компилятор C и C ++ проекта GNU (GNU project C and C++ compiler)
Параметры подробно (Options detail)
Controlling C++ Dialect
This section describes the command-line options that are only
meaningful for C++ programs. 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 -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for
C++ programs; you can use the other options with any language
supported by GCC.
Some options for compiling C programs, such as -std, are also
relevant for C++ programs.
Here is a list of options that are only for compiling C++
programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to
the C++ ABI specification. Therefore, the ABI obtained using
version 0 will change in different versions of G++ as ABI
bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared
in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared
in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as
a template argument.
Version 4, which first appeared in G++ 4.5, implements a
standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the
mangling of attribute const/volatile on function pointer
types, decltype of a plain decl, and use of a function
parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the
promotion behavior of C++11 scoped enums and the mangling of
template argument packs, const/static_cast, prefix ++ and --,
and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats
nullptr_t as a builtin type and corrects the mangling of
lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the
substitution behavior of function types with function-cv-
qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the
alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling
convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the
mangling of sizeof... expressions and operator names. For
multiple entities with the same name within a function, that
are declared in different scopes, the mangling now changes
starting with the twelfth occurrence. It also implies
-fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the
calling conventions for empty classes on the x86_64 target
and for classes with only deleted copy/move constructors. It
accidentally changes the calling convention for classes with
a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the
accidental change in version 12.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around
mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled
name. This switch specifies which ABI version to use for the
alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC
7 compatibility). If another ABI version is explicitly
selected, this defaults to 0. For compatibility with GCC
versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version
is used for compatibility aliases. If this option is
provided along with -Wabi (without the version), the version
from this option is used for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful
for working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more
alignment than "void* ::operator new(std::size_t)" provides.
A numeric argument such as "-faligned-new=32" can be used to
specify how much alignment (in bytes) is provided by that
function, but few users will need to override the default of
"alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++2a. This
includes the addition of a new "char8_t" fundamental type,
changes to the types of UTF-8 string and character literals,
new signatures for user-defined literals, associated standard
library updates, and new "__cpp_char8_t" and
"__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary
and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character
literals introduces incompatibilities with ISO C++11 and
later standards. For example, the following code is well-
formed under ISO C++11, but is ill-formed when -fchar8_t is
specified.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested
-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" only returns 0 if it is
declared "throw()", in which case the compiler always checks
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).
-fconcepts
Enable support for the C++ Extensions for Concepts Technical
Specification, ISO 19217 (2015), which allows code like
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless
recursion during constant expression evaluation. The minimum
specified by the standard is 512.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14
constexpr functions to n. A limit is needed to detect
infinite loops during constant expression evaluation. The
default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single
constexpr evaluation. Even when number of iterations of a
single loop is limited with the above limit, if there are
several nested loops and each of them has many iterations but
still smaller than the above limit, or if in a body of some
loop or even outside of a loop too many expressions need to
be evaluated, the resulting constexpr evaluation might take
too long. The default is 33554432 (1<<25).
-fdeduce-init-list
Enable deduction of a template type parameter as
"std::initializer_list" from a brace-enclosed initializer
list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}
This deduction was implemented as a possible extension to the
originally proposed semantics for the C++11 standard, but was
not part of the final standard, so it is disabled by default.
This option is deprecated, and may be removed in a future
version of G++.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that 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. This option also causes G++ to call trivial
member functions which otherwise would be expanded inline.
In C++17, the compiler is required to omit these temporaries,
but this option still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++
standard, but may be useful for reducing code size in
production builds, much like defining "NDEBUG". This does
not give user code permission to throw exceptions in
violation of the exception specifications; the compiler still
optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run
time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a variable
goes through a wrapper function that performs any necessary
initialization. When the use and definition of the variable
are in the same translation unit, this overhead can be
optimized away, but when the use is in a different
translation unit there is significant overhead even if the
variable doesn't actually need dynamic initialization. If
the programmer can be sure that no use of the variable in a
non-defining TU needs to trigger dynamic initialization
(either because the variable is statically initialized, or a
use of the variable in the defining TU will be executed
before any uses in another TU), they can avoid this overhead
with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol
aliases, the default is -fno-extern-tls-init.
-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. This option is implied by the strict
ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are
instantiated implicitly (i.e. by use); only emit code for
explicit instantiations. If you use this option, you must
take care to structure your code to include all the necessary
explicit instantiations to avoid getting undefined symbols at
link time.
-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
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
causes linker errors if these functions are not inlined
everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such
as implicit int and getting a pointer to member function via
non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11
constructor inheritance. This is part of C++17 but also
considered to be a Defect Report against C++11 and C++14.
This flag is enabled by default unless -fabi-version=10 or
lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template
template parameters and default arguments: this allows a
template with default template arguments as an argument for a
template template parameter with fewer template parameters.
This flag is enabled by default for -std=c++17.
-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.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception
specification. If the function has local variables of types
with non-trivial destructors, the exception specification
actually makes the function smaller because the EH cleanups
for those variables can be optimized away. The semantic
effect is that an exception thrown out of a function with
such an exception specification results in a call to
"terminate" rather than "unexpected".
-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 allows some
nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a
function template, the compiler normally prints the signature
of the template followed by the template arguments and any
typedefs or typenames in the signature (e.g. "void f(T) [with
T = int]" rather than "void f(int)") so that it's clear which
template is involved. When an error message refers to a
specialization of a class template, the compiler omits any
template arguments that match the default template arguments
for that template. If either of these behaviors make it
harder to understand the error message rather than easier,
you can use -fno-pretty-templates to disable them.
-frepo
Enable automatic template instantiation at link time. This
option also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with
virtual functions for use by the C++ run-time 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 G++ generates it as needed. The
"dynamic_cast" operator can still be used for casts that do
not require run-time type information, i.e. casts to "void *"
or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with
-fno-rtti may not work. For example, programs may fail to
link if a class compiled with -fno-rtti is used as a base for
a class compiled with -frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example, use the
size of the object to make deallocation faster. Enabled by
default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want to
add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a
value of enumerated type can only be one of the values of the
enumeration (as defined in the C++ standard; basically, a
value that can be represented in the minimum number of bits
needed to represent all the enumerators). This assumption
may not be valid if the program uses a cast to convert an
arbitrary integer value to the enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift
expressions in left-to-right order, and evaluate assignment
in right-to-left order, as adopted for C++17. Enabled by
default with -std=c++17. -fstrong-eval-order=some enables
just the ordering of member access and shift expressions, and
is the default without -std=c++17.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a
single warning or error to n. The default value is 10.
-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 (changed to 1024 in
C++11). The default value is 900, as the compiler can run
out of stack space before hitting 1024 in some situations.
-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 only works if
your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine.
This causes "std::uncaught_exception" to be incorrect, but is
necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to
compare pointers to inline functions or methods where the
addresses of the two functions are taken in different shared
objects.
The effect of this is that GCC may, effectively, mark inline
methods 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.
The behavior of this switch is not quite the same as marking
the methods as hidden directly, because it does not affect
static variables local to the function or cause the compiler
to deduce that the function is defined in only one shared
object.
You may mark a method as having a visibility explicitly to
negate the effect of the switch for that method. For
example, if you do want to compare pointers to a particular
inline method, you might mark it as having default
visibility. Marking the enclosing class with explicit
visibility has no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared
library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's
C++ linkage model compatible with that of Microsoft Visual
Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without
explicit visibility specifications that are defined in
more than one shared object: those declarations are
permitted if they are permitted when this option is not
used.
In new code it is better to use -fvisibility=hidden and
export those classes that are intended to be externally
visible. Unfortunately it is possible for code to rely,
perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects are different, so changing one does
not change the other; and that pointers to function members
defined in different shared objects may not compare equal.
When this flag is given, it is a violation of the ODR to
define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ uses weak symbols if they are
available. This option exists only for testing, and should
not be used by end-users; it results 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:
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn when G++ it generates code that is probably not
compatible with the vendor-neutral C++ ABI. Since G++ now
defaults to updating the ABI with each major release,
normally -Wabi will warn only if there is a check added later
in a release series for an ABI issue discovered since the
initial release. -Wabi will warn about more things if an
older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to
warn about compatibility with a particular -fabi-version
level, e.g. -Wabi=2 to warn about changes relative to
-fabi-version=2.
If an explicit version number is provided and
-fabi-compat-version is not specified, the version number
from this option is used for compatibility aliases. If no
explicit version number is provided with this option, but
-fabi-compat-version is specified, that version number is
used for ABI warnings.
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 is 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.
Known incompatibilities in -fabi-version=2 (which was the
default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of
reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute
((vector_size))" were mangled in a non-standard way that
does not allow for overloading of functions taking
vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as
type qualifiers, and "decltype" of a plain declaration
was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic
function are promoted like unscoped enumerators, causing
"va_arg" to complain. On most targets this does not
actually affect the parameter passing ABI, as there is no
way to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument
packs, "const_cast", "static_cast", prefix
increment/decrement, and a class scope function used as a
template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled
incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-
qualifiers, the un-qualified function type was
incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC
5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1,
leading to unaligned accesses. Note that this did not
affect the ABI of a function with a "nullptr_t"
parameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC
5.2.
* Target-specific attributes that affect the identity of a
type, such as ia32 calling conventions on a function type
(stdcall, regparm, etc.), did not affect the mangled
name, leading to name collisions when function pointers
were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC
6.1.
It also warns about psABI-related changes. The known psABI
changes at this point include:
* For SysV/x86-64, unions with "long double" members are
passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};
"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that
does not have that ABI tag. See C++ Attributes for more
information about ABI tags.
-Wctor-dtor-privacy (C++ and Objective-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. Also warn if
there are no non-private methods, and there's at least one
private member function that isn't a constructor or
destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class
that has virtual functions and non-virtual destructor. It is
unsafe to delete an instance of a derived class through a
pointer to a base class if the base class does not have a
virtual destructor. This warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or
copy assignment operator is deprecated if the class has a
user-provided copy constructor or copy assignment operator,
in C++11 and up. This warning is enabled by -Wextra. With
-Wdeprecated-copy-dtor, also deprecate if the class has a
user-provided destructor.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are
likely to result in dangling pointers. Since the underlying
array for an "initializer_list" is handled like a normal C++
temporary object, it is easy to inadvertently keep a pointer
to the array past the end of the array's lifetime. For
example:
* If a function returns a temporary "initializer_list", or
a local "initializer_list" variable, the array's lifetime
ends at the end of the return statement, so the value
returned has a dangling pointer.
* If a new-expression creates an "initializer_list", the
array only lives until the end of the enclosing full-
expression, so the "initializer_list" in the heap has a
dangling pointer.
* When an "initializer_list" variable is assigned from a
brace-enclosed initializer list, the temporary array
created for the right side of the assignment only lives
until the end of the full-expression, so at the next
statement the "initializer_list" variable has a dangling
pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from
the "initializer_list" argument, this doesn't extend the
lifetime of the array, so if a class variable is
constructed from a temporary "initializer_list", the
pointer is left dangling by the end of the variable
declaration statement.
-Wliteral-suffix (C++ and Objective-C++ only)
Warn when a string or character literal is followed by a ud-
suffix which does not begin with an underscore. As a
conforming extension, GCC treats such suffixes as separate
preprocessing tokens in order to maintain backwards
compatibility with code that uses formatting macros from
"<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
Additionally, warn when a user-defined literal operator is
declared with a literal suffix identifier that doesn't begin
with an underscore. Literal suffix identifiers that don't
begin with an underscore are reserved for future
standardization.
This warning is enabled by default.
-Wlto-type-mismatch
During the link-time optimization warn about type mismatches
in global declarations from different compilation units.
Requires -flto to be enabled. Enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are
diagnosed by default, as required by the standard. A
narrowing conversion from a constant produces an error, and a
narrowing conversion from a non-constant produces a warning,
but -Wno-narrowing suppresses the diagnostic. Note that this
does not affect the meaning of well-formed code; narrowing
conversions are still considered ill-formed in SFINAE
contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion
prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of
a call to a function that does not have a non-throwing
exception specification (i.e. "throw()" or "noexcept") but is
known by the compiler to never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a
function type changes the mangled name of a symbol relative
to C++14. Enabled by -Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls
"f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function
such as "memset" or "memcpy" is an object of class type, and
when writing into such an object might bypass the class non-
trivial or deleted constructor or copy assignment, violate
const-correctness or encapsulation, or corrupt virtual table
pointers. Modifying the representation of such objects may
violate invariants maintained by member functions of the
class. For example, the call to "memset" below is undefined
because it modifies a non-trivial class object and is,
therefore, diagnosed. The safe way to either initialize or
clear the storage of objects of such types is by using the
appropriate constructor or assignment operator, if one is
available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly
casting the pointer to the class object to "void *" or to a
type that can be safely accessed by the raw memory function
suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible
non-virtual destructor itself or in an accessible polymorphic
base class, in which case it is possible but unsafe to delete
an instance of a derived class through a pointer to the class
itself or base class. This warning is automatically enabled
if -Weffc++ is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier,
except when it is part of the GNU Explicit Register Variables
extension. The use of the "register" keyword as storage
class specifier has been deprecated in C++11 and removed in
C++17. Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-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 rearranges 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.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy
elision. A typical scenario when copy elision can occur is
when returning in a function with a class return type, when
the expression being returned is the name of a non-volatile
automatic object, and is not a function parameter, and has
the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy
elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that
is, when a move operation would have been performed even
without the "std::move" call. This happens because the
compiler is forced to treat the object as if it were an
rvalue in certain situations such as returning a local
variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++
implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the
type of the expression being returned and the function return
type differ, yet G++ treats the return value as if it were
designated by an rvalue.
This warning is enabled by -Wextra.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal
number suffixes as GNU extensions. When this option is
turned off these suffixes are treated as C++11 user-defined
literal numeric suffixes. This is on by default for all
pre-C++11 dialects and all GNU dialects: -std=c++98,
-std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off
by default for ISO C++11 onwards (-std=c++11, ...).
The following -W... options are not affected by -Wall.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from
Scott Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an
object.
* Distinguish between prefix and postfix forms of increment
and decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also
one of the effective C++ recommendations. However, the check
is extended to warn about the lack of virtual destructor in
accessible non-polymorphic bases classes too.
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.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn 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 rather than a null pointer, it is guaranteed to be
of the same size as a pointer. But this use is not portable
across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are
declared within a template. In very old versions of GCC that
predate implementation of the ISO standard, declarations such
as friend int foo(int), where the name of the friend is an
unqualified-id, could be interpreted as a particular
specialization of a template function; the warning exists to
diagnose compatibility problems, and is enabled by default.
-Wold-style-cast (C++ and Objective-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
("dynamic_cast", "static_cast", "reinterpret_cast", and
"const_cast") are less vulnerable to unintended effects and
much easier to search for.
-Woverloaded-virtual (C++ and Objective-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();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to
member function to a plain pointer.
-Wsign-promo (C++ and Objective-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++ tried to preserve unsignedness, but the
standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered.
Some coding rules disallow templates, and this may be used to
enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL.
One may also instantiate or specialize templates.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base
classes. Some coding rules disallow multiple inheritance,
and this may be used to enforce that rule. The warning is
inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that
indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base
class. Some coding rules disallow multiple inheritance, and
this may be used to enforce that rule. The warning is
inactive inside a system header file, such as the STL, so one
can still use the STL. One may also define classes that
indirectly use virtual inheritance.
-Wnamespaces
Warn when a namespace definition is opened. Some coding
rules disallow namespaces, and this may be used to enforce
that rule. The warning is inactive inside a system header
file, such as the STL, so one can still use the STL. One may
also use using directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will
immediately result in a call to "terminate".
-Wno-class-conversion (C++ and Objective-C++ only)
Disable the warning about the case when a conversion function
converts an object to the same type, to a base class of that
type, or to void; such a conversion function will never be
called.
