Default argument for a functor in a templated parameter

Question

I would like to have a default lambda for a functor argument in my function.

I am aware it is possible using a struct and operator() like this:

struct AddOne {
    int operator()(int a) {
        return a+1;
    }
};

template <typename Functor = AddOne>
int run_old(int x, Functor func = AddOne()) 
{
    return func(x);
}

But I was wondering if there was a modern way, given the changes in the standard in either c++14/17/20, to make this work?

template <typename Functor>
int run_new(int x, Functor func = [](int a){ return a+1; }) 
{
    return func(x);
}

I'm not sure what one would use as the default type to Functor, or if there is syntax i'm unaware of.

https://godbolt.org/z/Hs6vQs

Answer 1

From C++11 you can already do that:

template <typename Functor = int(int)>
int run_new(int x, Functor func = [](int a){ return a+1; }) 
{
    return func(x);
}