Passing any function as template parameter

Question

I want to pass a function value as a template parameter to a function. Currently the best I managed to do is :

template< typename F, F f >         


        

Answer 1

It's now possible in C++17 with template:

template
struct FuncWrapper final
{
    template
    auto operator()(Args &&... args) const
    {
        return Func(std::forward(args)...);
    }
};

int add(int a, int b)
{
    return a + b;
}

int main()
{
    FuncWrapper wrapper;
    return wrapper(12, 34);
}

Demo: https://godbolt.org/g/B7W56t

You can use #ifdef __cpp_nontype_template_parameter_auto to detect compiler support for this in your code.

If you are able to use C++20 and you want better error messages, you can also use concepts:

template
concept CanAddTwoNumbers = std::is_invocable_r_v;

template
    requires CanAddTwoNumbers
struct AddTwoNumbersWrapper final
{
    auto operator()(int a, int b) const
    -> int
    {
        return std::invoke(Func, a, b);
    }
};

int add(int a, int b)
{
    return a + b;
}

int main()
{
    AddTwoNumbersWrapper wrapper;
    return wrapper(12, 34);
    AddTwoNumbersWrapper<123> bad; //error: constraint failure
}

Demo: https://gcc.godbolt.org/z/ai3WGH