Question

I’ve been working on some solutions for some questions that have been recently posted where the original questioners have been trying to query if a method exists in a given class. I’ve been trying to develop a solution using a SFINAE approach, but unfortunately I keep coming up empty.

Here was my attempt at a solution that isn’t working for a class that allows us to detect if another class has a method called function():

#include <iostream>

using namespace std;

struct sample_class
{
    void function() {}
};


template<typename T>
class test_size_call
{
    private:
        typedef char yes;
        typedef char (&no)[2];
        int tester[1];

        template <unsigned int>
        struct helper { static const unsigned int value = 1; };

        template<typename R>
        static yes test(int (&a)[helper<sizeof(std::declval<R>().function(), 0)>::value]);

        /* template<typename R>
        static no test(...); */

    public:
        static const bool value = (sizeof(test<T>(tester)) == sizeof(yes));
};


int main()
{
    cout << "Has function() method: " << test_size_call<sample_class>::value << endl;
        return 0;
}

The results, if you uncomment the catch-all test function, keep coming up as false. With the function commented out, I get a compiler error that there is not version of test that will take a int (&)[1] argument. I’m curious why the declval<R>().function() doesn’t seem to be instantiating properly. For instance, if I changed it to something very explicit, like declval<T>().function() then it works. Unfortunately that’s not SFINAE, because if the class doesn’t have a function() method, rather than failing silently, I get a compiler error.

I’m sure there is something really simple I’m missing here. Thanks for any help you can provide.

Answer 1

Must be a problem with your compiler, Clang correctly prints 1 and 0 for the following code:

#include <utility>

template<typename T>
class test_size_call
{
    private:
        typedef char yes;
        typedef char (&no)[2];
        int tester[1];

        template <unsigned int>
        struct helper { static const unsigned int value = 1; };

        template<typename R>
        static yes test(int (&a)[helper<sizeof(std::declval<R>().function(), 0)>::value]);

        template<typename R>
        static no test(...);

    public:
        static const bool value = (sizeof(test<T>(tester)) == sizeof(yes));
};

#include <iostream>

struct sample_class
{
    void function() {}
};

struct sample_class2{};

int main()
{
    std::cout << "Has function() method: " << test_size_call<sample_class>::value << '\n';
    std::cout << "Has function() method 2: " << test_size_call<sample_class2>::value << '\n';
    return 0;
}

While GCC 4.5.1 does not. Note that it’s fixed in GCC 4.7, as pointed out here.