What is a C plus plus program to demonstrate the use of access specifiers?

Access specifiers apply to class and struct data types only. If a member is declared before an access specifier is declared, the default access is implied. Once an access specifier is declared, that specifier remains in force until another specifier is declared. Specifiers can be declared in any order and may be repeated as often as required.

The following demonstrates usage and purpose of each specifier.

class X {

friend void f(); // Friends can be declared anywhere.

private: // The default access specifier for class types (implied if omitted).

int a; // Only accessible to members of X and to friends of X.

protected:

int b; // Same as private, also accessible to derivatives of X.

public:

int c; // Accessible to any code where X is visible.

};

struct Y { friend void f(); // Friends can be declared anywhere.

public: // The default access specifier for struct types (implied if omitted).

int a; // Accessible to any code where Y is visible.

protected:

int b; // Same as private, also accessible to derivatives of Y.

private:

int c; // Only accessible to members of Y and friends of Y.

};

struct Z : X

{};

void f()

{

X x;

x.a = 42; // OK! X::a is private and f is a friend of X.

x.b = 42; // OK! X::b is protected and f is a friend of X.

x.c = 42; // OK! X::c is public and X is visible to f.

Y y;

y.a = 42; // OK! Y::a is public and Y is visible to f.

y.b = 42; // OK! Y::b is protected and f is a friend of Y.

y.c = 42; // OK! Y::c is private and f is a friend of Y.

Z z;

z.a = 42; // OK! Z::Y::a is public and Z is visible to f.

z.b = 42; // OK! Z::Y::b is protected and f is a friend of Y.

z.c = 42; // OK! Z::Y::c is private and f is a friend of Y.

}

int main()

{

X x;

x.a = 42; // error! X::a is private and main is not a friend of X.

x.b = 42; // error! X::b is protected and main does not derive from X.

x.c = 42; // OK! X::c is public and is X is visible to main.

Y y;

y.a = 42; // OK! Y::a is public and is Y is visible to main.

y.b = 42; // error! Y::b is protected and main does not derive from Y.

y.c = 42; // error! Y::c is private and main is not a friend of Y.

Z z;

z.a = 42; // OK! Z::Y::a is public and Z is visible to main.

z.b = 42; // error! Z::Y::b is protected and main is not derived from Y.

z.c = 42; // error! Z::Y::c is private and main is not a friend of Y.

}