mutable

C++ keyword enabling you to to have const member functions that DO change your class member variables.

When in a class, when you see the following member function:

//Declaration
double getValue() const;
//Definition
double MyClass::getValue() const
{
  return mValue;
}

you know its a function that does not alter the member variables or the class.

But imagine that getting this value is a very time-intensive process. Then you might want the class to also store the amount of times this function has been called. Then you have two options:

make the member function non-const
make the member variable mutable

Example using mutable

class MyClass
{
  //Stuff..
  mutable int mRequests;
  double getValue() const;
  //More stuff..
}

double MyClass::getValue() const
{
  ++mRequests; //A member variable
  return mValue;
}

Example using non-const member function

class MyClass
{
  //Stuff..
  int mRequests;
  double getValue();
  //More stuff..
}

double MyClass::getValue()
{
  ++mRequests; //A member variable
  return mValue;
}

last edited (August 4, 2005) by bilderbikkel, Number of views: 1594, Current Rev: 1

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