Does the 'mutable' keyword have any purpos

Question

A while ago I came across some code that marked a member variable of a class with the mutable keyword. As far as I can see it simply allows you to modify a variable in a const method:

class Foo  
{  
private:  
    mutable bool done_;  
public:  
    void doSomething() const { ...; done_ = true; }  
};

Is this the only use of this keyword or is there more to it than meets the eye? I have since used this technique in a class, marking a boost::mutex as mutable allowing const functions to lock it for thread-safety reasons, but, to be honest, it feels like a bit of a hack.

Answer 1

Mutable is for marking specific attribute as modifiable from within const methods. That is its only purpose. Think carefully before using it, because your code will probably be cleaner and more readable if you change the design rather than use mutable.

http://www.highprogrammer.com/alan/rants/mutable.html

So if the above madness isn't what mutable is for, what is it for? Here's the subtle case: mutable is for the case where an object is logically constant, but in practice needs to change. These cases are few and far between, but they exist.

Examples the author gives include caching and temporary debugging variables.

Answer 2

It's useful in situations where you have hidden internal state such as a cache. For example:

class HashTable
{
...
public:
    string lookup(string key) const
    {
        if(key == lastKey)
            return lastValue;

        string value = lookupInternal(key);

        lastKey = key;
        lastValue = value;

        return value;
    }

private:
    mutable string lastKey, lastValue;
};

And then you can have a const HashTable object still use its lookup() method, which modifies the internal cache.

Answer 3

Well, yeah, that's what it does. I use it for members that are modified by methods that do not logically change the state of a class - for instance, to speed up lookups by implementing a cache:

class CIniWrapper
{
public:
   CIniWrapper(LPCTSTR szIniFile);

   // non-const: logically modifies the state of the object
   void SetValue(LPCTSTR szName, LPCTSTR szValue);

   // const: does not logically change the object
   LPCTSTR GetValue(LPCTSTR szName, LPCTSTR szDefaultValue) const;

   // ...

private:
   // cache, avoids going to disk when a named value is retrieved multiple times
   // does not logically change the public interface, so declared mutable
   // so that it can be used by the const GetValue() method
   mutable std::map<string, string> m_mapNameToValue;
};

Now, you must use this with care - concurrency issues are a big concern, as a caller might assume that they are thread safe if only using const methods. And of course, modifying mutable data shouldn't change the behavior of the object in any significant fashion, something that could be violated by the example i gave if, for instance, it was expected that changes written to disk would be immediately visible to the app.

Answer 4

Your use of it isn't a hack, though like many things in C++, mutable can be hack for a lazy programmer who doesn't want to go all the way back and mark something that shouldn't be const as non-const.

Answer 5

Your use with boost::mutex is exactly what this keyword is intended for. Another use is for internal result caching to speed access.

Basically, 'mutable' applies to any class attribute that does not affect the externally visible state of the object.

In the sample code in your question, mutable might be inappropriate if the value of done_ affects external state, it depends on what is in the ...; part.

Answer 6

One of the best example where we use mutable is, in deep copy. in copy constructor we send const &obj as argument. So the new object created will be of constant type. If we want to change (mostly we won't change, in rare case we may change) the members in this newly created const object we need to declare it as mutable.

mutable storage class can be used only on non static non const data member of a class. Mutable data member of a class can be modified even if it's part of an object which is declared as const.

class Test
{
public:
    Test(): x(1), y(1) {};
    mutable int x;
    int y;
};

int main()
{
    const Test object;
    object.x = 123;
    //object.y = 123;
    /* 
    * The above line if uncommented, will create compilation error.
    */   

    cout<< "X:"<< object.x << ", Y:" << object.y;
    return 0;
}

Output:-
X:123, Y:1

In the above example, we are able to change the value of member variable x though it's part of an object which is declared as const. This is because the variable x is declared as mutable. But if you try to modify the value of member variable y, compiler will throw an error.