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

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

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The classic example  as mentioned in other answers  and the only situation I have seen the mutable keyword used in so far  is for caching the result of a complicated Get method  where the cache is implemented as a data member of the class and not as a static variable in the method  for reasons of sharing between several functions or plain cleanliness    In general  the alternatives to using the mutable keyword are usually a static variable in the method or the const cast trick   Another detailed explanation is in here

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The mutable can be handy when you are overriding a const virtual function and want to modify your child class member variable in that function  In most of the cases you would not want to alter the interface of the base class  so you have to use mutable member variable of your own

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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

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The very keyword  mutable  is actually a reserved keyword often it is used to vary the value of constant variable If you want to have multiple values of a constsnt use the keyword mutable     Prototype  class tag name                                                      mutable var name

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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

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Mutable is used when you have a variable inside the class that is only used within that class to signal things like for example a mutex or a lock  This variable does not change the behaviour of the class  but is necessary in order to implement thread safety of the class itself  Thus if without  mutable   you would not be able to have  const  functions because this variable will need to be changed in all functions that are available to the outside world  Therefore  mutable was introduced in order to make a member variable writable even by a const function       The mutable specified informs both the compiler and the reader that it   is safe and expected that a member variable may be modified within a const   member function

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One of the best example where we use mutable is  in deep copy  in copy constructor we send const  amp 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 lt  lt   X   lt  lt  object x  lt  lt     Y    lt  lt  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

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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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In some cases  like poorly designed iterators   the class needs to keep a count or some other incidental value  that doesn t really affect the major  state  of the class   This is most often where I see mutable used   Without mutable  you d be forced to sacrifice the entire const-ness of your design   It feels like a hack most of the time to me as well   Useful in a very very few situations

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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

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mutable is mainly used on an implementation detail of the class  The user of the class doesn t need to know about it  therefore method s he thinks  should  be const can be  Your example of having a mutex be mutable is a good canonical example

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Use  mutable  when for things that are LOGICALLY stateless to the user  and thus should have  const  getters in the public class  APIs  but are NOT stateless in the underlying IMPLEMENTATION  the code in your  cpp    The cases I use it most frequently are lazy initialization of state-less  plain old data  members  Namely  it is ideal in the narrow cases when such members are expensive to either build  processor  or carry around  memory  and many users of the object will never ask for them  In that situation you want lazy construction on the back end for performance  since 90  of the objects built will never need to build them at all  yet you still need to present the correct stateless API for public consumption

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Mutable changes the meaning of const from bitwise const to logical const for the class   This means that classes with mutable members are longer be bitwise const and will no longer appear in read-only sections of the executable   Furthermore  it modifies type-checking by allowing const member functions to change mutable members without using const cast   class Logical       mutable int var   public      Logical    var 0         void set int x  const   var   x        class Bitwise       int var   public      Bitwise    var 0         void set int x  const           const cast lt Bitwise  gt  this - gt var   x            const Logical logical     Not put in read-only  const Bitwise bitwise     Likely put in read-only   int main void        logical set 5      Well defined      bitwise set 5      Undefined      See the other answers for more details but I wanted to highlight that it isn t merely for type-saftey and that it affects the compiled result

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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

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The mutable keyword is very useful when creating stubs for class test purposes  You can stub a const function and still be able to increase  mutable  counters or whatever test functionality you have added to your stub  This keeps the interface of the stubbed class intact

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mutable does exist as you infer to allow one to modify data in an otherwise constant function   The intent is that you might have a function that  does nothing  to the internal state of the object  and so you mark the function const  but you might really need to modify some of the objects state in ways that don t affect its correct functionality   The keyword may act as a hint to the compiler -- a theoretical compiler could place a constant object  such as a global  in memory that was marked read-only  The presence of mutable hints that this should not be done   Here are some valid reasons to declare and use mutable data    Thread safety  Declaring a mutable boost  mutex is perfectly reasonable  Statistics  Counting the number of calls to a function  given some or all of its arguments  Memoization  Computing some expensive answer  and then storing it for future reference rather than recomputing it again

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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 lt string  string gt  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

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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

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

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