What is a mutable storage class specifier?

A mutable storage class specifier is a keyword that can be used in the declaration of a class member variable to indicate that the variable's value can be modified even if the containing class is marked as "const".

A class is a user-defined data type that can contain data members (also known as member variables or fields) and member functions (also known as methods) that operate on those data members. By default, a class's data members are considered to be constant and cannot be modified once the class is instantiated. However, sometimes it is useful to allow certain data members to be modified even if the class is marked as "const".

This is where the "mutable" keyword comes in. When a data member is declared with the "mutable" keyword, it can be modified even if the containing class is marked as "const". This allows for more flexibility in the design of a class, as it allows for certain data members to be changed while still maintaining the overall const-correctness of the class.

For example, consider a class that represents a point in 2D space. The class might have data members to represent the x and y coordinates of the point. By default, these data members would be considered constant and could not be modified after the class is instantiated. However, it might be useful to allow the x and y coordinates to be modified even if the class is marked as "const", to allow for easy manipulation of the point's position. In this case, the data members could be declared as "mutable" to allow for modification even if the class is const.

It's also worth noting that, a mutable member can only be changed from non-const member functions of the class.

In summary, a mutable storage class specifier is a keyword that allows for certain data members of a class to be modified even if the class is marked as "const". This can be useful for allowing certain data members to be changed while still maintaining the overall const-correctness of the class.