What is std::function and how does it relate to lambdas?

std::function is a type-erased, general-purpose polymorphic wrapper that can hold any callable (lambda, function pointer, functor) matching a given signature. It's useful for storing lambdas in containers or class members where the exact closure type can't be named, at the cost of some overhead versus auto or templates.

std::function<int(int)> f = [](int x){ return x*x; };
std::vector<std::function<void()>> callbacks;