Propose to the user to rewrite functions with lambda - c++

Question

I'm doing tests to understand lambdas. I am trying to offer to the users the ability to rewrite a function directly in the main function. Let me explain:
#include <iostream>
using namespace std;
class A {
public:
virtual bool execute() {};
};
class B : public A {
public:
bool execute() { cout << "Execute in B" << endl; }
};
int main() {
B newB;
newB.execute();
newB.execute() { cout << "Execute in New B" << endl; } ;
newB.execute();
return 0;
}
This source code doesn't work because it's illegal to rewrite a function like that. What would be for you, the best way to rewrite a function like that in C ++ 14? With lambda? Without lambda?
I want do like in Javascript, overloading a function like that: newB.somefunction = function(...) { ... };. I want the function to be written in source code by a user of my Library. In a way a callback function.
my question is the following: How to write a callback function or Lambda expressions to rewrite a method outside a class/object?
Solution proposed by Exagon with variable :
#include <iostream>
#include <functional>
class B {
public:
int global=0;
std::function<void()> execute{
[](){
std::cout << "Hello World" << std::endl;
}
};
};
int main() {
B newB;
newB.execute();
newB.execute();
newB.execute = [newB](){std::cout << newB.global << " = FOOBAR\n";};
newB.execute();
return 0;
}

Answer 1

You can do this using std::function from the functional header.
Then make a std::function member and create a setter for this member.
the execute member function need to call this std::function member.
you can pass a lambda into the setter method.
here is my approach:
#include <iostream>
#include <functional>
class B {
public:
void execute() {_f();}
void setFunction(std::function<void()> f){ _f = f;}
private:
std::function<void()> _f{[](){std::cout << "Hello World" << std::endl;}};
};
int main() {
B newB;
newB.execute();
newB.execute();
newB.setFunction([](){std::cout << "FOOBAR\n";});
newB.execute();
return 0;
}
the output is:
Hello World
Hello World
FOOBAR
Since you are after something "JavaScript-like" you could do it like this:
#include <iostream>
#include <functional>
class B {
public:
std::function<void()> execute{
[](){
std::cout << "Hello World" << std::endl;
}
};
};
int main() {
B newB;
newB.execute();
newB.execute();
newB.execute = [](){std::cout << "FOOBAR\n";};
newB.execute();
return 0;
}
Which has the same output.
here is a live demo