When have you used C++ 'mutable' keyword? [closed]

Answer 1

I use mutable for class members that are initialized on demand, especially from a database or source external to the program. This allows the "getter" functions to create the object on demand otherwise it is a constant method.

Answer 2

I use it when locking a mutex for thread-safety. The mutex is marked as mutable so the methods that lock it can remain const.

Answer 3

In mock objects to capture the value of arguments of const functions in a member variable.

class Source
{
public:
    virtual ~Source() {}
    virtual std::string read(int count) const=0;
};

class SourceMock : public Source
{
public:
    mutable std::vector<int> arguments;
    std::string read(int count) const {
        arguments.push_back(count);
        return "...";
    }
};

//TEST....
SourceMock mock;
//...
VERIFY(mock.arguments.size()==2);
VERIFY(mock.arguments[0]==3);
//...

Answer 4

It can be used in many scenarios e.g

Loggers, timing, access counters etc.

These can be called in const-qualified accessors without changing the state of the data.

Answer 5

http://msdn.microsoft.com/en-us/library/4h2h0ktk%28v=vs.80%29.aspx would be the best example. Hey, I have learned something today!

Answer 6

My template class implements reference-counter pattern. When it is passed to functions as argument with const modifier it is possible that reference can be increased anyway. So instead of const_cast mutable can be used.