A mutator is a function or method that changes the state of the object it belongs to; this is in contrast to an "accessor," which only "reads" the state of the object it belongs to.

In object-oriented programs, the program's data typically comprises "objects" that represent things or concepts in the real world. Objects not only have data, sometimes called "state" or "identity"; but they also have behavior, or things that they can do, either on their own data or data given to them by other objects. An object that represents an egg-timer might contain data about how long to boil an egg, and it might have a function to ring a bell once the egg has been boiling for long enough.

When a function that is associated with an object actually changes some of the data in the object, the function is referred to as a "mutator function" because it quite literally "mutates" the object into a different state.

As an example, consider a simple C++ eggTimer class:

• class eggTimer {
• public:
• eggTimer() : _hasBeenSet(false), _timeVal(0) {} // constructor
• void setTimer(int t) { _timeVal = t;} // mutator
• int getTimer() const { _hasBeenSet = true; return _timeVal;} // accessor
• private:
• int _timeVal;
• mutable bool _hasBeenSet; // can be changed in a const function
• };

In the above code, the function setTimer() is a mutator function because it changes the data in the eggTimer object by setting its _timeVal member to a given value. But the setTimer() function is an accessor function because it does not change anything about the object.

To make the compiler enforce this, getTimer() has been declared as "const," which means that if a programmer mistakenly did something in writing the function that did change the object, the compiler would detect this and issue an error.

The interesting thing to note is that even though getTimer() has been declared "const," the member variable _hasBeenSet is being changed. This is allowed because _hasBeenSet has been declared as "mutable," which tells the compiler that it is allowed to change the value of _hasBeenSet in a "const" function.