What's the difference between __PRETTY_FUNCTION__, __FUNCTION__, __func__?
What's the difference between __PRETTY_FUNCTION__
, __FUNCTION__
, __func__
, and where are they documented? How do I decide which on开发者_Python百科e to use?
__func__
is an implicitly declared identifier that expands to a character array variable containing the function name when it is used inside of a function. It was added to C in C99. From C99 §6.4.2.2/1:
The identifier
__func__
is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declarationstatic const char __func__[] = "function-name";
appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.
Note that it is not a macro and it has no special meaning during preprocessing.
__func__
was added to C++ in C++11, where it is specified as containing "an implementation-defined string" (C++11 §8.4.1[dcl.fct.def.general]/8), which is not quite as useful as the specification in C. (The original proposal to add __func__
to C++ was N1642).
__FUNCTION__
is a pre-standard extension that some C compilers support (including gcc and Visual C++); in general, you should use __func__
where it is supported and only use __FUNCTION__
if you are using a compiler that does not support it (for example, Visual C++, which does not support C99 and does not yet support all of C++0x, does not provide __func__
).
__PRETTY_FUNCTION__
is a gcc extension that is mostly the same as __FUNCTION__
, except that for C++ functions it contains the "pretty" name of the function including the signature of the function. Visual C++ has a similar (but not quite identical) extension, __FUNCSIG__
.
For the nonstandard macros, you will want to consult your compiler's documentation. The Visual C++ extensions are included in the MSDN documentation of the C++ compiler's "Predefined Macros". The gcc documentation extensions are described in the gcc documentation page "Function Names as Strings."
Despite not fully answering the original question, this is probably what most people googling this wanted to see.
For GCC:
$ cat test.cpp
#include <iostream>
int main(int argc, char **argv)
{
std::cout << __func__ << std::endl
<< __FUNCTION__ << std::endl
<< __PRETTY_FUNCTION__ << std::endl;
}
$ g++ test.cpp
$ ./a.out
main
main
int main(int, char**)
__PRETTY_FUNCTION__
handles C++ features: classes, namespaces, templates and overload
main.cpp
#include <iostream>
namespace N {
class C {
public:
template <class T>
static void f(int i) {
(void)i;
std::cout << "__func__ " << __func__ << std::endl
<< "__FUNCTION__ " << __FUNCTION__ << std::endl
<< "__PRETTY_FUNCTION__ " << __PRETTY_FUNCTION__ << std::endl;
}
template <class T>
static void f(double f) {
(void)f;
std::cout << "__PRETTY_FUNCTION__ " << __PRETTY_FUNCTION__ << std::endl;
}
};
}
int main() {
N::C::f<char>(1);
N::C::f<void>(1.0);
}
Compile and run:
g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out
Output:
__func__ f
__FUNCTION__ f
__PRETTY_FUNCTION__ static void N::C::f(int) [with T = char]
__PRETTY_FUNCTION__ static void N::C::f(double) [with T = void]
You may also be interested in stack traces with function names: print call stack in C or C++
Tested in Ubuntu 19.04, GCC 8.3.0.
C++20 std::source_location::function_name
main.cpp
#include <iostream>
#include <string_view>
#include <source_location>
void log(std::string_view message,
const std::source_location& location = std::source_location::current()
) {
std::cout << "info:"
<< location.file_name() << ":"
<< location.line() << ":"
<< location.function_name() << " "
<< message << '\n';
}
int f(int i) {
log("Hello world!"); // Line 16
return i + 1;
}
int f(double i) {
log("Hello world!"); // Line 21
return i + 1.0;
}
int main() {
f(1);
f(1.0);
}
Compile and run:
g++ -std=c++20 -Wall -Wextra -pedantic -o main.out main.cpp
./main.out
Output:
info:source_location.cpp:16:int f(int) Hello world!
info:source_location.cpp:21:int f(double) Hello world!
so note how this returns the caller information, and is therefore perfect for usage in logging, see also: Is there a way to get function name inside a C++ function?
The proposal: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1208r5.pdf
The documentation says:
constexpr const char* function_name() const noexcept;
6 Returns: If this object represents a position in the body of a function, returns an implementation-defined NTBS that should correspond to the function name. Otherwise, returns an empty string.
where NTBS means "Null Terminated Byte String".
Tested on GCC 11.3 Ubuntu 22.04. It was not on GCC 9.1.0 with g++-9 -std=c++2a
.
__func__
is documented in the C++0x standard at section 8.4.1. In this case it's a predefined function local variable of the form:
static const char __func__[] = "function-name ";
where "function name" is implementation specfic. This means that whenever you declare a function, the compiler will add this variable implicitly to your function. The same is true of __FUNCTION__
and __PRETTY_FUNCTION__
. Despite their uppercasing, they aren't macros. Although __func__
is an addition to C++0x
g++ -std=c++98 ....
will still compile code using __func__
.
__PRETTY_FUNCTION__
and __FUNCTION__
are documented here http://gcc.gnu.org/onlinedocs/gcc-4.5.1/gcc/Function-Names.html#Function-Names. __FUNCTION__
is just another name for __func__
. __PRETTY_FUNCTION__
is the same as __func__
in C but in C++ it contains the type signature as well.
For those, who wonder how it goes in VS.
MSVC 2015 Update 1, cl.exe version 19.00.24215.1:
#include <iostream>
template<typename X, typename Y>
struct A
{
template<typename Z>
static void f()
{
std::cout << "from A::f():" << std::endl
<< __FUNCTION__ << std::endl
<< __func__ << std::endl
<< __FUNCSIG__ << std::endl;
}
};
void main()
{
std::cout << "from main():" << std::endl
<< __FUNCTION__ << std::endl
<< __func__ << std::endl
<< __FUNCSIG__ << std::endl << std::endl;
A<int, float>::f<bool>();
}
output:
from main(): main main int __cdecl main(void) from A::f(): A<int,float>::f f void __cdecl A<int,float>::f<bool>(void)
Using of __PRETTY_FUNCTION__
triggers undeclared identifier error, as expected.
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