Question

I know Thinking in C++ by Bruce Eckel is not a reference book but I found a strange paragraph and I don’t understand if it’s still applicable today:

Making a structure nested doesn’t automatically give it access to
private members. To accomplish this, you must follow a particular
form: first, declare (without defining) the nested structure, then
declare it as a friend, and finally define the structure. The
structure definition must be separate from the friend declaration,
otherwise it would be seen by the compiler as a non-member.

I actually tried this without declaring the nested structure as a friend and it worked:

struct myStruct{
private:
    int bar;
public:
    struct nestedStruct{
        void foo(myStruct *);
    }a;
};

void myStruct::nestedStruct::foo(myStruct * p){
    p->bar = 20;
}

Is there still a need to declare a nested structure friend in order to modify the private members of the base class?

Answer 1

That quote is wrong. A nested inner class-type has access to all members (including private) of the enclosing class-type.

This was not the case in C++98, and your edition probably refers to that version of the standard. In C++03 and C++11 the quote doesn’t apply.

11.7 Nested classes [class.access.nest]

1 A nested class is a member and as such has the same access rights as any other member. The members of
an enclosing class have no special access to members of a nested class; the usual access rules (Clause 11)
shall be obeyed.