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@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,

@c 1999, 2000, 2001, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

@c Free Software Foundation, Inc.

@c This is part of the GCC manual.

@c For copying conditions, see the file gcc.texi.

@node Trouble

@chapter Known Causes of Trouble with GCC

@cindex bugs, known

@cindex installation trouble

@cindex known causes of trouble

This section describes known problems that affect users of GCC@.  Most

of these are not GCC bugs per se---if they were, we would fix them.

But the result for a user may be like the result of a bug.

Some of these problems are due to bugs in other software, some are

missing features that are too much work to add, and some are places

where people's opinions differ as to what is best.

@menu

* Actual Bugs::         Bugs we will fix later.

* Cross-Compiler Problems:: Common problems of cross compiling with GCC.

* Interoperation::      Problems using GCC with other compilers,

                        and with certain linkers, assemblers and debuggers.

* Incompatibilities::   GCC is incompatible with traditional C.

* Fixed Headers::       GCC uses corrected versions of system header files.

                        This is necessary, but doesn't always work smoothly.

* Standard Libraries::  GCC uses the system C library, which might not be

                        compliant with the ISO C standard.

* Disappointments::     Regrettable things we can't change, but not quite bugs.

* C++ Misunderstandings:: Common misunderstandings with GNU C++.

* Non-bugs::            Things we think are right, but some others disagree.

* Warnings and Errors:: Which problems in your code get warnings,

                        and which get errors.

@end menu

@node Actual Bugs

@section Actual Bugs We Haven't Fixed Yet

@itemize @bullet

@item

The @code{fixincludes} script interacts badly with automounters; if the

directory of system header files is automounted, it tends to be

unmounted while @code{fixincludes} is running.  This would seem to be a

bug in the automounter.  We don't know any good way to work around it.

@end itemize

@node Cross-Compiler Problems

@section Cross-Compiler Problems

You may run into problems with cross compilation on certain machines,

for several reasons.

@itemize @bullet

@item

At present, the program @file{mips-tfile} which adds debug

support to object files on MIPS systems does not work in a cross

compile environment.

@end itemize

@node Interoperation

@section Interoperation

This section lists various difficulties encountered in using GCC

together with other compilers or with the assemblers, linkers,

libraries and debuggers on certain systems.

@itemize @bullet

@item

On many platforms, GCC supports a different ABI for C++ than do other

compilers, so the object files compiled by GCC cannot be used with object

files generated by another C++ compiler.

An area where the difference is most apparent is name mangling.  The use

of different name mangling is intentional, to protect you from more subtle

problems.

Compilers differ as to many internal details of C++ implementation,

including: how class instances are laid out, how multiple inheritance is

implemented, and how virtual function calls are handled.  If the name

encoding were made the same, your programs would link against libraries

provided from other compilers---but the programs would then crash when

run.  Incompatible libraries are then detected at link time, rather than

at run time.

@item

On some BSD systems, including some versions of Ultrix, use of profiling

causes static variable destructors (currently used only in C++) not to

be run.

@item

On some SGI systems, when you use @option{-lgl_s} as an option,

it gets translated magically to @samp{-lgl_s -lX11_s -lc_s}.

Naturally, this does not happen when you use GCC@.

You must specify all three options explicitly.

@item

On a SPARC, GCC aligns all values of type @code{double} on an 8-byte

boundary, and it expects every @code{double} to be so aligned.  The Sun

compiler usually gives @code{double} values 8-byte alignment, with one

exception: function arguments of type @code{double} may not be aligned.

As a result, if a function compiled with Sun CC takes the address of an

argument of type @code{double} and passes this pointer of type

@code{double *} to a function compiled with GCC, dereferencing the

pointer may cause a fatal signal.

One way to solve this problem is to compile your entire program with GCC@.

Another solution is to modify the function that is compiled with

Sun CC to copy the argument into a local variable; local variables

are always properly aligned.  A third solution is to modify the function

that uses the pointer to dereference it via the following function

@code{access_double} instead of directly with @samp{*}:

@smallexample

inline double

access_double (double *unaligned_ptr)

@{

  union d2i @{ double d; int i[2]; @};

  union d2i *p = (union d2i *) unaligned_ptr;

  union d2i u;

  u.i[0] = p->i[0];

  u.i[1] = p->i[1];

  return u.d;

@}

@end smallexample

@noindent

Storing into the pointer can be done likewise with the same union.

@item

On Solaris, the @code{malloc} function in the @file{libmalloc.a} library

may allocate memory that is only 4 byte aligned.  Since GCC on the

SPARC assumes that doubles are 8 byte aligned, this may result in a

fatal signal if doubles are stored in memory allocated by the

@file{libmalloc.a} library.

The solution is to not use the @file{libmalloc.a} library.  Use instead

@code{malloc} and related functions from @file{libc.a}; they do not have

this problem.

@item

On the HP PA machine, ADB sometimes fails to work on functions compiled

with GCC@.  Specifically, it fails to work on functions that use

@code{alloca} or variable-size arrays.  This is because GCC doesn't

generate HP-UX unwind descriptors for such functions.  It may even be

impossible to generate them.

@item

Debugging (@option{-g}) is not supported on the HP PA machine, unless you use

the preliminary GNU tools.

@item

Taking the address of a label may generate errors from the HP-UX

PA assembler.  GAS for the PA does not have this problem.

@item

Using floating point parameters for indirect calls to static functions

will not work when using the HP assembler.  There simply is no way for GCC

to specify what registers hold arguments for static functions when using

the HP assembler.  GAS for the PA does not have this problem.

@item

In extremely rare cases involving some very large functions you may

receive errors from the HP linker complaining about an out of bounds

unconditional branch offset.  This used to occur more often in previous

versions of GCC, but is now exceptionally rare.  If you should run

into it, you can work around by making your function smaller.

@item

GCC compiled code sometimes emits warnings from the HP-UX assembler of

the form:

@smallexample

(warning) Use of GR3 when

  frame >= 8192 may cause conflict.

@end smallexample

These warnings are harmless and can be safely ignored.

@item

In extremely rare cases involving some very large functions you may

receive errors from the AIX Assembler complaining about a displacement

that is too large.  If you should run into it, you can work around by

making your function smaller.

@item

The @file{libstdc++.a} library in GCC relies on the SVR4 dynamic

linker semantics which merges global symbols between libraries and

applications, especially necessary for C++ streams functionality.

This is not the default behavior of AIX shared libraries and dynamic

linking.  @file{libstdc++.a} is built on AIX with ``runtime-linking''

enabled so that symbol merging can occur.  To utilize this feature,

the application linked with @file{libstdc++.a} must include the

@option{-Wl,-brtl} flag on the link line.  G++ cannot impose this

because this option may interfere with the semantics of the user

program and users may not always use @samp{g++} to link his or her

application.  Applications are not required to use the

@option{-Wl,-brtl} flag on the link line---the rest of the

@file{libstdc++.a} library which is not dependent on the symbol

merging semantics will continue to function correctly.

@item

An application can interpose its own definition of functions for

functions invoked by @file{libstdc++.a} with ``runtime-linking''

enabled on AIX@.  To accomplish this the application must be linked

with ``runtime-linking'' option and the functions explicitly must be

exported by the application (@option{-Wl,-brtl,-bE:exportfile}).

@item

AIX on the RS/6000 provides support (NLS) for environments outside of

the United States.  Compilers and assemblers use NLS to support

locale-specific representations of various objects including

floating-point numbers (@samp{.} vs @samp{,} for separating decimal

fractions).  There have been problems reported where the library linked

with GCC does not produce the same floating-point formats that the

assembler accepts.  If you have this problem, set the @env{LANG}

environment variable to @samp{C} or @samp{En_US}.

@item

@opindex fdollars-in-identifiers

Even if you specify @option{-fdollars-in-identifiers},

you cannot successfully use @samp{$} in identifiers on the RS/6000 due

to a restriction in the IBM assembler.  GAS supports these

identifiers.

@end itemize

@node Incompatibilities

@section Incompatibilities of GCC

@cindex incompatibilities of GCC

@opindex traditional

There are several noteworthy incompatibilities between GNU C and K&R

(non-ISO) versions of C@.

@itemize @bullet

@cindex string constants

@cindex read-only strings

@cindex shared strings

@item

GCC normally makes string constants read-only.  If several

identical-looking string constants are used, GCC stores only one

copy of the string.

@cindex @code{mktemp}, and constant strings

One consequence is that you cannot call @code{mktemp} with a string

constant argument.  The function @code{mktemp} always alters the

string its argument points to.

@cindex @code{sscanf}, and constant strings

@cindex @code{fscanf}, and constant strings

@cindex @code{scanf}, and constant strings

Another consequence is that @code{sscanf} does not work on some very

old systems when passed a string constant as its format control string

or input.  This is because @code{sscanf} incorrectly tries to write

into the string constant.  Likewise @code{fscanf} and @code{scanf}.

The solution to these problems is to change the program to use

@code{char}-array variables with initialization strings for these

purposes instead of string constants.

@item

@code{-2147483648} is positive.

This is because 2147483648 cannot fit in the type @code{int}, so

(following the ISO C rules) its data type is @code{unsigned long int}.

Negating this value yields 2147483648 again.

@item

GCC does not substitute macro arguments when they appear inside of

string constants.  For example, the following macro in GCC

@smallexample

#define foo(a) "a"

@end smallexample

@noindent

will produce output @code{"a"} regardless of what the argument @var{a} is.

@cindex @code{setjmp} incompatibilities

@cindex @code{longjmp} incompatibilities

@item

When you use @code{setjmp} and @code{longjmp}, the only automatic

variables guaranteed to remain valid are those declared

@code{volatile}.  This is a consequence of automatic register

allocation.  Consider this function:

@smallexample

jmp_buf j;

foo ()

@{

  int a, b;

  a = fun1 ();

  if (setjmp (j))

    return a;

  a = fun2 ();

  /* @r{@code{longjmp (j)} may occur in @code{fun3}.} */

  return a + fun3 ();

@}

@end smallexample

Here @code{a} may or may not be restored to its first value when the

@code{longjmp} occurs.  If @code{a} is allocated in a register, then

its first value is restored; otherwise, it keeps the last value stored

in it.

@opindex W

If you use the @option{-W} option with the @option{-O} option, you will

get a warning when GCC thinks such a problem might be possible.

@item

Programs that use preprocessing directives in the middle of macro

arguments do not work with GCC@.  For example, a program like this

will not work:

@smallexample

@group

foobar (

#define luser

        hack)

@end group

@end smallexample

ISO C does not permit such a construct.

@item

K&R compilers allow comments to cross over an inclusion boundary

(i.e.@: started in an include file and ended in the including file).

@cindex external declaration scope

@cindex scope of external declarations

@cindex declaration scope

@item

Declarations of external variables and functions within a block apply

only to the block containing the declaration.  In other words, they

have the same scope as any other declaration in the same place.

In some other C compilers, an @code{extern} declaration affects all the

rest of the file even if it happens within a block.

@item

In traditional C, you can combine @code{long}, etc., with a typedef name,

as shown here:

@smallexample

typedef int foo;

typedef long foo bar;

@end smallexample

In ISO C, this is not allowed: @code{long} and other type modifiers

require an explicit @code{int}.

@cindex typedef names as function parameters

@item

PCC allows typedef names to be used as function parameters.

@item

Traditional C allows the following erroneous pair of declarations to

appear together in a given scope:

@smallexample

typedef int foo;

typedef foo foo;

@end smallexample

@item

GCC treats all characters of identifiers as significant.  According to

K&R-1 (2.2), ``No more than the first eight characters are significant,

although more may be used.''.  Also according to K&R-1 (2.2), ``An

identifier is a sequence of letters and digits; the first character must

be a letter.  The underscore _ counts as a letter.'', but GCC also

allows dollar signs in identifiers.

@cindex whitespace

@item

PCC allows whitespace in the middle of compound assignment operators

such as @samp{+=}.  GCC, following the ISO standard, does not

allow this.

@cindex apostrophes

@cindex '

@item

GCC complains about unterminated character constants inside of

preprocessing conditionals that fail.  Some programs have English

comments enclosed in conditionals that are guaranteed to fail; if these

comments contain apostrophes, GCC will probably report an error.  For

example, this code would produce an error:

@smallexample

#if 0

You can't expect this to work.

#endif

@end smallexample

The best solution to such a problem is to put the text into an actual

C comment delimited by @samp{/*@dots{}*/}.

@item

Many user programs contain the declaration @samp{long time ();}.  In the

past, the system header files on many systems did not actually declare

@code{time}, so it did not matter what type your program declared it to

return.  But in systems with ISO C headers, @code{time} is declared to

return @code{time_t}, and if that is not the same as @code{long}, then

@samp{long time ();} is erroneous.

The solution is to change your program to use appropriate system headers

(@code{<time.h>} on systems with ISO C headers) and not to declare

@code{time} if the system header files declare it, or failing that to

use @code{time_t} as the return type of @code{time}.

@cindex @code{float} as function value type

@item

When compiling functions that return @code{float}, PCC converts it to

a double.  GCC actually returns a @code{float}.  If you are concerned

with PCC compatibility, you should declare your functions to return

@code{double}; you might as well say what you mean.

@cindex structures

@cindex unions

@item

When compiling functions that return structures or unions, GCC

output code normally uses a method different from that used on most

versions of Unix.  As a result, code compiled with GCC cannot call

a structure-returning function compiled with PCC, and vice versa.

The method used by GCC is as follows: a structure or union which is

1, 2, 4 or 8 bytes long is returned like a scalar.  A structure or union

with any other size is stored into an address supplied by the caller

(usually in a special, fixed register, but on some machines it is passed

on the stack).  The target hook @code{TARGET_STRUCT_VALUE_RTX}

tells GCC where to pass this address.

By contrast, PCC on most target machines returns structures and unions

of any size by copying the data into an area of static storage, and then

returning the address of that storage as if it were a pointer value.

The caller must copy the data from that memory area to the place where

the value is wanted.  GCC does not use this method because it is

slower and nonreentrant.

On some newer machines, PCC uses a reentrant convention for all

structure and union returning.  GCC on most of these machines uses a

compatible convention when returning structures and unions in memory,

but still returns small structures and unions in registers.

@opindex fpcc-struct-return

You can tell GCC to use a compatible convention for all structure and

union returning with the option @option{-fpcc-struct-return}.

@cindex preprocessing tokens

@cindex preprocessing numbers

@item

GCC complains about program fragments such as @samp{0x74ae-0x4000}

which appear to be two hexadecimal constants separated by the minus

operator.  Actually, this string is a single @dfn{preprocessing token}.

Each such token must correspond to one token in C@.  Since this does not,

GCC prints an error message.  Although it may appear obvious that what

is meant is an operator and two values, the ISO C standard specifically

requires that this be treated as erroneous.

A @dfn{preprocessing token} is a @dfn{preprocessing number} if it

begins with a digit and is followed by letters, underscores, digits,

periods and @samp{e+}, @samp{e-}, @samp{E+}, @samp{E-}, @samp{p+},

@samp{p-}, @samp{P+}, or @samp{P-} character sequences.  (In strict C90

mode, the sequences @samp{p+}, @samp{p-}, @samp{P+} and @samp{P-} cannot

appear in preprocessing numbers.)

To make the above program fragment valid, place whitespace in front of

the minus sign.  This whitespace will end the preprocessing number.

@end itemize

@node Fixed Headers

@section Fixed Header Files

GCC needs to install corrected versions of some system header files.

This is because most target systems have some header files that won't

work with GCC unless they are changed.  Some have bugs, some are

incompatible with ISO C, and some depend on special features of other

compilers.

Installing GCC automatically creates and installs the fixed header

files, by running a program called @code{fixincludes}.  Normally, you

don't need to pay attention to this.  But there are cases where it

doesn't do the right thing automatically.

@itemize @bullet

@item

If you update the system's header files, such as by installing a new

system version, the fixed header files of GCC are not automatically

updated.  They can be updated using the @command{mkheaders} script

installed in

@file{@var{libexecdir}/gcc/@var{target}/@var{version}/install-tools/}.

@item

On some systems, header file directories contain

machine-specific symbolic links in certain places.  This makes it

possible to share most of the header files among hosts running the

same version of the system on different machine models.

The programs that fix the header files do not understand this special

way of using symbolic links; therefore, the directory of fixed header

files is good only for the machine model used to build it.

It is possible to make separate sets of fixed header files for the

different machine models, and arrange a structure of symbolic links so

as to use the proper set, but you'll have to do this by hand.

@end itemize

@node Standard Libraries

@section Standard Libraries

@opindex Wall

GCC by itself attempts to be a conforming freestanding implementation.

@xref{Standards,,Language Standards Supported by GCC}, for details of

what this means.  Beyond the library facilities required of such an

implementation, the rest of the C library is supplied by the vendor of

the operating system.  If that C library doesn't conform to the C

standards, then your programs might get warnings (especially when using

@option{-Wall}) that you don't expect.

For example, the @code{sprintf} function on SunOS 4.1.3 returns

@code{char *} while the C standard says that @code{sprintf} returns an

@code{int}.  The @code{fixincludes} program could make the prototype for

this function match the Standard, but that would be wrong, since the

function will still return @code{char *}.

If you need a Standard compliant library, then you need to find one, as

GCC does not provide one.  The GNU C library (called @code{glibc})

provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for

GNU/Linux and HURD-based GNU systems; no recent version of it supports

other systems, though some very old versions did.  Version 2.2 of the

GNU C library includes nearly complete C99 support.  You could also ask

your operating system vendor if newer libraries are available.

@node Disappointments

@section Disappointments and Misunderstandings

These problems are perhaps regrettable, but we don't know any practical

way around them.

@itemize @bullet

@item

Certain local variables aren't recognized by debuggers when you compile

with optimization.

This occurs because sometimes GCC optimizes the variable out of

existence.  There is no way to tell the debugger how to compute the

value such a variable ``would have had'', and it is not clear that would

be desirable anyway.  So GCC simply does not mention the eliminated

variable when it writes debugging information.

You have to expect a certain amount of disagreement between the

executable and your source code, when you use optimization.

@cindex conflicting types

@cindex scope of declaration

@item

Users often think it is a bug when GCC reports an error for code

like this:

@smallexample

int foo (struct mumble *);

struct mumble @{ @dots{} @};

int foo (struct mumble *x)

@{ @dots{} @}

@end smallexample

This code really is erroneous, because the scope of @code{struct

mumble} in the prototype is limited to the argument list containing it.

It does not refer to the @code{struct mumble} defined with file scope

immediately below---they are two unrelated types with similar names in

different scopes.

But in the definition of @code{foo}, the file-scope type is used

because that is available to be inherited.  Thus, the definition and

the prototype do not match, and you get an error.

This behavior may seem silly, but it's what the ISO standard specifies.

It is easy enough for you to make your code work by moving the

definition of @code{struct mumble} above the prototype.  It's not worth

being incompatible with ISO C just to avoid an error for the example

shown above.

@item

Accesses to bit-fields even in volatile objects works by accessing larger

objects, such as a byte or a word.  You cannot rely on what size of

object is accessed in order to read or write the bit-field; it may even

vary for a given bit-field according to the precise usage.

If you care about controlling the amount of memory that is accessed, use

volatile but do not use bit-fields.

@item

GCC comes with shell scripts to fix certain known problems in system

header files.  They install corrected copies of various header files in

a special directory where only GCC will normally look for them.  The

scripts adapt to various systems by searching all the system header

files for the problem cases that we know about.

If new system header files are installed, nothing automatically arranges

to update the corrected header files.  They can be updated using the

@command{mkheaders} script installed in

@file{@var{libexecdir}/gcc/@var{target}/@var{version}/install-tools/}.

@item

@cindex floating point precision

On 68000 and x86 systems, for instance, you can get paradoxical results

if you test the precise values of floating point numbers.  For example,

you can find that a floating point value which is not a NaN is not equal

to itself.  This results from the fact that the floating point registers

hold a few more bits of precision than fit in a @code{double} in memory.

Compiled code moves values between memory and floating point registers

at its convenience, and moving them into memory truncates them.

@opindex ffloat-store

You can partially avoid this problem by using the @option{-ffloat-store}

option (@pxref{Optimize Options}).

@item

On AIX and other platforms without weak symbol support, templates

need to be instantiated explicitly and symbols for static members

of templates will not be generated.

@item

On AIX, GCC scans object files and library archives for static

constructors and destructors when linking an application before the

linker prunes unreferenced symbols.  This is necessary to prevent the

AIX linker from mistakenly assuming that static constructor or

destructor are unused and removing them before the scanning can occur.

All static constructors and destructors found will be referenced even

though the modules in which they occur may not be used by the program.

This may lead to both increased executable size and unexpected symbol

references.

@end itemize

@node C++ Misunderstandings

@section Common Misunderstandings with GNU C++

@cindex misunderstandings in C++

@cindex surprises in C++

@cindex C++ misunderstandings

C++ is a complex language and an evolving one, and its standard

definition (the ISO C++ standard) was only recently completed.  As a

result, your C++ compiler may occasionally surprise you, even when its

behavior is correct.  This section discusses some areas that frequently

give rise to questions of this sort.

@menu

* Static Definitions::  Static member declarations are not definitions

* Name lookup::         Name lookup, templates, and accessing members of base classes

* Temporaries::         Temporaries may vanish before you expect

* Copy Assignment::     Copy Assignment operators copy virtual bases twice

@end menu

@node Static Definitions

@subsection Declare @emph{and} Define Static Members

@cindex C++ static data, declaring and defining

@cindex static data in C++, declaring and defining

@cindex declaring static data in C++

@cindex defining static data in C++

When a class has static data members, it is not enough to @emph{declare}

the static member; you must also @emph{define} it.  For example:

@smallexample

class Foo

@{

  @dots{}

  void method();

  static int bar;

@};

@end smallexample

This declaration only establishes that the class @code{Foo} has an

@code{int} named @code{Foo::bar}, and a member function named

@code{Foo::method}.  But you still need to define @emph{both}

@code{method} and @code{bar} elsewhere.  According to the ISO

standard, you must supply an initializer in one (and only one) source

file, such as:

@smallexample

int Foo::bar = 0;

@end smallexample

Other C++ compilers may not correctly implement the standard behavior.

As a result, when you switch to @command{g++} from one of these compilers,

you may discover that a program that appeared to work correctly in fact

does not conform to the standard: @command{g++} reports as undefined

symbols any static data members that lack definitions.

@node Name lookup

@subsection Name lookup, templates, and accessing members of base classes

@cindex base class members

@cindex two-stage name lookup

@cindex dependent name lookup

The C++ standard prescribes that all names that are not dependent on

template parameters are bound to their present definitions when parsing

a template function or class.@footnote{The C++ standard just uses the

term ``dependent'' for names that depend on the type or value of

template parameters.  This shorter term will also be used in the rest of

this section.}  Only names that are dependent are looked up at the point

of instantiation.  For example, consider

@smallexample

  void foo(double);

  struct A @{

    template <typename T>

    void f () @{

      foo (1);        // @r{1}

      int i = N;      // @r{2}

      T t;

      t.bar();        // @r{3}

      foo (t);        // @r{4}

    @}

    static const int N;

  @};

@end smallexample

Here, the names @code{foo} and @code{N} appear in a context that does

not depend on the type of @code{T}.  The compiler will thus require that

they are defined in the context of use in the template, not only before

the point of instantiation, and will here use @code{::foo(double)} and

@code{A::N}, respectively.  In particular, it will convert the integer

value to a @code{double} when passing it to @code{::foo(double)}.

Conversely, @code{bar} and the call to @code{foo} in the fourth marked

line are used in contexts that do depend on the type of @code{T}, so

they are only looked up at the point of instantiation, and you can

provide declarations for them after declaring the template, but before

instantiating it.  In particular, if you instantiate @code{A::f<int>},

the last line will call an overloaded @code{::foo(int)} if one was

provided, even if after the declaration of @code{struct A}.

This distinction between lookup of dependent and non-dependent names is

called two-stage (or dependent) name lookup.  G++ implements it

since version 3.4.

Two-stage name lookup sometimes leads to situations with behavior

different from non-template codes.  The most common is probably this:

@smallexample

  template <typename T> struct Base @{

    int i;

  @};

  template <typename T> struct Derived : public Base<T> @{

    int get_i() @{ return i; @}

  @};

@end smallexample

In @code{get_i()}, @code{i} is not used in a dependent context, so the

compiler will look for a name declared at the enclosing namespace scope

(which is the global scope here).  It will not look into the base class,

since that is dependent and you may declare specializations of

@code{Base} even after declaring @code{Derived}, so the compiler can't

really know what @code{i} would refer to.  If there is no global

variable @code{i}, then you will get an error message.

In order to make it clear that you want the member of the base class,

you need to defer lookup until instantiation time, at which the base

class is known.  For this, you need to access @code{i} in a dependent

context, by either using @code{this->i} (remember that @code{this} is of

type @code{Derived<T>*}, so is obviously dependent), or using

@code{Base<T>::i}.  Alternatively, @code{Base<T>::i} might be brought

into scope by a @code{using}-declaration.

Another, similar example involves calling member functions of a base

class:

@smallexample

  template <typename T> struct Base @{

      int f();

  @};

  template <typename T> struct Derived : Base<T> @{

      int g() @{ return f(); @};

  @};

@end smallexample

Again, the call to @code{f()} is not dependent on template arguments

(there are no arguments that depend on the type @code{T}, and it is also

not otherwise specified that the call should be in a dependent context).

Thus a global declaration of such a function must be available, since

the one in the base class is not visible until instantiation time.  The

compiler will consequently produce the following error message:

@smallexample

  x.cc: In member function `int Derived<T>::g()':

  x.cc:6: error: there are no arguments to `f' that depend on a template

     parameter, so a declaration of `f' must be available

  x.cc:6: error: (if you use `-fpermissive', G++ will accept your code, but

     allowing the use of an undeclared name is deprecated)

@end smallexample

To make the code valid either use @code{this->f()}, or

@code{Base<T>::f()}.  Using the @option{-fpermissive} flag will also let

the compiler accept the code, by marking all function calls for which no

declaration is visible at the time of definition of the template for

later lookup at instantiation time, as if it were a dependent call.

We do not recommend using @option{-fpermissive} to work around invalid

code, and it will also only catch cases where functions in base classes

are called, not where variables in base classes are used (as in the

example above).

Note that some compilers (including G++ versions prior to 3.4) get these

examples wrong and accept above code without an error.  Those compilers

do not implement two-stage name lookup correctly.

@node Temporaries

@subsection Temporaries May Vanish Before You Expect

@cindex temporaries, lifetime of

@cindex portions of temporary objects, pointers to

It is dangerous to use pointers or references to @emph{portions} of a

temporary object.  The compiler may very well delete the object before

you expect it to, leaving a pointer to garbage.  The most common place

where this problem crops up is in classes like string classes,

especially ones that define a conversion function to type @code{char *}

or @code{const char *}---which is one reason why the standard

@code{string} class requires you to call the @code{c_str} member

function.  However, any class that returns a pointer to some internal

structure is potentially subject to this problem.

For example, a program may use a function @code{strfunc} that returns

@code{string} objects, and another function @code{charfunc} that

operates on pointers to @code{char}:

@smallexample

string strfunc ();

void charfunc (const char *);

void

f ()

@{

  const char *p = strfunc().c_str();

  @dots{}

  charfunc (p);

  @dots{}

  charfunc (p);

@}

@end smallexample

@noindent

In this situation, it may seem reasonable to save a pointer to the C

string returned by the @code{c_str} member function and use that rather

than call @code{c_str} repeatedly.  However, the temporary string

created by the call to @code{strfunc} is destroyed after @code{p} is

initialized, at which point @code{p} is left pointing to freed memory.

Code like this may run successfully under some other compilers,

particularly obsolete cfront-based compilers that delete temporaries

along with normal local variables.  However, the GNU C++ behavior is

standard-conforming, so if your program depends on late destruction of

temporaries it is not portable.

The safe way to write such code is to give the temporary a name, which

forces it to remain until the end of the scope of the name.  For

example:

@smallexample

const string& tmp = strfunc ();

charfunc (tmp.c_str ());

@end smallexample

@node Copy Assignment

@subsection Implicit Copy-Assignment for Virtual Bases

When a base class is virtual, only one subobject of the base class

belongs to each full object.  Also, the constructors and destructors are

invoked only once, and called from the most-derived class.  However, such

objects behave unspecified when being assigned.  For example:

@smallexample

struct Base@{

  char *name;

  Base(char *n) : name(strdup(n))@{@}

  Base& operator= (const Base& other)@{

   free (name);

   name = strdup (other.name);

  @}

@};

struct A:virtual Base@{

  int val;

  A():Base("A")@{@}

@};

struct B:virtual Base@{

  int bval;

  B():Base("B")@{@}

@};

struct Derived:public A, public B@{

  Derived():Base("Derived")@{@}

@};

void func(Derived &d1, Derived &d2)

@{

  d1 = d2;

@}

@end smallexample

The C++ standard specifies that @samp{Base::Base} is only called once

when constructing or copy-constructing a Derived object.  It is

unspecified whether @samp{Base::operator=} is called more than once when

the implicit copy-assignment for Derived objects is invoked (as it is

inside @samp{func} in the example).

G++ implements the ``intuitive'' algorithm for copy-assignment: assign all

direct bases, then assign all members.  In that algorithm, the virtual

base subobject can be encountered more than once.  In the example, copying

proceeds in the following order: @samp{val}, @samp{name} (via

@code{strdup}), @samp{bval}, and @samp{name} again.

If application code relies on copy-assignment, a user-defined

copy-assignment operator removes any uncertainties.  With such an

operator, the application can define whether and how the virtual base

subobject is assigned.

@node Non-bugs

@section Certain Changes We Don't Want to Make

This section lists changes that people frequently request, but which

we do not make because we think GCC is better without them.