std::function as a std::map key

Question

I want to insert std::function as a std::map key, but the code doesnt compile, any thoughts?

int sum(int a, float b){
    return a+b;
}

int main()
{
    std::function<int(int, float)>f = sum;
    std::map <std::function<int(int, float)>, std::string> mapa;
    mapa.insert(std::make_pair(f, "sum"));
}

but the code doesnt compile - what does it do if it doesn't compile? Any error? — 273K, Aug 09 '23 at 19:55
could it be that you want string to be the key and function to be the value? — Raildex, Aug 09 '23 at 20:01
This is very unusual. Can you describe what you want the program to do? We may be able to offer alternatives. — user4581301, Aug 09 '23 at 20:20
[yx problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)? Could it be that you want a `std::map` ? So that you can look up the function by its name? — 463035818_is_not_an_ai, Aug 09 '23 at 20:51

score 4 · Accepted Answer · answered Aug 09 '23 at 19:55

std::map requires ordered keys.

std::function does not support an order.

They can't work together.

Adding ordering requirements to a std::function isn't trivial. I'd advise against trying.

Note that the reversed map has no problems.

std::function doesn't even support ==, let alone < or (for the unordered map) std::hash.

Reconsider your design.

std::function as a std::map key

1 Answers1