Whole document tree

2.5 Options Controlling C++ Dialect

This section describes the command-line options that are only meaningful for C++ programs; but you can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file firstClass.C like this:

g++ -g -frepo -O -c firstClass.C

In this example, only `-frepo' is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Here is a list of options that are only for compiling C++ programs:

-fno-access-control

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

-fcheck-new

Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. The current Working Paper requires that operator new never return a null pointer, so this check is normally unnecessary.

An alternative to using this option is to specify that your operator new does not throw any exceptions; if you declare it `throw()', g++ will check the return value. See also `new (nothrow)'.

-fconserve-space

Put uninitialized or runtime-initialized global variables into the common segment, as C does. This saves space in the executable at the cost of not diagnosing duplicate definitions. If you compile with this flag and your program mysteriously crashes after main() has completed, you may have an object that is being destroyed twice because two definitions were merged.

This option is no longer useful on most targets, now that support has been added for putting variables into BSS without making them common.

-fdollars-in-identifiers

Accept `$' in identifiers. You can also explicitly prohibit use of `$' with the option `-fno-dollars-in-identifiers'. (GNU C allows `$' by default on most target systems, but there are a few exceptions.) Traditional C allowed the character `$' to form part of identifiers. However, ANSI C and C++ forbid `$' in identifiers.

-fno-elide-constructors

The C++ standard allows an implementation to omit creating a temporary which is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces g++ to call the copy constructor in all cases.

-fexternal-templates

Cause template instantiations to obey `#pragma interface' and `implementation'; template instances are emitted or not according to the location of the template definition. See section 5.5 Where's the Template?, for more information.

This option is deprecated.

-falt-external-templates

Similar to -fexternal-templates, but template instances are emitted or not according to the place where they are first instantiated. See section 5.5 Where's the Template?, for more information.

This option is deprecated.

-ffor-scope

-fno-for-scope

If -ffor-scope is specified, the scope of variables declared in a for-init-statement is limited to the `for' loop itself, as specified by the draft C++ standard. If -fno-for-scope is specified, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope, as was the case in old versions of gcc, and other (traditional) implementations of C++.

The default if neither flag is given to follow the standard, but to allow and give a warning for old-style code that would otherwise be invalid, or have different behavior.

-fno-gnu-keywords

Do not recognize classof, headof, signature, sigof or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __classof__, __headof__, __signature__, __sigof__, and __typeof__ instead. `-ansi' implies `-fno-gnu-keywords'.

-fguiding-decls

Treat a function declaration with the same type as a potential function template instantiation as though it declares that instantiation, not a normal function. If a definition is given for the function later in the translation unit (or another translation unit if the target supports weak symbols), that definition will be used; otherwise the template will be instantiated. This behavior reflects the C++ language prior to September 1996, when guiding declarations were removed.

This option implies `-fname-mangling-version-0', and will not work with other name mangling versions. Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

-fhandle-signatures

Recognize the signature and sigof keywords for specifying abstract types. The default (`-fno-handle-signatures') is not to recognize them. See section Type Abstraction using Signatures.

-fhonor-std

Treat the namespace std as a namespace, instead of ignoring it. For compatibility with earlier versions of g++, the compiler will, by default, ignore namespace-declarations, using-declarations, using-directives, and namespace-names, if they involve std.

-fhuge-objects

Support virtual function calls for objects that exceed the size representable by a `short int'. Users should not use this flag by default; if you need to use it, the compiler will tell you so.

This flag is not useful when compiling with -fvtable-thunks.

Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

-fno-implicit-templates

Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See section 5.5 Where's the Template?, for more information.

-fno-implicit-inline-templates

Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization will need the same set of explicit instantiations.

-finit-priority

Support `__attribute__ ((init_priority (n)))' for controlling the order of initialization of file-scope objects. On ELF targets, this requires GNU ld 2.10 or later.

-fno-implement-inlines

To save space, do not emit out-of-line copies of inline functions controlled by `#pragma implementation'. This will cause linker errors if these functions are not inlined everywhere they are called.

-fname-mangling-version-n

Control the way in which names are mangled. Version 0 is compatible with versions of g++ before 2.8. Version 1 is the default. Version 1 will allow correct mangling of function templates. For example, version 0 mangling does not mangle foo<int, double> and foo<int, char> given this declaration:

template <class T, class U> void foo(T t);

Like all options that change the ABI, all C++ code, including libgcc must be built with the same setting of this option.

-foperator-names

Recognize the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms for the symbols they refer to. `-ansi' implies `-foperator-names'.

-fno-optional-diags

Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by g++ is the one for a name having multiple meanings within a class.

-fpermissive

Downgrade messages about nonconformant code from errors to warnings. By default, g++ effectively sets `-pedantic-errors' without `-pedantic'; this option reverses that. This behavior and this option are superceded by `-pedantic', which works as it does for GNU C.

-frepo

Enable automatic template instantiation. This option also implies `-fno-implicit-templates'. See section 5.5 Where's the Template?, for more information.

-fno-rtti

Disable generation of the information used by C++ runtime type identification features (`dynamic_cast' and `typeid'). If you don't use those parts of the language (or exception handling, which uses `dynamic_cast' internally), you can save some space by using this flag.

-fstrict-prototype

Within an `extern "C"' linkage specification, treat a function declaration with no arguments, such as `int foo ();', as declaring the function to take no arguments. Normally, such a declaration means that the function foo can take any combination of arguments, as in C. `-pedantic' implies `-fstrict-prototype' unless overridden with `-fno-strict-prototype'.

Specifying this option will also suppress implicit declarations of functions.

This flag no longer affects declarations with C++ linkage.

-fsquangle

-fno-squangle

`-fsquangle' will enable a compressed form of name mangling for identifiers. In particular, it helps to shorten very long names by recognizing types and class names which occur more than once, replacing them with special short ID codes. This option also requires any C++ libraries being used to be compiled with this option as well. The compiler has this disabled (the equivalent of `-fno-squangle') by default.

Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option.

-ftemplate-depth-n

Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17.

-fthis-is-variable

Permit assignment to this. The incorporation of user-defined free store management into C++ has made assignment to `this' an anachronism. Therefore, by default it is invalid to assign to this within a class member function; that is, GNU C++ treats `this' in a member function of class X as a non-lvalue of type `X *'. However, for backwards compatibility, you can make it valid with `-fthis-is-variable'.

-fvtable-thunks=thunks-version

Use `thunks' to implement the virtual function dispatch table (`vtable'). The traditional (cfront-style) approach to implementing vtables was to store a pointer to the function and two offsets for adjusting the `this' pointer at the call site. Newer implementations store a single pointer to a `thunk' function which does any necessary adjustment and then calls the target function.

The original implementation of thunks (version 1) had a bug regarding virtual base classes; this bug is fixed with version 2 of the thunks implementation. With setting the version to 2, compatibility to the version 1 thunks is provided, at the cost of extra machine code. Version 3 does not include this compatibility.

This option also enables a heuristic for controlling emission of vtables; if a class has any non-inline virtual functions, the vtable will be emitted in the translation unit containing the first one of those.

Like all options that change the ABI, all C++ code, including libgcc.a must be built with the same setting of this option. Since version 1 and version 2 are also incompatible (for classes with virtual bases defining virtual functions), all code must also be compiled with the same version.

In this version of gcc, there are no targets for which version 2 thunks are the default. On all targets, not giving the option will use the traditional implementation, and -fvtable-thunks will produce version 2 thunks.

-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 the C++ library.)

In addition, these optimization, warning, and code generation options have meanings only for C++ programs:

-fno-default-inline

Do not assume `inline' for functions defined inside a class scope. See section Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.

-Wctor-dtor-privacy (C++ only)

Warn when a class seems unusable, because all the constructors or destructors in a class are private and the class has no friends or public static member functions.

-Wnon-virtual-dtor (C++ only)

Warn when a class declares a non-virtual destructor that should probably be virtual, because it looks like the class will be used polymorphically.

-Wreorder (C++ only)

Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

struct A { int i; int j; A(): j (0), i (1) { } };

Here the compiler will warn that the member initializers for `i' and `j' will be rearranged to match the declaration order of the members.

The following `-W...' options are not affected by `-Wall'.

-Weffc++ (C++ only)

Warn about violations of various style guidelines from Scott Meyers' Effective C++ books. If you use this option, you should be aware that the standard library headers do not obey all of these guidelines; you can use `grep -v' to filter out those warnings.

-Wno-deprecated (C++ only)

Do not warn about usage of deprecated features. See section 4.41 Deprecated Features.

-Wno-non-template-friend (C++ only)

Disable warnings when non-templatized friend functions are declared within a template. With the advent of explicit template specification support in g++, if the name of the friend is an unqualified-id (ie, `friend foo(int)'), the C++ language specification demands that the friend declare or define an ordinary, nontemplate function. (Section 14.5.3). Before g++ implemented explicit specification, unqualified-ids could be interpreted as a particular specialization of a templatized function. Because this non-conforming behavior is no longer the default behavior for g++, `-Wnon-template-friend' allows the compiler to check existing code for potential trouble spots, and is on by default. This new compiler behavior can also be turned off with the flag `-fguiding-decls', which activates the older, non-specification compiler code, or with `-Wno-non-template-friend' which keeps the conformant compiler code but disables the helpful warning.

-Wold-style-cast (C++ only)

Warn if an old-style (C-style) cast is used within a C++ program. The new-style casts (`static_cast', `reinterpret_cast', and `const_cast') are less vulnerable to unintended effects.

-Woverloaded-virtual (C++ only)

Warn when a derived class function declaration may be an error in defining a virtual function. In a derived class, the definitions of virtual functions must match the type signature of a virtual function declared in the base class. With this option, the compiler warns when you define a function with the same name as a virtual function, but with a type signature that does not match any declarations from the base class.

-Wno-pmf-conversions (C++ only)

Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

-Wsign-promo (C++ only)

Warn when overload resolution chooses a promotion from unsigned or enumeral type to a signed type over a conversion to an unsigned type of the same size. Previous versions of g++ would try to preserve unsignedness, but the standard mandates the current behavior.

-Wsynth (C++ only)

Warn when g++'s synthesis behavior does not match that of cfront. For instance:

struct A { operator int (); A& operator = (int); }; main () { A a,b; a = b; }

In this example, g++ will synthesize a default `A& operator = (const A&);', while cfront will use the user-defined `operator ='.