C++ Creating a function, like a variable, change its body later and then call it later

I'm trying to create a dummy function inside of a class, whichs' body will be changed later in int main(). And then I'd like to call this body changed func in the class. Is there a way to achieve this?

Something like this:

class Animation {
public:

    //Don't know what to write at the next line
    function<void>/*?*/ whenCompleted = []() mutable { /* Dummy func. */ };
    .
    .
    .
    void startAnimation() { /* Do stuff, then */ animationEnded(); }
    void animationEnded() { whenCompleted(); }

}score;


int main(){
    score.whenCompleted = { /* new body for whenCompleted() */ }
    score.startAnimation();
}