Mutable Data Members (C++)
This keyword can only be applied to non-static, non-const, and non-reference data members of a class. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function.
Syntax
mutable member-variable-declaration;
Remarks
For example, the following code will compile without error because m_accessCount has been declared to be mutable, and therefore can be modified by GetFlag even though GetFlag is a const member function.
// mutable.cpp
class X
{
public:
bool GetFlag() const
{
m_accessCount++;
return m_flag;
}
private:
bool m_flag;
mutable int m_accessCount;
};