Inferring the call signature of a lambda or arbitrary callable for “make_function”

Question

In some situations it\'s desirable to be able to type-erase a callable (e.g. function, function pointer, object instance with operator(), lambda, mem_fn

Answer 1

For non-variadic non-generic captureless lambda functions as well as simple free functions one can use following approach:

#include 

#include 

template< typename L, typename R, typename ...A >
constexpr
auto // std::function< R (A...) >
to_function_pointer(L l, R (L::*)(A...) const)
{
    return static_cast< R (*)(A...) >(l);
}

template< typename L, typename R, typename ...A >
constexpr
auto // std::function< R (A...) >
to_function_pointer(L l, R (L::*)(A...)) // for mutable lambda
{
    return static_cast< R (*)(A...) >(l);
}

template< typename L >
constexpr
auto
to_function_pointer(L l)
{
    return to_function_pointer(l, &L::operator ());
}

template< typename R, typename ...A >
constexpr
auto // std::function< R (A...) >
to_function_pointer(R (* fp)(A...))
{
    return fp;
}

namespace
{

void f() { std::cout << __PRETTY_FUNCTION__ << std::endl; }

}

int
main()
{
    to_function_pointer([] () { std::cout << __PRETTY_FUNCTION__ << std::endl; })();
    //to_function_pointer([&] () { std::cout << __PRETTY_FUNCTION__ << std::endl; })(); // can't cast from non-captureless lambda to function pointer
    to_function_pointer([] () mutable { std::cout << __PRETTY_FUNCTION__ << std::endl; })();
    to_function_pointer(f)();
    to_function_pointer(&f)();
    return EXIT_SUCCESS;
}