Function passed as a class argument - c++

Question

In C++, we can pass a function/functor to a function like so:
template <typename F>
void doOperation(int a, int b, F f){
std::cout << "Result: " << f(a,b) << std::endl;
}
we can then use both functions and functors:
int add(const int &a, const int &b){ return a+b; }
struct subtract(){
void operator() (const int &a, const int &b) { return a-b; }
};
and use it in the following manner:
doOperation(1,2,add);
doOperation(5,2,subtract());
My question is, can I do something similar with a class and pass a function as an argument to a class, store it and use it later? E.g.
template <typename F>
class doOperation{
public:
doOperation(int &a, int &b, F f) : a(a), b(b), f(f) {};
void setOperands(int &a, int &b) { this->a = a; this->b = b };
void performCalculation(){
std::cout << "Result: " << f(a,b) << std::endl;
}
private:
int a,b;
F f;
}
So that we may assign it a function once and then later use it:
doOperation summing(1,2,add);
summing.setOperands(2,3);
summing.performCalculation();
doOperation subtraction(7,3,subtract());
subtraction.performCalculation();
If my examples are valid, I would appreciate the explanation for the mechanics here as I seem to be getting a bit lost. In case I missed something, I am looking for hints on whether this can be achieved.
Lastly, how would I then use such a class doOperation in other functions and classes. For example, would defining something like this inside a member function require me to template the new class, its member function, and how would it be declared and used:
class higherFunctionality{
public:
higherFunctionality() {...}
void coolThings(){
doOperation *myOperation = operationFactory( ... );
myOperation->setOperands(4,5);
myOperation->performCalculation();
}
};

Answer 1

Yes, but you have to supply the type when you instantiate template classes. The usual way to deal with this is to create a helper function:
template < typename Fun > struct operation_class
{
operation_class(Fun f) : fun{f} {}
Fun fun;
};
template < typename Fun >
operation_class<Fun> operation(Fun fun) { return operation_class<Fun>{fun}; }
int main()
{
auto op0 = operation(some_fun);
auto op1 = operation(some_functor{});
}
Frankly though, you are better off just using lambda:
auto op0 = [a,b]() { return sum(a,b); };
auto op1 = [a,b]() { return subtract{a,b}(); }
// C++17:
auto op2 = [op=subtract{a,b}] { return op(); };