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4.5. 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:

-fabi-version=n

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.

-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. 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).

-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.

-fno-const-strings

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, unless you also use -fwritable-strings.

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 *.

-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.

-fno-enforce-eh-specs

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.

-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 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.

-fno-gnu-keywords

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.

-fno-implicit-templates

Never emit code for non-inline templates which are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. Section 7.6 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.

-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.

-fms-extensions

Disable pedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

-fno-nonansi-builtins

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.

-fno-operator-names

Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords.

-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 some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive will allow some nonconforming code to compile.

-frepo

Enable automatic template instantiation at link time. This option also implies -fno-implicit-templates. Section 7.6 Where's the Template?, for more information.

-fno-rtti

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.

-fstats

Emit statistics about front-end processing at the end of the compilation. This information is generally only useful to the G++ development team.

-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.

-fno-threadsafe-statics

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.

-fuse-cxa-atexit

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.

-fvisibility-inlines-hidden

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.

-fno-weak

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++.

-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. Section 4.10 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:

  • Incorrect handling of tail-padding for bit-fields. G++ may attempt to pack data into the same byte as a base class. For example:

    struct A { virtual void f(); int f1 : 1; };
    struct B : public A { int f2 : 1; };

    In this case, G++ will place B::f2 into the same byte asA::f1; other compilers will not. You can avoid this problem by explicitly padding A so that its size is a multiple of the byte size on your platform; that will cause G++ and other compilers to layout B identically.

  • Incorrect handling of tail-padding for virtual bases. G++ does not use tail padding when laying out virtual bases. For example:

    struct A { virtual void f(); char c1; };
    struct B { B(); char c2; };
    struct C : public A, public virtual B {};

    In this case, G++ will not place B into the tail-padding for A; other compilers will. You can avoid this problem by explicitly padding A so that its size is a multiple of its alignment (ignoring virtual base classes); that will cause G++ and other compilers to layout C identically.

  • Incorrect handling of bit-fields with declared widths greater than that of their underlying types, when the bit-fields appear in a union. For example:

    union U { int i : 4096; };

    Assuming that an int does not have 4096 bits, G++ will make the union too small by the number of bits in an int.

  • Empty classes can be placed at incorrect offsets. For example:

    struct A {};
    
    struct B {
      A a;
      virtual void f ();
    };
    
    struct C : public B, public A {};

    G++ will place the A base class of C at a nonzero offset; it should be placed at offset zero. G++ mistakenly believes that the A data member of B is already at offset zero.

  • Names of template functions whose types involve typename or template template parameters can be mangled incorrectly.

    template <typename Q>
    void f(typename Q::X) {}
    
    template <template <typename> class Q>
    void f(typename Q<int>::X) {}

    Instantiations of these templates may be mangled incorrectly.

-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:

  • Item 11: Define a copy constructor and an assignment operator for classes with dynamically allocated memory.

  • Item 12: Prefer initialization to assignment in constructors.

  • Item 14: Make destructors virtual in base classes.

  • Item 15: Have operator= return a reference to *this.

  • Item 23: Don't try to return a reference when you must return an object.

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

  • Item 6: Distinguish between prefix and postfix forms of increment and decrement operators.

  • Item 7: Never overload &&, ||, or ,.

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. Section 7.12 Deprecated Features.

-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;
b->f();

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.

-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 =.

 
 
  Published under the terms of the GNU General Public License Design by Interspire