mardi 3 février 2015

Checking for existence of an (overloaded) member function

There are a number of answered questions about checking whether a member function exists: for example, Is it possible to write a C++ template to check for a function's existence?


But this method fails, if the function is overloaded. Here is a slightly modified code from that question's top-rated answer.



#include <iostream>
#include <vector>

struct Hello
{
int helloworld(int x) { return 0; }
int helloworld(std::vector<int> x) { return 0; }
};

struct Generic {};


// SFINAE test
template <typename T>
class has_helloworld
{
typedef char one;
typedef long two;

template <typename C> static one test( decltype(&C::helloworld) ) ;
template <typename C> static two test(...);


public:
enum { value = sizeof(test<T>(0)) == sizeof(char) };
};


int
main(int argc, char *argv[])
{
std::cout << has_helloworld<Hello>::value << std::endl;
std::cout << has_helloworld<Generic>::value << std::endl;
return 0;
}


This code prints out:



0
0


But:



1
0


if the second helloworld() is commented out.


So my question is whether it's possible to check whether a member function exists, regardless of whether it's overloaded.


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