Nonstatic member as a default argument of a nonstatic member function

Question

struct X
{
   X():mem(42){}
   void f(int param = mem) //ERROR
   {
      //do something
   }
private: 
   int mem;
};

Can anyone give me just one

Answer 1

Default arguments are evaluated in two distinct steps, in different contexts.
First, the name lookup for the default argument is performed in the context of the declaration.
Secondly, the evaluation of the default argument is performed in the context of the actual function call.

To keep the implementation from becoming overly complicated, some restrictions are applied to the expressions that can be used as default arguments.

• Variables with non-static lifetime can't be used, because they might not exist at the time of the call.
• Non-static member variables can't be used, because they need an (implicit) this-> qualification, which can typically not be evaluated at the call site.