What are the advantages of using “mutable” in C++?

The mutable keyword in C++ is used to modify class member variables, allowing them to be changed in const member functions. Its main advantages include:

1. Flexibility: The mutable keyword allows modifying class member variables in const member functions, which is very useful for situations where internal state changes need to occur in const member functions. For example, if a class has an internal cache that needs to be updated in a const member function, it can be declared as mutable.
2. Simplify code: Using the ‘mutable’ keyword can simplify code and avoid creating many similar but different member functions. Without using ‘mutable’, you may need to add a non-const member function in each const member function to modify member variables.
3. Improving readability: using the mutable keyword can explicitly indicate the intention to modify member variables in const member functions, which can enhance the readability of the code and make it easier for others to understand and maintain.

It is important to note that while the mutable keyword allows for the modification of member variables in const member functions, it should be used cautiously. In well-designed classes, const member functions should not modify the class’s state. Therefore, using mutable should be an exception rather than the norm.