C++ SFINAE example: how to detect if a class contains ToString method

sfinae_tostring_ex.cpp

// SFINAE, enable_if example
// based on http://stackoverflow.com/questions/257288/is-it-possible-to-write-a-c-template-to-check-for-a-functions-existence


#include <iostream>
#include <type_traits>

class ClassWithToString
{
public:
    std::string ToString() { return "description of ClassWithToString object"; }
};

class ClassNoToString
{
public:
    int a;
};


// SFINAE test
template <typename T>
class HasToString
{
private:
    typedef char YesType[1];
    typedef char NoType[2];

    template <typename C> static YesType& test( decltype(&C::ToString) ) ;
    template <typename C> static NoType& test(...);


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

template<typename T> 
typename std::enable_if<HasToString<T>::value, std::string>::type
CallToString(T * t) {
   /* something when T has toString ... */
   return t->ToString();
}

/*
template<typename T> 
typename std::enable_if<std::is_class<T>::value, std::string>::type
CallToString(T * t) {
   return "a class without ToString() method!";
}
*/

std::string CallToString(...)
{
    return "undefined object, cannot call ToString() method here";
}


int main(int argc, char *argv[])
{
    std::cout << HasToString<ClassWithToString>::value << std::endl;
    std::cout << HasToString<ClassNoToString>::value << std::endl;
    std::cout << HasToString<int>::value << std::endl;
    
    ClassWithToString c1;
    std::cout << CallToString(&c1) << std::endl;
    
    ClassNoToString c2;
    std::cout << CallToString(&c2) << std::endl;
    
    int c3 = 0;
    std::cout << CallToString(&c3) << std::endl;
    
    return 0;
}

Clean implementation. Note that this only works for public members, since decltype will fail on private members. This can either be a bug or a feature depending on what you're trying to do.

good

Clean and concise, thank you!