Function Objects

First of all, don’t call them functors. That’s a well-defined term from a branch of mathematics called category theory.

Function objects are objects that behave like functions. They achieve this by implementing the function call operator. As function objects are objects, they can have attributes and, therefore, state.

struct Square{
  void operator()(int& i){i= i*i;}
};

std::vector<int> myVec{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

std::for_each(myVec.begin(), myVec.end(), Square());

for (auto v: myVec) std::cout << v << " "; // 1 4 9 16 25 36 49 64 81 100

Instantiate function objects to use them

It’s a common error that the name of the function object (Square) is used in an algorithm instead of the instance of function object (Square()) itself: std::for_each(myVec.begin(), myVec.end(), Square). Of course, that’s a typical error. You have to use the instance: std::for_each(myVec.begin(), myVec.end...