Friend and other identifiers

Friends can see your privates

friend keyword

Can be applied to classes or methods to allow access to private data and methods from an outside source.

Example:

     class Matrix {
       public:
         // output operator
         friend ostream& operator<< ( ostream& os, const Matrix& m );
     };

allows the output operator full access to the private methods of the Matrix class.

static keyword

A class variable that is declared static is allocated only once for all instances of the class and each instance can access the value of this variable. This variable does not get initialized with the rest of the class but is done once when the program is started.
```
     class Matrix {
       public:
         static int num;
     };

     int Matrix::num = 1;
    
```
A class method which is declared static only has access to variables which are also declared static and any data passed into it. It does not implicitly have access to other class data.

const keyword

A class variable or class defined as const cannot be directly modified without causing a compiler error. The compiler will also throw errors for any attempt to pass a const object into a non-const function

A class method defined as const is safe to operate on objects as long as it does not modify the object. The const keyword follows the function definition as in
```
         double getVal ( int row, int col ) const;
    
```