andy andy - 1 year ago 106
C++ Question

Is it possible to write a template to check for a function's existence?

Is it possible to write a template that changes behavior depending on if a certain member function is defined on a class?

Here's a simple example of what I would want to write:

template<class T>
std::string optionalToString(T* obj)
if (FUNCTION_EXISTS(T->toString))
return obj->toString();
return "toString not defined";

So, if
class T
defined, then it uses it; otherwise, it doesn't. The magical part that I don't know how to do is the "FUNCTION_EXISTS" part.

Answer Source

Yes, with SFINAE you can check if a given class does provide a certain method. Here's the working code:

#include <iostream>

struct Hello
    int helloworld() { return 0; }

struct Generic {};    

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

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

    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;

I've just tested it with Linux and gcc 4.1/4.3. I don't know if it's portable to other platforms running different compilers.

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