Function Objects

At first, don’t call them functors. That’s a well-defined term from the category theory.

Function objects are objects that behave like functions. They achieve this due to their call operator being implemented. As functions 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 only the name of the function object (Square) and not the instance of function object (Square()) is used in an algorithm: std::for_each(myVec.begin(), myVec.end(), Square). That’s of course an error. You have to use the instance: std::for_each(myVec.begin(), myVec.end(), Square())

Predefined Function Objects...