c++0x: overloading on lambda arity

Question

I\'m trying to create a function which can be called with a lambda that takes either 0, 1 or 2 arguments. Since I need the code to work on both g++ 4.5 and vs2010(which doe

Answer 1

Compile time means of obtaining the arity of a function or a function object, including that of a lambda:

int main (int argc, char ** argv) {
    auto f0 = []() {};
    auto f1 = [](int) {};
    auto f2 = [](int, void *) {};

    std::cout << Arity::value << std::endl; // 0
    std::cout << Arity::value << std::endl; // 1
    std::cout << Arity::value << std::endl; // 2

    std::cout << Arity::value << std::endl; // 2
}

---

template 
class Arity {
private:
    struct Any {
        template 
        operator T ();
    };

    template 
    struct Id {
        typedef T type;
    };

    template 
    struct Size {
        enum { value = N };
    };

    template 
    static Size<0> match (
        F f,
        decltype(f()) * = nullptr);

    template 
    static Size<1> match (
        F f,
        decltype(f(Any())) * = nullptr,
        decltype(f(Any())) * = nullptr);

    template 
    static Size<2> match (
        F f,
        decltype(f(Any(), Any())) * = nullptr,
        decltype(f(Any(), Any())) * = nullptr,
        decltype(f(Any(), Any())) * = nullptr);

public:
    enum { value = Id(Any())))>::type::value };
};