Question

I want to do something like this:

int a = 9, b = 3;
map<char,operator> m;
m['+'] = +;
m['-'] = -;
m['*'] = *;
m['/'] = /;
for(map<char,operator>::iterator it = m.begin(); it != m.end(); ++it) {
    cout << func(a,b,it -> second) << endl;
}

With the output being something like this:

12
6
27
3

How can I do it?

Answer 1

You can use the premade functors in <functional>:

int a = 9, b = 3;
std::map<char, std::function<int(int, int)>> m;

m['+'] = std::plus<int>();
m['-'] = std::minus<int>();
m['*'] = std::multiplies<int>();
m['/'] = std::divides<int>();

for(std::map<char, std::function<int(int, int)>>::iterator it = m.begin(); it != m.end(); ++it) {
    std::cout << it->second(a, b) << std::endl;
}

Each one is a class with an operator() that takes two arguments and returns the result of a mathematical operation on those two arguments. For example, std::plus<int>()(3, 4) is basically the same as 3 + 4. Each is stored as a function wrapper object of the signature int(int, int) and then called with the two numbers as needed.