How to pass lamda as a function pointer argument of member function? - c++

Question

I learned how to pass member functions to another member function as a function pointer argument.
Now, I'm trying to pass lamda as a function pointer argument of member function.
My Code:
#include <iostream>
using namespace std;
class Test
{
public:
int add(int a, int b)
{
return a + b;
}
int sub(int a, int b)
{
return a - b;
}
typedef int (Test::*funcPtr)(int a, int b);
int myFunc(funcPtr func, int a, int b)
{
return (this->*func)(a, b);
}
void setup()
{
cout << myFunc(&Test::add, 5, 3) << endl;
cout << myFunc(&Test::sub, 5, 3) << endl;
cout << myFunc([](int a, int b) {return a * b;}, 5, 3) << endl; //ERROR!!!
}
};
int main()
{
Test test;
test.setup();
}
Result:
Error: : No viable conversion from lambda to 'Test::funcPtr' (aka 'int
(Test::*)(int, int)')
Expected Result:
8
2
15
How should I correct my code so I can get the expected result?

Answer 1

one option is making your functions static and then use std::function as type:
using funcType = std::function<int(int, int)>;
int myFunc(funcType func, int a, int b)
{
return func(a, b);
}
void setup()
{
cout << myFunc(Test::add, 5, 3) << endl;
cout << myFunc(Test::sub, 5, 3) << endl;
cout << myFunc([](int a, int b) {return a * b;}, 5, 3) << endl;
}
Live
Thanks to #holyBlackCat the other option is using regular function pointer(member functions need to be static):
typedef int (*funcPtr)(int a, int b);
//or:
//using funcPtr = int (*)(int a, int b);
int myFunc(funcPtr func, int a, int b)
{
return (*func)(a, b);
}
and also template:
template<typename funcType>
int myFunc(funcType func, int a, int b)
{
return func(a, b);
}
void setup()
{
cout << myFunc(Test::add, 5, 3) << endl;
cout << myFunc(Test::sub, 5, 3) << endl;
cout << myFunc([](int a, int b) {return a * b;}, 5, 3) << endl;
}
regular function pointer live, template live
EDIT
The examples provided above are working only with static member function. To invoke non-static member functions you can use pointer to member function type
using funcPtr = int(Test::*)(int a, int b);
int myFunc(funcPtr func, int a, int b)
{
return invoke(func, this, a, b);
}
//..
// call:
cout << myFunc(&Test::add, 5, 3) << endl;
pointer to non-static member function live