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 -felide-constructors -O -c firstClass.C

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

   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.

`-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.

`-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.

`-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.

`-fdollars-in-identifiers'
     Accept `$' in identifiers.  You can also explicitly prohibit use of
     `$' with the option `-fno-dollars-in-identifiers'.  (GNU C++
     allows `$' by default on some target systems but not others.)
     Traditional C allowed the character `$' to form part of
     identifiers.  However, ANSI C and C++ forbid `$' in identifiers.

`-fenum-int-equiv'
     Anachronistically permit implicit conversion of `int' to
     enumeration types.  Current C++ allows conversion of `enum' to
     `int', but not the other way around.

`-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.  *Note Template
     Instantiation::, for more information.

`-falt-external-templates'
     Similar to -fexternal-templates, but template instances are
     emitted or not according to the place where they are first
     instantiated.  See Template Instantiation, for more
     information.

`-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'.

`-fno-implicit-templates'
     Never emit code for templates which are instantiated implicitly
     (i.e. by use); only emit code for explicit instantiations.  
See Template Instantiation, for more information.

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

`-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.  If
     you compile any of your code with this flag, you must compile
     *all* of your code with this flag (including libg++, if you use
     it).

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

`-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.

`-fmemoize-lookups'
`-fsave-memoized'
     Use heuristics to compile faster.  These heuristics are not
     enabled by default, since they are only effective for certain
     input files.  Other input files compile more slowly.

     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.
     `-fmemoize-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-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 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.

     The code that implements these flags has rotted; you should
     probably avoid using them.

`-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'.

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

`-fno-nonnull-objects'
     Don't assume that a reference is initialized to refer to a valid
     object.  Although the current C++ Working Paper prohibits null
     references, some old code may rely on them, and you can use
     `-fno-nonnull-objects' to turn on checking.

     At the moment, the compiler only does this checking for
     conversions to virtual base classes.

`-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'.

`-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'
     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.

     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.

`-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++.)

`-traditional'
     For C++ programs (in addition to the effects that apply to both C
     and C++), this has the same effect as `-fthis-is-variable'.  
See Options Controlling C Dialect: C Dialect Options.

   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 Options That Control Optimization: Optimize Options.

`-Wenum-clash'
`-Woverloaded-virtual'
`-Wtemplate-debugging'
     Warnings that apply only to C++ programs.  *Note Options to
     Request or Suppress Warnings: Warning Options.

`+eN'
     Control how virtual function definitions are used, in a fashion
     compatible with `cfront' 1.x.  *Note Options for Code Generation
     Conventions: Code Gen Options.