C++: How to make a container indexed by std::function?

Question

For indexing I use std::unordered_map and std::map. Both of them throws compiling errors when using as follow:

std::unordered_map<std::function<bool(Ent*)>, int> var;

std::unordered_map fails due referencing deleted function

std::map fails due no < operator

The ideal solution for me would be to use a type of map, but if is a must to use another type of container, then it shouldn't be a problem

Answer 1

One way of having functions as container key is to wrap them into functor structure

#include <unordered_map>
#include <typeinfo>

struct FunctorSum {
    int operator()(int x, int y) {
        return x + y;
    }
};
struct FunctorMult {
    int operator()(int x, int y) {
        return x * y;
    }
};

int main() {
    std::unordered_map<size_t, int> funcToInt;
    funcToInt[typeid(FunctorSum).hash_code()] = 0;
    funcToInt[typeid(FunctorMult).hash_code()] = 1;

    return 0;
}

Here I used typeid as hash, but it can also be hardcoded into functor struct.

Another way is to use std::function::target_type to calculate hash of the function, which will work only with lambdas. But you can always wrap any function into lambda.

#include <iostream>
#include <functional>

using FuncType = std::function<bool(int)>;
bool x(int v) { return v == 0; }

std::string hash(FuncType f) {
    return f.target_type().name();
}

int main() {
    auto y = [](int v) { return v == 1; };
    auto z = [](int v) { return v == 2; };

    std::cout << "x: " << hash(x) << std::endl;
    std::cout << "y: " << hash(y) << std::endl;
    std::cout << "z: " << hash(z) << std::endl;

    return 0;
}

Output

x: PFbiE
y: Z4mainEUliE_
z: Z4mainEUliE0_