Index of Section 1 Manual Pages
|Interix / SUA||g++.1||Interix / SUA
g++, c++ - GNU project C++ compiler
g++ [option|filename] ...
c++ [option|filename] ...
The C and C++ compilers are integrated; g++(1) is a script to call gcc(1)
with options to recognize C++. The gcc(1) utility processes input files
through one or more of four stages: preprocessing, compilation, assembly,
and linking. This topic 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
C++ source files use one of the suffixes .C, .cc, or .cxx; preprocessed
C++ files use the suffix .ii.
There are many command-line options, including options to control details
of optimization, warnings, and code generation, which are common to both
gcc(1) and g++(1). 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.
Compile or assemble the source files, but do not link. The compiler
output is an object file corresponding to each source file.
Define macro with the string -1 as its definition.
Define macro as defn.
Stop after the preprocessing stage; do not run the compiler proper.
The output is preprocessed source code, which is sent to the standard
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
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.
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 (although on some platforms,
it is enabled by default for GNU C++ as well).
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.
Usually, GNU C++ allows conversion of enum to int, but does not allow
conversion from int to enum. Use this option if you want GNU C++ to
allow conversion of int to enum.
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
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.
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 ensure that the system linker includes
constructors and destructors. (Collect2 is included in the GNU CC
distribution.) For systems that must use Collect2, the compiler driver
gcc(1) is configured to do this automatically.
These flags 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:
* determine whether the class implements member functions of that
* resolve which member function to call (which involves figuring
out what sorts of type conversions need to be made); and
* check the visibility of the member function to the caller.
All of this results in slower compilation. Usually, 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 the following code:
cout << "This " << p << " has " << n << " legs.0;
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 that must be implemented, and
so incurs its own overhead. The -fmemoize-lookups option enables the
Because access privileges (visibility) to members and member functions
can differ from one function context to the next, g++(1) 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 that are friends of
other classes, each member function has exactly the same access
privileges as every other, and the cache need not be flushed.
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;
that is, you need not add "inline" in front of the member function
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; that is, int foo (); is equivalent to saying int foo (...);.
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 ();
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 does
not require the default checking.
These options control the recognition of the signature and sigof
constructs for specifying abstract types. By default, these constructs
are not recognized.
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.
Produce debugging information in the operating system's native format
(for DBX or SDB or DWARF). gdb can also 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 -g with -O. The
shortcuts taken by optimized code can occasionally produce surprising
results: some variables you declared might not exist at all; flow of
control may briefly move where you did not expect it; some statements
might not be executed because they compute constant results or their
values were already at hand; and some statements might 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.
Append directory dir to the list of directories searched for include
Add directory dir to the list of directories to be searched for -l.
Use the library named library when linking. (C++ programs often
require -lg++ for successful linking.)
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.
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++.)
Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
Place output in file.
Stop after the stage of compilation proper; do not assemble. The
output is an assembler code file for each non-assembler input file
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
supplied as strings, 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 must 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
For C++ programs only (not C), -traditional has one additional effect:
assignment to this is permitted. This is the same as the effect of -
Issue warnings for conditions which pertain to usage that we recommend
avoiding and which we believe is easy to avoid, even in conjunction
Warn when converting between different enumeration types.
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 is defined with the same name
as a virtual function in the base class, but with a type signature
that does not match any virtual functions from the base class.
When using templates in a C++ program, warn if debugging is not yet
Inhibit all warning messages.
Control how virtual function definitions are used, in a fashion
compatible with cfront(1) 1.x.
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
Use this directive in header files that define object classes to save
space in most 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. 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
#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 base name 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
There is no way to split up the contents of a single header file into
multiple implementation files.
C header (preprocessor) file
Preprocessed C source file
C++ source file
C++ source file
C++ source file
Assembly language file
Link edited output
Linker front end needed on some computers
GCC subroutine library
Additional start-up routine for C++
Standard C library, see intro(3)
Standard directory for #include files
Standard gcc directory for #include files
Additional g++ directory for #include
LIBDIR is usually /usr/local/lib/machine/version. On Interix, however, it
TMPDIR comes from the environment variable TMPDIR (default /usr/tmp if
available, otherwise /tmp).
The gcc, cpp, as, ld, and gdb entries in info(1).
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.
For instructions on how to report bugs, see the GCC manual.
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
See the GNU CC Manual for the contributors to GNU CC.