callback in C++ struct - c++

Question

I have been trying to implement a callback function in c++. Within a class, I have a struct, a number of methods, and a method that creates an instance of the struct with one of the other methods as its argument.
The struct has many other variables, but an illustration is depicted here:
class MYCLASS
{
public:
MYCLASS();
struct TEST{
std::function<int(int)> foo;
};
int plus(int x){
return x + 1;
}
int minus(int x){
return x - 1;
}
void sim(){
TEST T; // make an instance of TEST
T.foo = plus(5); // assign TEST.foo a function (plus or minus)
T.foo(); // call the method we assigned
}
};
Within the sim method, I want to create an instance of test and give it either plus or minus, depending on some criterion. Both lines where I try and give the instance T a plus function and subsequently call it are incorrect.

Answer 1

If you want to delay the call to T.foo, then you could use a lambda like this:
T.foo = [this](int x) { return plus(x); };
T.foo(5);

Answer 2

Option - 1
If the member functions plus() and minus() are simple enough like you have shown, you can make them as lambda functions inside the struct TEST.
Since the capture-less lambdas can be stored in typed function pointers, the following will do what you want.
See live demo
#include <iostream>
class MYCLASS
{
int m_var = 5; // just for demonstration
public:
MYCLASS() = default;
struct TEST
{
using fPtrType = int(*)(int); // function pointer type
const fPtrType foo1 = [](int x) { return x + 1; }; // plus function
const fPtrType foo2 = [](int x) { return x - 1; }; // minus function
};
void sim()
{
TEST T;
std::cout << "Answer from int PLUS(int): " << T.foo1(m_var) << std::endl;
std::cout << "Answer from int MINUS(int): " << T.foo2(m_var) << std::endl;
}
};
Option - 2
If the above alter a lot in your code, use typed function pointer again for member functions and do as follows; which will avoid unnecessary copying(by capturing) the class instance to the lambda and template instantiation and other performance issues comes along with std::function as well.
See live demo
#include <iostream>
class MYCLASS
{
using fPtrType = int(MYCLASS::*)(int); // class member function pointer type
public:
MYCLASS() = default;
struct TEST { fPtrType foo = nullptr; };
int plus(int x) { return x + 1; }
int minus(int x) { return x - 1; }
void sim()
{
TEST T;
T.foo = &MYCLASS::plus; // now you can
std::cout << "Answer from int PLUS(int): " << (this->*T.MYCLASS::TEST::foo)(5) << std::endl;
//^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ syntax would be a bit ugly
// later same ptr variable for minus()
T.foo = &MYCLASS::minus;
int answer = (this->*T.MYCLASS::TEST::foo)(5);
std::cout << "Answer from int MINUS(int): " << answer << std::endl;
}
};
int main()
{
MYCLASS obj;
obj.sim();
return 0;
}
Output:
Answer from int PLUS(int): 6
Answer from int MINUS(int): 4