Test if Callable points at specific member function - c++

Question

I want to test if a _Callable __f is a (pointer to a) specific non-static member function memberfuncA.
I can test that with __f == &MyClass::memberfuncA, but only if I get no memberfuncB in the way, as in the example below, which results in a compiler error.
I tried to use std::is_convertible to make sure the cast will compile, but that didn't work; same with typeid().
#include <type_traits>
#include <typeinfo>
class A {};
class B {};
class MyClass {
public:
MyClass() {
A a;
B b;
push_exec(&MyClass::memberfuncA, this, a);
push_exec(&MyClass::memberfuncB, this, b);
}
template<typename _Callable, typename... _Args>
void push_exec(_Callable&& __f, _Args&&... __args)
{
// Test if typeid is the same
if(typeid(__f) == typeid(&MyClass::memberfuncA))
// Test if types are castable
if(std::is_convertible<typeof __f, typeof &MyClass::memberfuncA>::value)
// Test if __f is actually pointing to a specific member function (compiler error)
if(__f == &MyClass::memberfuncA)
return;
}
void memberfuncA(A a) { }
void memberfuncB(B b) { }
};
int main() {
MyClass mc;
}
Compiler output is
g++ --std=gnu++11 -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/callable_test.d" -MT"src/callable_test.o" -o "src/callable_test.o" "../src/callable_test.cpp"
../src/callable_test.cpp: In instantiation of ‘void MyClass::push_exec(_Callable&&, _Args&& ...) [with _Callable = void (MyClass::*)(B); _Args = {MyClass* const, B&}]’:
../src/callable_test.cpp:13:49: required from here
../src/callable_test.cpp:24:24: error: comparison between distinct pointer types ‘void (MyClass::*)(B)’ and ‘void (MyClass::*)(A)’ lacks a cast [-fpermissive]
if(__f == &MyClass::memberfuncA)
^
../src/callable_test.cpp:24:24: error: cannot convert ‘__f’ from type ‘void (MyClass::*)(B)’ to type ‘void MyClass::*’
../src/callable_test.cpp:24:24: error: cannot convert ‘&MyClass::memberfuncA’ from type ‘void (MyClass::*)(A)’ to type ‘void MyClass::*’
make: *** [src/callable_test.o] Error 1
Any idea how I can check if __f == &MyClass::memberfuncA?

Answer 1

If you really want compare __f and &MyClass::memberfunc, you can cast they to void *
if( ((void*)__f) == ((void*)&MyClass::memberfuncA) )
return;
Checking with std::is_convertible don't work because it's a compilation problem, not a run-time error. I mean: even if std::is_convertible value is false, the compiler try to compile the following if.
You could try std::is_convertible with SFINAE; something like this (EDIT: corrected version):
template<typename _Callable, typename... _Args>
typename std::enable_if<true == std::is_convertible<_Callable, void (MyClass::*)(A)>::value, void>::type push_exec(_Callable && __f, _Args && ... __args)
{
// do something
}
template<typename _Callable, typename... _Args>
typename std::enable_if<false == std::is_convertible<_Callable, void (MyClass::*)(A)>::value, void>::type push_exec(_Callable && __f, _Args && ... __args)
{
// do something else
}