G++

NAME
SYNOPSIS
DESCRIPTION
OPTIONS
PRAGMAS
FILES
SEE ALSO
BUGS
COPYING
AUTHORS

NAME

g++ − GNU project C++ Compiler

SYNOPSIS

g++ [option | filename ]...

DESCRIPTION

The C and C++ compilers are integrated; g++ is a script to call gcc with options to recognize C++. gcc processes input files through one or more of four stages: preprocessing, compilation, assembly, and linking. This man page contains full descriptions for only C++ specific aspects of the compiler, though it also contains summaries of some general-purpose options. For a fuller explanation of the compiler, see gcc(1).

C++ source files use one of the suffixes ‘.C’, ‘.cc’, ‘.cxx’, ‘.cpp’, or ‘.c++’; preprocessed C++ files use the suffix ‘.ii’.

OPTIONS

There are many command-line options, including options to control details of optimization, warnings, and code generation, which are common to both gcc and g++. For full information on all options, see gcc(1).

Options must be separate: ‘−dr’ is quite different from ‘−d −r ’.

Most ‘−f’ and ‘−W’ options have two contrary forms: −fname and −fno−name (or −Wname and −Wno−name). Only the non-default forms are shown here.

−c

Compile or assemble the source files, but do not link. The compiler output is an object file corresponding to each source file.

−Dmacro

Define macro macro with the string ‘1’ as its definition.

−Dmacro=defn

Define macro macro as defn.

−E

Stop after the preprocessing stage; do not run the compiler proper. The output is preprocessed source code, which is sent to the standard output.

−fall−virtual

Treat all possible member functions as virtual, implicitly. All member functions (except for constructor functions and new or delete member operators) are treated as virtual functions of the class where they appear.

This does not mean that all calls to these member functions will be made through the internal table of virtual functions. Under some circumstances, the compiler can determine that a call to a given virtual function can be made directly; in these cases the calls are direct in any case.

−fdollars−in−identifiers

Permit the use of ‘$’ in identifiers. Traditional C allowed the character ‘$’ to form part of identifiers; by default, GNU C also allows this. However, ANSI C forbids ‘$’ in identifiers, and GNU C++ also forbids it by default on most platforms (though on some platforms it’s enabled by default for GNU C++ as well).

−felide−constructors

Use this option to instruct the compiler to be smarter about when it can elide constructors. Without this flag, GNU C++ and cfront both generate effectively the same code for:

A foo ();
A x (foo ()); // x initialized by ‘foo ()’, no ctor called
A y = foo (); // call to ‘foo ()’ heads to temporary,
// y is initialized from the temporary.

Note the difference! With this flag, GNU C++ initializes ‘y’ directly from the call to foo () without going through a temporary.

−fenum−int−equiv

Normally GNU C++ allows conversion of enum to int, but not the other way around. Use this option if you want GNU C++ to allow conversion of int to enum as well.

−fexternal−templates

Produce smaller code for template declarations, by generating only a single copy of each template function where it is defined. To use this option successfully, you must also mark all files that use templates with either ‘#pragma implementation’ (the definition) or ‘#pragma interface’ (declarations).

When your code is compiled with ‘−fexternal−templates’, all template instantiations are external. You must arrange for all necessary instantiations to appear in the implementation file; you can do this with a typedef that references each instantiation needed. Conversely, when you compile using the default option ‘−fno−external−templates’, all template instantiations are explicitly internal.

−fno−gnu−linker

Do not output global initializations (such as C++ constructors and destructors) in the form used by the GNU linker (on systems where the GNU linker is the standard method of handling them). Use this option when you want to use a non-GNU linker, which also requires using the collect2 program to make sure the system linker includes constructors and destructors. (collect2 is included in the GNU CC distribution.) For systems which must use collect2, the compiler driver gcc is configured to do this automatically.

−fmemoize−lookups

−fsave−memoized

These flags are used to get the compiler to compile programs faster using heuristics. They are not on by default since they are only effective about half the time. The other half of the time programs compile more slowly (and take more memory).

The first time the compiler must build a call to a member function (or reference to a data member), it must (1) determine whether the class implements member functions of that name; (2) resolve which member function to call (which involves figuring out what sorts of type conversions need to be made); and (3) check the visibility of the member function to the caller. All of this adds up to slower compilation. Normally, the second time a call is made to that member function (or reference to that data member), it must go through the same lengthy process again. This means that code like this

cout << "This " << p << " has " << n << " legs.\n";

makes six passes through all three steps. By using a software cache, a ‘‘hit’’ significantly reduces this cost. Unfortunately, using the cache introduces another layer of mechanisms which must be implemented, and so incurs its own overhead. ‘−fmemoize−lookups’ enables the software cache.

Because access privileges (visibility) to members and member functions may differ from one function context to the next, g++ may need to flush the cache. With the ‘−fmemoize−lookups’ flag, the cache is flushed after every function that is compiled. The ‘−fsave−memoized’ flag enables the same software cache, but when the compiler determines that the context of the last function compiled would yield the same access privileges of the next function to compile, it preserves the cache. This is most helpful when defining many member functions for the same class: with the exception of member functions which are friends of other classes, each member function has exactly the same access privileges as every other, and the cache need not be flushed.

−fno−default−inline

Do not make member functions inline by default merely because they are defined inside the class scope. Otherwise, when you specify −O, member functions defined inside class scope are compiled inline by default; i.e., you don’t need to add ‘inline’ in front of the member function name.

−fno−strict−prototype

Consider the declaration int foo ();. In C++, this means that the function foo takes no arguments. In ANSI C, this is declared int foo(void);. With the flag ‘−fno−strict−prototype’, declaring functions with no arguments is equivalent to declaring its argument list to be untyped, i.e., int foo (); is equivalent to saying int foo (...);.

−fnonnull−objects

Normally, GNU C++ makes conservative assumptions about objects reached through references. For example, the compiler must check that ‘a’ is not null in code like the following:
obj &a = g ();
a.f (2);
Checking that references of this sort have non-null values requires extra code, however, and it is unnecessary for many programs. You can use ‘−fnonnull−objects’ to omit the checks for null, if your program doesn’t require the default checking.

−fhandle−signatures

−fno−handle−signatures

These options control the recognition of the signature and sigof constructs for specifying abstract types. By default, these constructs are not recognized.

−fthis−is−variable

The incorporation of user-defined free store management into C++ has made assignment to this an anachronism. Therefore, by default GNU C++ treats the type of this in a member function of class X to be X *const. In other words, it is illegal to assign to this within a class member function. However, for backwards compatibility, you can invoke the old behavior by using ‘−fthis−is−variable’.

−g

Produce debugging information in the operating system’s native format (for DBX or SDB or DWARF). GDB also can work with this debugging information. On most systems that use DBX format, ‘−g’ enables use of extra debugging information that only GDB can use.

Unlike most other C compilers, GNU CC allows you to use ‘−g’ with ‘−O’. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.

Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

−Idir

Append directory dir to the list of directories searched for include files.

−Ldir

Add directory dir to the list of directories to be searched for ‘−l’.

−llibrary

Use the library named library when linking. (C++ programs often require ‘−lg++’ for successful linking.)

−nostdinc

Do not search the standard system directories for header files. Only the directories you have specified with −I options (and the current directory, if appropriate) are searched.

−nostdinc++

Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building libg++.)

−O

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

−o file

Place output in file file.

−S

Stop after the stage of compilation proper; do not assemble. The output is an assembler code file for each non-assembler input file specified.

−traditional

Attempt to support some aspects of traditional C compilers.

Specifically, for both C and C++ programs:

In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.

In the preprocessor, macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline.

The preprocessor does not predefine the macro __STDC__ when you use ‘−traditional’, but still predefines__GNUC__ (since the GNU extensions indicated by __GNUC__ are not affected by ‘−traditional’). If you need to write header files that work differently depending on whether ‘−traditional’ is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ANSI C compilers, and other old C compilers.

String ‘‘constants’’ are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately.

For C++ programs only (not C), ‘−traditional’ has one additional effect: assignment to this is permitted. This is the same as the effect of ‘−fthis−is−variable’.

−Umacro

Undefine macro macro.

−Wall

Issue warnings for conditions which pertain to usage that we recommend avoiding and that we believe is easy to avoid, even in conjunction with macros.

−Wenum−clash

Warn when converting between different enumeration types.

−Woverloaded−virtual

In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. Use this option to request warnings when a derived class declares a function that may be an erroneous attempt to define a virtual function: that is, warn when a function with the same name as a virtual function in the base class, but with a type signature that doesn’t match any virtual functions from the base class.

−Wtemplate−debugging

When using templates in a C++ program, warn if debugging is not yet fully available.

−w

Inhibit all warning messages.

+eN

Control how virtual function definitions are used, in a fashion compatible with cfront 1.x.

PRAGMAS

Two ‘#pragma’ directives are supported for GNU C++, to permit using the same header file for two purposes: as a definition of interfaces to a given object class, and as the full definition of the contents of that object class.

#pragma interface

Use this directive in header files that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing ‘#pragma interface’ is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses ‘#pragma implementation’). Instead, the object files will contain references to be resolved at link time.

#pragma implementation

#pragma implementation "objects.h"

Use this pragma in a main input file, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use ‘#pragma interface’. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files.

If you use ‘#pragma implementation’ with no argument, it applies to an include file with the same basename as your source file; for example, in ‘allclass.cc’, ‘#pragma implementation’ by itself is equivalent to ‘#pragma implementation "allclass.h"’. Use the string argument if you want a single implementation file to include code from multiple header files.

There is no way to split up the contents of a single header file into multiple implementation files.

FILES

file.h

C header (preprocessor) file

file.i

preprocessed C source file

file.C

C++ source file

file.cc

C++ source file

file.cxx

C++ source file

file.s

assembly language file

file.o

object file

a.out

link edited output

TMPDIR/cc∗

temporary files

LIBDIR/cpp

preprocessor

LIBDIR/cc1plus

compiler

LIBDIR/collect

linker front end needed on some machines

LIBDIR/libgcc.a

GCC subroutine library

/lib/crt[01n].o

start-up routine

LIBDIR/ccrt0

additional start-up routine for C++

/lib/libc.a

standard C library, see intro(3)

/usr/include

standard directory for #include files

LIBDIR/include

standard gcc directory for #include files

LIBDIR/g++−include

additional g++ directory for #include

LIBDIR is usually /usr/local/lib/machine/version.
TMPDIR
comes from the environment variable TMPDIR (default /usr/tmp if available, else /tmp).

SEE ALSO

gcc(1), cpp(1), as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1).
gcc’, ‘cpp’, ‘as’,ld’, and ‘gdb’ entries in info.
Using and Porting GNU CC (for version 2.0)
, Richard M. Stallman; The C Preprocessor, Richard M. Stallman; Debugging with GDB: the GNU Source-Level Debugger, Richard M. Stallman and Roland H. Pesch; Using as: the GNU Assembler, Dean Elsner, Jay Fenlason & friends; gld: the GNU linker, Steve Chamberlain and Roland Pesch.

BUGS

For instructions on how to report bugs, see the GCC manual.

COPYING

Copyright (c) 1991, 1992, 1993 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English.

AUTHORS

See the GNU CC Manual for the contributors to GNU CC.