constexpr_if.cpp

#include <iostream>

void foo()
{
    std::cout << "foo" << std::endl;
}

void bar(int x)
{
    std::cout << "bar" << std::endl;
}

// template traits class
template<class F>
struct function_traits;

// specialization using function pointer
template<class... Args>
struct function_traits<void (*)(Args...)>
{
    static constexpr std::size_t number_parameters = sizeof...(Args);
};

template <typename Func>
void call(Func f)
{
    if constexpr (function_traits<Func>::number_parameters == 0)
        f();
    else
        f(0);
}

// Old way
template <typename Func,
    typename std::enable_if_t<
        function_traits<Func>::number_parameters == 0>* = nullptr>
void call_old(Func f)
{
    f();
}

template <typename Func,
    typename std::enable_if_t<
        function_traits<Func>::number_parameters == 1>* = nullptr>
void call_old(Func f)
{
    f(0);
}

int main()
{
    call(foo);
    call(bar);

    call_old(foo);
    call_old(bar);
    return 0;
}