CODEWITHSUNDEEP
c++functionparameter-passing
northerner
northerner

Reputation: 794

What is the difference between different ways of passing a function as an argument to another function?

I have the situation where one function calls one of several possible functions. This seems like a good place to pass a function as a parameter. In this Quoara answer by Zubkov there are three ways to do this.

int g(int x(int)) { return x(1); }
int g(int (*x)(int)) { return x(1); }
int g(int (&x)(int)) { return x(1); }
...
int f(int n) { return n*2; }
g(f); // all three g's above work the same

When should which method be used? What are there differences? I prefer the simplest approach so why shouldn't the first way always be used?

For my situation, the function is only called once and I'd like to keep it simple. I have it working with pass by pointer and I just call it with g(myFunc) where myFunc is the function that gets called last.

Upvotes: 8

Views: 385

Answers (1)

Eric
Eric

Reputation: 97591

Expanding on L.F.'s comment, it's often better to eschew function pointers entirely, and work in terms of invocable objects (things which define operator()). All of the following allow you to do that:

#include <type_traits>

// (1) unrestricted template parameter, like <algorithm> uses
template<typename Func>
int g(Func x) { return x(1); }

// (2) restricted template parameter to produce possibly better errors
template<
    typename Func,
    typename=std::enable_if_t<std::is_invocable_r_v<int, Func, int>>
>
int g(Func x) { return std::invoke(x, 1); }

// (3) template-less, trading a reduction in code size for runtime overhead and heap use
int g(std::function<int(int)> x) { return x(1); }

Importantly, all of these can be used on lambda functions with captures, unlike any of your options:

int y = 2;
int ret = g([y](int v) {
    return y + v;
});

Upvotes: 3

Related Questions