What is the copy-and-swap idiom

Question

What is this idiom and when should it be used  Which problems does it solve  Does the idiom change when C  11 is used   Although it s been mentioned in many places  we didn t have any singular  what is it  question and answer  so here it is  Here is a partial list of places where it was previously mentioned    What are your favorite C   Coding Style idioms   Copy-swap Copy constructor and   operator overload in C    is a common function possible  What is copy elision and how it optimizes copy-and-swap idiom C    dynamically allocating an array of objects

User · Answer

I would like to add a word of warning when you are dealing with C++11-style allocator-aware containers. Swapping and assignment have subtly different semantics.

For concreteness, let us consider a container std::vector<T, A>, where A is some stateful allocator type, and we'll compare the following functions:

void fs(std::vector<T, A> & a, std::vector<T, A> & b)
{ 
    a.swap(b);
    b.clear(); // not important what you do with b
}

void fm(std::vector<T, A> & a, std::vector<T, A> & b)
{
    a = std::move(b);
}

The purpose of both functions fs and fm is to give a the state that b had initially. However, there is a hidden question: What happens if a.get_allocator() != b.get_allocator()? The answer is: It depends. Let's write AT = std::allocator_traits<A>.

If AT::propagate_on_container_move_assignment is std::true_type, then fm reassigns the allocator of a with the value of b.get_allocator(), otherwise it does not, and a continues to use its original allocator. In that case, the data elements need to be swapped individually, since the storage of a and b is not compatible.
If AT::propagate_on_container_swap is std::true_type, then fs swaps both data and allocators in the expected fashion.
If AT::propagate_on_container_swap is std::false_type, then we need a dynamic check.
- If a.get_allocator() == b.get_allocator(), then the two containers use compatible storage, and swapping proceeds in the usual fashion.
- However, if a.get_allocator() != b.get_allocator(), the program has undefined behaviour (cf. [container.requirements.general/8].

The upshot is that swapping has become a non-trivial operation in C++11 as soon as your container starts supporting stateful allocators. That's a somewhat "advanced use case", but it's not entirely unlikely, since move optimizations usually only become interesting once your class manages a resource, and memory is one of the most popular resources.

User · Answer

Assignment  at its heart  is two steps  tearing down the object s old state and building its new state as a copy of some other object s state    Basically  that s what the destructor and the copy constructor do  so the first idea would be to delegate the work to them  However  since destruction mustn t fail  while construction might  we actually want to do it the other way around  first perform the constructive part and  if that succeeded  then do the destructive part  The copy-and-swap idiom is a way to do just that  It first calls a class  copy constructor to create a temporary object  then swaps its data with the temporary s  and then lets the temporary s destructor destroy the old state  Since swap   is supposed to never fail  the only part which might fail is the copy-construction  That is performed first  and if it fails  nothing will be changed in the targeted object    In its refined form  copy-and-swap is implemented by having the copy performed by initializing the  non-reference  parameter of the assignment operator    T amp  operator  T tmp        this- gt swap tmp       return  this

User · Answer

This answer is more like an addition and a slight modification to the answers above   In some versions of Visual Studio  and possibly other compilers  there is a bug that is really annoying and doesn t make sense  So if you declare define your swap function like this   friend void swap A amp  first  A amp  second         std  swap first size  second size       std  swap first arr  second arr            the compiler will yell at you when you call the swap function      This has something to do with a friend function being called and this object being passed as a parameter     A way around this is to not use friend keyword and redefine the swap function   void swap A amp  other         std  swap size  other size       std  swap arr  other arr        This time  you can just call swap and pass in other  thus making the compiler happy       After all  you don t need to use a friend function to swap 2 objects  It makes just as much sense to make swap a member function that has one other object as a parameter   You already have access to this object  so passing it in as a parameter is technically redundant

User · Answer

There are some good answers already   I ll focus mainly on what I think they lack - an explanation of the  cons  with the copy-and-swap idiom         What is the copy-and-swap idiom    A way of implementing the assignment operator in terms of a swap function   X amp  operator  X rhs        swap rhs       return  this      The fundamental idea is that    the most error-prone part of assigning to an object is ensuring any resources the new state needs are acquired  e g  memory  descriptors  that acquisition can be attempted before modifying the current state of the object  i e   this  if a copy of the new value is made  which is why rhs is accepted by value  i e  copied  rather than by reference swapping the state of the local copy rhs and  this is usually relatively easy to do without potential failure exceptions  given the local copy doesn t need any particular state afterwards  just needs state fit for the destructor to run  much as for an object being moved from in    C  11       When should it be used    Which problems does it solve   create       When you want the assigned-to objected unaffected by an assignment that throws an exception  assuming you have or can write a swap with strong exception guarantee  and ideally one that can t fail throw      When you want a clean  easy to understand  robust way to define the assignment operator in terms of  simpler  copy constructor  swap and destructor functions    Self-assignment done as a copy-and-swap avoids oft-overlooked edge cases       When any performance penalty or momentarily higher resource usage created by having an extra temporary object during the assignment is not important to your application            swap throwing  it s generally possible to reliably swap data members that the objects track by pointer  but non-pointer data members that don t have a throw-free swap  or for which swapping has to be implemented as X tmp   lhs  lhs   rhs  rhs   tmp  and copy-construction or assignment may throw  still have the potential to fail leaving some data members swapped and others not   This potential applies even to C  03 std  string s as James comments on another answer       wilhelmtell  In C  03  there is no mention of exceptions potentially thrown by std  string  swap  which is called by std  swap   In C  0x  std  string  swap is noexcept and must not throw exceptions      James McNellis Dec 22  10 at 15 24          assignment operator implementation that seems sane when assigning from a distinct object can easily fail for self-assignment   While it might seem unimaginable that client code would even attempt self-assignment  it can happen relatively easily during algo operations on containers  with x   f x   code where f is  perhaps only for some  ifdef branches  a macro ala  define f x  x or a function returning a reference to x  or even  likely inefficient but concise  code like x   c1   x   2   c2   x   2   x     For example   struct X       T  p       size t size       X amp  operator  const X amp  rhs                delete   p       OUCH          p    new T size    rhs size            std  copy p   rhs p   rhs p    rhs size                       On self-assignment  the above code delete s x p    points p  at a newly allocated heap region  then attempts to read the uninitialised data therein  Undefined Behaviour   if that doesn t do anything too weird  copy attempts a self-assignment to every just-destructed  T        The copy-and-swap idiom can introduce inefficiencies or limitations due to the use of an extra temporary  when the operator s parameter is copy-constructed    struct Client       IP Address ip address       int socket       X const X amp  rhs          ip address  rhs ip address    socket  connect rhs ip address                 Here  a hand-written Client  operator  might check if  this is already connected to the same server as rhs  perhaps sending a  reset  code if useful   whereas the copy-and-swap approach would invoke the copy-constructor which would likely be written to open a distinct socket connection then close the original one   Not only could that mean a remote network interaction instead of a simple in-process variable copy  it could run afoul of client or server limits on socket resources or connections    Of course this class has a pretty horrid interface  but that s another matter  -P

User · Answer

Overview  Why do we need the copy-and-swap idiom   Any class that manages a resource  a wrapper  like a smart pointer  needs to implement The Big Three  While the goals and implementation of the copy-constructor and destructor are straightforward  the copy-assignment operator is arguably the most nuanced and difficult  How should it be done  What pitfalls need to be avoided   The copy-and-swap idiom is the solution  and elegantly assists the assignment operator in achieving two things  avoiding code duplication  and providing a strong exception guarantee   How does it work   Conceptually  it works by using the copy-constructor s functionality to create a local copy of the data  then takes the copied data with a swap function  swapping the old data with the new data  The temporary copy then destructs  taking the old data with it  We are left with a copy of the new data   In order to use the copy-and-swap idiom  we need three things  a working copy-constructor  a working destructor  both are the basis of any wrapper  so should be complete anyway   and a swap function   A swap function is a non-throwing function that swaps two objects of a class  member for member  We might be tempted to use std  swap instead of providing our own  but this would be impossible  std  swap uses the copy-constructor and copy-assignment operator within its implementation  and we d ultimately be trying to define the assignment operator in terms of itself    Not only that  but unqualified calls to swap will use our custom swap operator  skipping over the unnecessary construction and destruction of our class that std  swap would entail      An in-depth explanation  The goal  Let s consider a concrete case  We want to manage  in an otherwise useless class  a dynamic array  We start with a working constructor  copy-constructor  and destructor    include  lt algorithm gt     std  copy  include  lt cstddef gt     std  size t  class dumb array   public          default  constructor     dumb array std  size t size   0            mSize size             mArray mSize   new int mSize      nullptr                      copy-constructor     dumb array const dumb array amp  other            mSize other mSize             mArray mSize   new int mSize    nullptr                    note that this is non-throwing  because of the data            types being used  more attention to detail with regards            to exceptions must be given in a more general case  however         std  copy other mArray  other mArray   mSize  mArray                 destructor      dumb array                 delete    mArray         private      std  size t mSize      int  mArray       This class almost manages the array successfully  but it needs operator  to work correctly   A failed solution  Here s how a naive implementation might look      the hard part dumb array amp  operator  const dumb array amp  other        if  this     amp other      1                   get rid of the old data            delete    mArray      2          mArray   nullptr      2    see footnote for rationale                 and put in the new         mSize   other mSize      3          mArray   mSize   new int mSize    nullptr      3          std  copy other mArray  other mArray   mSize  mArray       3             return  this      And we say we re finished  this now manages an array  without leaks  However  it suffers from three problems  marked sequentially in the code as  n     The first  is the self-assignment test  This check serves two purposes  it s an easy way to prevent us from running needless code on self-assignment  and it protects us from subtle bugs  such as deleting the array only to try and copy it   But in all other cases it merely serves to slow the program down  and act as noise in the code  self-assignment rarely occurs  so most of the time this check is a waste  It would be better if the operator could work properly without it  The second is that it only provides a basic exception guarantee  If new int mSize  fails   this will have been modified   Namely  the size is wrong and the data is gone   For a strong exception guarantee  it would need to be something akin to   dumb array amp  operator  const dumb array amp  other        if  this     amp other      1                   get the new data ready before we replace the old         std  size t newSize   other mSize          int  newArray   newSize   new int newSize      nullptr      3          std  copy other mArray  other mArray   newSize  newArray       3              replace the old data  all are non-throwing          delete    mArray          mSize   newSize          mArray   newArray             return  this     The code has expanded  Which leads us to the third problem  code duplication  Our assignment operator effectively duplicates all the code we ve already written elsewhere  and that s a terrible thing    In our case  the core of it is only two lines  the allocation and the copy   but with more complex resources this code bloat can be quite a hassle  We should strive to never repeat ourselves    One might wonder  if this much code is needed to manage one resource correctly  what if my class manages more than one  While this may seem to be a valid concern  and indeed it requires non-trivial try catch clauses  this is a non-issue  That s because a class should manage one resource only    A successful solution  As mentioned  the copy-and-swap idiom will fix all these issues  But right now  we have all the requirements except one  a swap function  While The Rule of Three successfully entails the existence of our copy-constructor  assignment operator  and destructor  it should really be called  The Big Three and A Half   any time your class manages a resource it also makes sense to provide a swap function   We need to add swap functionality to our class  and we do that as follows      class dumb array   public                  friend void swap dumb array amp  first  dumb array amp  second     nothrow                  enable ADL  not necessary in our case  but good practice          using std  swap              by swapping the members of two objects             the two objects are effectively swapped         swap first mSize  second mSize           swap first mArray  second mArray                           Here is the explanation why public friend swap   Now not only can we swap our dumb array s  but swaps in general can be more efficient  it merely swaps pointers and sizes  rather than allocating and copying entire arrays  Aside from this bonus in functionality and efficiency  we are now ready to implement the copy-and-swap idiom   Without further ado  our assignment operator is   dumb array amp  operator  dumb array other      1        swap  this  other       2       return  this      And that s it  With one fell swoop  all three problems are elegantly tackled at once   Why does it work   We first notice an important choice  the parameter argument is taken by-value  While one could just as easily do the following  and indeed  many naive implementations of the idiom do    dumb array amp  operator  const dumb array amp  other        dumb array temp other       swap  this  temp        return  this      We lose an important optimization opportunity  Not only that  but this choice is critical in C  11  which is discussed later   On a general note  a remarkably useful guideline is as follows  if you re going to make a copy of something in a function  let the compiler do it in the parameter list       Either way  this method of obtaining our resource is the key to eliminating code duplication  we get to use the code from the copy-constructor to make the copy  and never need to repeat any bit of it  Now that the copy is made  we are ready to swap   Observe that upon entering the function that all the new data is already allocated  copied  and ready to be used  This is what gives us a strong exception guarantee for free  we won t even enter the function if construction of the copy fails  and it s therefore not possible to alter the state of  this   What we did manually before for a strong exception guarantee  the compiler is doing for us now  how kind    At this point we are home-free  because swap is non-throwing  We swap our current data with the copied data  safely altering our state  and the old data gets put into the temporary  The old data is then released when the function returns   Where upon the parameter s scope ends and its destructor is called    Because the idiom repeats no code  we cannot introduce bugs within the operator  Note that this means we are rid of the need for a self-assignment check  allowing a single uniform implementation of operator    Additionally  we no longer have a performance penalty on non-self-assignments    And that is the copy-and-swap idiom   What about C  11   The next version of C    C  11  makes one very important change to how we manage resources  the Rule of Three is now The Rule of Four  and a half   Why  Because not only do we need to be able to copy-construct our resource  we need to move-construct it as well   Luckily for us  this is easy   class dumb array   public                     move constructor     dumb array dumb array amp  amp  other  noexcept                  dumb array      initialize via default constructor  C  11 only               swap  this  other                          What s going on here  Recall the goal of move-construction  to take the resources from another instance of the class  leaving it in a state guaranteed to be assignable and destructible   So what we ve done is simple  initialize via the default constructor  a C  11 feature   then swap with other  we know a default constructed instance of our class can safely be assigned and destructed  so we know other will be able to do the same  after swapping    Note that some compilers do not support constructor delegation  in this case  we have to manually default construct the class  This is an unfortunate but luckily trivial task    Why does that work   That is the only change we need to make to our class  so why does it work  Remember the ever-important decision we made to make the parameter a value and not a reference   dumb array amp  operator  dumb array other       1    Now  if other is being initialized with an rvalue  it will be move-constructed  Perfect  In the same way C  03 let us re-use our copy-constructor functionality by taking the argument by-value  C  11 will automatically pick the move-constructor when appropriate as well   And  of course  as mentioned in previously linked article  the copying moving of the value may simply be elided altogether    And so concludes the copy-and-swap idiom     Footnotes   Why do we set mArray to null  Because if any further code in the operator throws  the destructor of dumb array might be called  and if that happens without setting it to null  we attempt to delete memory that s already been deleted  We avoid this by setting it to null  as deleting null is a no-operation      There are other claims that we should specialize std  swap for our type  provide an in-class swap along-side a free-function swap  etc  But this is all unnecessary  any proper use of swap will be through an unqualified call  and our function will be found through ADL  One function will do      The reason is simple  once you have the resource to yourself  you may swap and or move it  C  11  anywhere it needs to be  And by making the copy in the parameter list  you maximize optimization         The move constructor should generally be noexcept  otherwise some code  e g  std  vector resizing logic  will use the copy constructor even when a move would make sense  Of course  only mark it noexcept if the code inside doesn t throw exceptions

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