UNIX/LINUX Command – g++


g++—GNU project C++ Compiler


g++ [ option | filename ]. ..


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.


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.

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 nondefault forms are shown here.

Options Description
–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–memorized 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. –fmemorize– 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–memorized 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; that is, 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, that is, 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 –0. 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 nonassembler 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.
In the preprocessor, comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.
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 that 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.

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