Lambda expressions

In C++, the regular function syntax is extended with the concept of a callable, short for callable entity—a callable is something that can be called in the same way as a function. Some examples of callables are functions (of course), function pointers, or objects with the operator(), also known as functors:

void f(int i); struct G {
  void operator()(int i);
};
f(5);            // Function
G g; g(5);        // Functor

It is often useful to define a callable entity in a local context, right next to the place it is used. For example, to sort a sequence of objects, we may want to define a custom comparison function. We can use an ordinary function for this:

bool compare(int i, int j) { return i < j; }
void do_work() {
  std::vector<int> v;
  .....
  std::sort(v.begin(), v.end(), compare);
}

However, in...