f78
Options that Control C++ Dialect
Contents|Index|Previous|Next
Options
that control C++ dialect
The following documentation
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 uses. For instance, you might compile a file, firstClass.C,
like the following example.
g++ -g -felide-constructors -O -c firstClass.C
In the previous 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. The following lists 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 ANSI C++ 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 sa
ffb
ves 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 the dollar sign ($)
in identifiers. You can also explicitly prohibit use of $
with the -fno-dollars-in-identifiers
option. 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++ forbids $
in identifiers.
-
-fembedded-cxx
Incompliance with
the Embedded C++ specification, make illegal the use of templates, exception
handling, multiple inheritance, or RTTI. Attempts to use namespsaces are
also not allowed. This makes the use of some keywords result in warnings
by default: template,
typename,
catch,
throw,
try,
using,
namespace,
dynamic_cast,
static_cast,
reinterpret_cast,
const_cast,
and typeid.
To make warnings not be given as errors, add the -pedantic-errors
flag.
-
-fenum-int-equiv
Anachronistically permit
implicit conversion of int
to enumeration types. Current C++ allows conversion of enum
to int,
but not the reverse.
-
-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 Wheres
the template? for more explanation of templates.
-
-falt-external-templates
Similar to -fexternal-templates,
but template instances are emitted or not according to the place where
they are first instantiated. See Wheres
the template? for more explanation on templates.
-
-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. Instead,
use the keywords, __classof__,
__ headof__,
__signature__,
__sigof__,
and __typeof__.
-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.
-fno-implicit-templates
Never emit code for templates
which are instantiated implicitly (i.e., by use); only emit code for explicit
instantiations. See Wheres the template?
for more explanation on templates.
-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.
-
-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 the following makes six
passes through all three steps.
cout << "This " << p << " has " << n << " legs.\n";
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.
-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 man
ffb
gling does not mangle foo<int,
double> and foo<int, char> given this declaration:
template <class T, class U> void foo(T t);
-fno-nonnull-objects
Dont 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; 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.
-frepo
Enable automatic template
instantiation. This option also implies -fno-implicit-templates.
See Wheres the template? for
more explanation of templates.
-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.
-fthis-is-variable
Permit assignment to this.
The incorporation of user-define
ffb
d 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.
-
-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.
-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.)
-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.
In addition, the following options
for optimization, warning, and code generation have meanings only for C++
programs.
-
-fno-default-inline
-
-Wold-style-cast
-
-Woverloaded-virtual
-
-Wtemplate-debugging
-
-Weffc++
Warn about violation
of some style rules from Effective C++ by Scott Myers.
-
+en
Control how virtual function
definitions are used, in a fashion compatible with cfront
1.x. For more
explanation of compatibility conventions for this option, see the description
for +e
in Options
for code generation conventions.
0