When making class how to access private variable

[xek12345]

lets say the class has a private variable double a
and only one public function double Calculate(double b, double c) that returns
a*b*c.

I want to change the value of private variable.

How do you access private variable?

[zb0S216C]

External methods and variables cannot access private members of a given class. However, integrating two class methods that return and modify the private member is possible. An example of this is below:

class MyClass
{
    public:
        MyClass( void )
        {
             PrivateMember = 0;
        }

        int GetPrivateMember( void ) const
        {
             return PrivateMember;
        }

        void SetPrivateMember( const int New = 0 )
        {
             PrivateMember = New;
             return;
        }

    private:
        int PrivateMember;
} NewClass;

int ExternalVariable( NewClass.GetPrivateMember( ) );   // This is allowed.
int ExternalVariable( NewClass.PrivateMember );         // This is not allowed. 

I hope this helps.