Nested Classes

Nested class works exactly the same as any other class, similar to declared outside the outer class.

class Outer {
public:
    class Inner {
    public:
        void hello();
    };
};

void Outer::Inner::hello() {
    // ....
}

Outer::Inner obj;
obj.hello();

Access rules

None of the classes can access other's members directly.

class Outer {
public:
    int x;
    
    class Inner {
    public:
        void f() {
            x = 10;  // ❌ ERROR
        }
    };
};

class Outer {
public:
    class Inner {
    public:
        int y = 5;
    };

    void f() {
        y = 10; // ❌ ERROR
    }
};

It needs object of the class, to access the members.

class Outer {
public:
    int x = 10;    
    class Inner {
    public:
        void f() {
            Outer o;
            o.x = 20; // ✅ allowed if access permits
        }
    };
};

class Outer {
public:
    class Inner {
    public:
        int y;
    };

    void f() {
        Inner i;
        i.y = 20;   // ✅ allowed if access permits
    }
};

Static Members

Both (Outer and Inner class) can directly access the static members of each other.

class Outer {
public:
    static int s;

    class Inner {
    public:
        static void f() {
            Outer::s = 10;   // ✅ direct access, no Outer object
        }
    };
};

int Outer::s = 0;

class Outer {
public:
    class Inner {
    public:
        static int y;
    };

    static void f() {
        Inner::y = 7;   // ✅ direct access, no Inner object
    }
};

int Outer::Inner::y = 0;

Access specifiers of nested classes

A nested class itself can be public, protected, or private.

class Outer {
private:
    class Secret {};
public:
    class PublicOne {};
};

Usage:

Outer::Secret s; // ❌ private
Outer::PublicOne p; // ✅

Implicitly Static Nested Class

In C++, nested classes behave like static members of the outer class, even though you never write the word static.

Meaning:
- They do not need an instance of Outer
- They do not carry outer object state

class Outer {
public:
    class Inner {
    public:
        void f() {}
    };
};

Outer::Inner obj;
obj.f();

Inner is not attached to any Outer object.

In case of Java language:

Outer outer = new Outer();
Outer.Inner inner = outer.new Inner();

Last modified: 08 February 2026