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3.5 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:

Use version n of the C++ ABI. Version 2 is the version of the C++ ABI that first appeared in G++ 3.4. Version 1 is the version of the C++ ABI that first appeared in G++ 3.2. Version 0 will always be the version that conforms most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change as ABI bugs are fixed.

The default is version 2.

Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new will only return 0 if it is declared `throw()', in which case the compiler will always check the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also `new (nothrow)'.
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.

Give string constants type char * instead of type const char *. By default, G++ uses type const char * as required by the standard. Even if you use -fno-const-strings, you cannot actually modify the value of a string constant.

This option might be removed in a future release of G++. For maximum portability, you should structure your code so that it works with string constants that have type const char *.

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.
Don't check for violation of exception specifications at runtime. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining `NDEBUG'. The compiler will still optimize based on the exception specifications.
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 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 G++, 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.

Do not recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. -ansi implies -fno-gnu-keywords.
Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. See Template Instantiation, for more information.
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.
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.
Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.
Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.
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.
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.
Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. See Template Instantiation, for more information.
Disable generation of information about every class with virtual functions for use by the C++ runtime type identification features (`dynamic_cast' and `typeid'). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but it will generate it as needed.
Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.
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.
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
Register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but will only work if your C library supports __cxa_atexit.
Causes all inlined methods to be marked with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. While it can cause bloating through duplication of code within each DSO where it is used, often the wastage is less than the considerable space occupied by a long symbol name in the export table which is typical when using templates and namespaces. For even more savings, combine with the -fvisibility=hidden switch.
Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++.
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:

Do not assume `inline' for functions defined inside a class scope. See Options That Control Optimization. Note that these functions will have linkage like inline functions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated will be compatible.

You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

The known incompatibilities at this point include:

-Wctor-dtor-privacy (C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a virtual destructor, yet it declares a non-virtual one. This warning is enabled by -Wall.
-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) { }

The compiler will rearrange the member initializers for `i' and `j' to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.

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

-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ book:

Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use `grep -v' to filter out those warnings.

-Wno-deprecated (C++ only)
Do not warn about usage of deprecated features. See Deprecated Features.
-Wstrict-null-sentinel (C++ only)
Warn also about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant not a null pointer, it is guaranteed to of the same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared within a template. Since the advent of explicit template specification support in G++, if the name of the friend is an unqualified-id (i.e., `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 be turned off 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 to a non-void type is used within a C++ program. The new-style casts (`static_cast', `reinterpret_cast', and `const_cast') are less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in:
          struct A {
            virtual void f();
          struct B: public A {
            void f(int);

the A class version of f is hidden in B, and code like:

          B* b;

will fail to compile.

-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 enumerated 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.
          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 ='.