C++ Language/Std/Stl/CallableObjects/Lambda

Just like an int has a literal form (e.g., 123), a callable-object has a literal form called a "lambda". In this example, the literal is being assigned to a callable-object variable:
std::function<int(int,int)> DoMath;
DoMath = [](int x, int y) {
    return x + y;
  };

Additional information about lambdas (includes interactive examples)