 3.1), so that the following code is now rejected, see [14.6]:
<blockquote><pre>
template <typename> struct A
{
typedef int X;
};
template <typename T> struct B
{
A<T>::X x; // error
typename A<T>::X y; // OK
};
B<void> b;
</pre></blockquote></li>
<li>For similar reasons, the following code now requires the
<code>template</code> keyword, see [14.2]:
<blockquote><pre>
template <typename> struct A
{
template <int> struct X {};
};
template <typename T> struct B
{
typename A<T>::X<0> x; // error
typename A<T>::template X<0> y; // OK
};
B<void> b;
</pre></blockquote></li>
<li>We now have two-stage name-lookup, so that the following code is
rejected, see [14.6]/9:
<blockquote><pre>
template <typename T> int foo()
{
return i; // error
}
</pre></blockquote></li>
<li>This also affects members of base classes, see [14.6.2]:
<blockquote><pre>
template <typename> struct A
{
int i, j;
};
template <typename T> struct B : A<T>
{
int foo1() { return i; } // error
int foo2() { return this->i; } // OK
int foo3() { return B<T>::i; } // OK
int foo4() { return A<T>::i; } // OK
using A<T>::j;
int foo5() { return j; } // OK
};
</pre></blockquote></li>
</ul>
<p>In addition to the problems listed above, the manual contains a section on
<a href="http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Misunderstandings.html">
Common Misunderstandings with GNU C++</a>.</p>
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