Does the 'mutable' keyword have any purpose other than allowing the variable to be modified by a const function?


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  
    mutable bool done_;  
    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.

12/9/2012 4:15:58 PM

Accepted Answer

It allows the differentiation of bitwise const and logical const. Logical const is when an object doesn't change in a way that is visible through the public interface, like your locking example. Another example would be a class that computes a value the first time it is requested, and caches the result.

Since c++11 mutable can be used on a lambda to denote that things captured by value are modifiable (they aren't by default):

int x = 0;
auto f1 = [=]() mutable {x = 42;};  // OK
auto f2 = [=]()         {x = 42;};  // Error: a by-value capture cannot be modified in a non-mutable lambda
4/6/2013 2:10:53 PM

The mutable keyword is a way to pierce the const veil you drape over your objects. If you have a const reference or pointer to an object, you cannot modify that object in any way except when and how it is marked mutable.

With your const reference or pointer you are constrained to:

  • only read access for any visible data members
  • permission to call only methods that are marked as const.

The mutable exception makes it so you can now write or set data members that are marked mutable. That's the only externally visible difference.

Internally those const methods that are visible to you can also write to data members that are marked mutable. Essentially the const veil is pierced comprehensively. It is completely up to the API designer to ensure that mutable doesn't destroy the const concept and is only used in useful special cases. The mutable keyword helps because it clearly marks data members that are subject to these special cases.

In practice you can use const obsessively throughout your codebase (you essentially want to "infect" your codebase with the const "disease"). In this world pointers and references are const with very few exceptions, yielding code that is easier to reason about and understand. For a interesting digression look up "referential transparency".

Without the mutable keyword you will eventually be forced to use const_cast to handle the various useful special cases it allows (caching, ref counting, debug data, etc.). Unfortunately const_cast is significantly more destructive than mutable because it forces the API client to destroy the const protection of the objects (s)he is using. Additionally it causes widespread const destruction: const_casting a const pointer or reference allows unfettered write and method calling access to visible members. In contrast mutable requires the API designer to exercise fine grained control over the const exceptions, and usually these exceptions are hidden in const methods operating on private data.

(N.B. I refer to to data and method visibility a few times. I'm talking about members marked as public vs. private or protected which is a totally different type of object protection discussed here.)

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