Can inner classes access private variables?

问题:

class Outer {

    class Inner {
    public:
        Inner() {}
        void func() ;
    };

private:
    static const char* const MYCONST;
    int var;
};

void Outer::Inner::func() {
    var = 1;
}

const char* const Outer::MYCONST = "myconst";

This errors out when I compile with class Outer::Inner' has no member named `var'

回答1:

An inner class is a friend of the class it is defined within.
So, yes; an object of type Outer::Inner can access the member variable var of an object of type Outer.

Unlike Java though, there is no correlation between an object of type Outer::Inner and an object of the parent class. You have to make the parent child relationship manually.

#include <string>
#include <iostream>

class Outer
{
    class Inner
    {
        public:
            Inner(Outer& x): parent(x) {}
            void func()
            {
                std::string a = "myconst1";
                std::cout << parent.var << std::endl;

                if (a == MYCONST)
                {   std::cout << "string same" << std::endl;
                }
                else
                {   std::cout << "string not same" << std::endl;
                }
            }
        private:
            Outer&  parent;
    };

    public:
        Outer()
            :i(*this)
            ,var(4)
        {}
        Outer(Outer& other)
            :i(other)
            ,var(22)
        {}
        void func()
        {
            i.func();
        }
    private:
        static const char* const MYCONST;
        Inner i;
        int var;
};

const char* const Outer::MYCONST = "myconst";

int main()
{

    Outer           o1;
    Outer           o2(o1);
    o1.func();
    o2.func();
}

回答2:

An inner class has access to all members of the outer class, but it does not have an implicit reference to a parent class instance (unlike some weirdness with Java). So if you pass a reference to the outer class to the inner class, it can reference anything in the outer class instance.

回答3:

Anything that is part of Outer should have access to all of Outer's members, public or private.

Edit: your compiler is correct, var is not a member of Inner. But if you have a reference or pointer to an instance of Outer, it could access that.

回答4:

var is not a member of inner class.

To access var, a pointer or reference to an outer class instance should be used. e.g. pOuter->var will work if the inner class is a friend of outer, or, var is public, if one follows C++ standard strictly.

Some compilers treat inner classes as the friend of the outer, but some may not. See this document for IBM compiler:

"A nested class is declared within the scope of another class. The name of a nested class is local to its enclosing class. Unless you use explicit pointers, references, or object names, declarations in a nested class can only use visible constructs, including type names, static members, and enumerators from the enclosing class and global variables.

Member functions of a nested class follow regular access rules and have no special access privileges to members of their enclosing classes. Member functions of the enclosing class have no special access to members of a nested class."