#include <iostream>
#include <functional>

class Foo
{
  public:
    std::function<int(int)> doit = [](int val){return ++val;};
};

int main() {
    Foo foo;
    std::cout << foo.doit(10) << '\n';
    foo.doit = [](int val){return val*2;};
    std::cout << foo.doit(10);
}