C++ How affects mutable keyword to the performance of container?

Question

I want to know how mutable affects a container (map, vector, list, ...). In addition, what do I have to bear in mind?

Answer 1

mutable , like const , is just a compile-time thing. It just allows you to modify that variable in a constant context. At runtime, there is no difference wether you declared the container mutable or not.

class Foo{
  mutable int i;

public:
  void foo() const{
    // constant context, but you can modify `i`
    i = 5;
  }
};