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 <algorithm> // std::copy
#include <cstddef> // 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& 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& operator=(const dumb_array& other)
{
if (this != &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& operator=(const dumb_array& other)
{
if (this != &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& first, dumb_array& 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& 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& operator=(const dumb_array& 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&& 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& 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.
Introduction
C++ treats variables of user-defined types with value semantics.
This means that objects are implicitly copied in various contexts,
and we should understand what "copying an object" actually means.
Let us consider a simple example:
class person
{
std::string name;
int age;
public:
person(const std::string& name, int age) : name(name), age(age)
{
}
};
int main()
{
person a("Bjarne Stroustrup", 60);
person b(a); // What happens here?
b = a; // And here?
}
(If you are puzzled by the name(name), age(age)
part,
this is called a member initializer list.)
Special member functions
What does it mean to copy a person
object?
The main
function shows two distinct copying scenarios.
The initialization person b(a);
is performed by the copy constructor.
Its job is to construct a fresh object based on the state of an existing object.
The assignment b = a
is performed by the copy assignment operator.
Its job is generally a little more complicated
because the target object is already in some valid state that needs to be dealt with.
Since we declared neither the copy constructor nor the assignment operator (nor the destructor) ourselves,
these are implicitly defined for us. Quote from the standard:
The [...] copy constructor and copy assignment operator, [...] and destructor are special member functions.
[ Note: The implementation will implicitly declare these member functions
for some class types when the program does not explicitly declare them.
The implementation will implicitly define them if they are used. [...] end note ]
[n3126.pdf section 12 §1]
By default, copying an object means copying its members:
The implicitly-defined copy constructor for a non-union class X performs a memberwise copy of its subobjects.
[n3126.pdf section 12.8 §16]
The implicitly-defined copy assignment operator for a non-union class X performs memberwise copy assignment
of its subobjects.
[n3126.pdf section 12.8 §30]
Implicit definitions
The implicitly-defined special member functions for person
look like this:
// 1. copy constructor
person(const person& that) : name(that.name), age(that.age)
{
}
// 2. copy assignment operator
person& operator=(const person& that)
{
name = that.name;
age = that.age;
return *this;
}
// 3. destructor
~person()
{
}
Memberwise copying is exactly what we want in this case:
name
and age
are copied, so we get a self-contained, independent person
object.
The implicitly-defined destructor is always empty.
This is also fine in this case since we did not acquire any resources in the constructor.
The members' destructors are implicitly called after the person
destructor is finished:
After executing the body of the destructor and destroying any automatic objects allocated within the body,
a destructor for class X calls the destructors for X's direct [...] members
[n3126.pdf 12.4 §6]
Managing resources
So when should we declare those special member functions explicitly?
When our class manages a resource, that is,
when an object of the class is responsible for that resource.
That usually means the resource is acquired in the constructor
(or passed into the constructor) and released in the destructor.
Let us go back in time to pre-standard C++.
There was no such thing as std::string
, and programmers were in love with pointers.
The person
class might have looked like this:
class person
{
char* name;
int age;
public:
// the constructor acquires a resource:
// in this case, dynamic memory obtained via new[]
person(const char* the_name, int the_age)
{
name = new char[strlen(the_name) + 1];
strcpy(name, the_name);
age = the_age;
}
// the destructor must release this resource via delete[]
~person()
{
delete[] name;
}
};
Even today, people still write classes in this style and get into trouble:
"I pushed a person into a vector and now I get crazy memory errors!"
Remember that by default, copying an object means copying its members,
but copying the name
member merely copies a pointer, not the character array it points to!
This has several unpleasant effects:
- Changes via
a
can be observed via b
.
- Once
b
is destroyed, a.name
is a dangling pointer.
- If
a
is destroyed, deleting the dangling pointer yields undefined behavior.
- Since the assignment does not take into account what
name
pointed to before the assignment,
sooner or later you will get memory leaks all over the place.
Explicit definitions
Since memberwise copying does not have the desired effect, we must define the copy constructor and the copy assignment operator explicitly to make deep copies of the character array:
// 1. copy constructor
person(const person& that)
{
name = new char[strlen(that.name) + 1];
strcpy(name, that.name);
age = that.age;
}
// 2. copy assignment operator
person& operator=(const person& that)
{
if (this != &that)
{
delete[] name;
// This is a dangerous point in the flow of execution!
// We have temporarily invalidated the class invariants,
// and the next statement might throw an exception,
// leaving the object in an invalid state :(
name = new char[strlen(that.name) + 1];
strcpy(name, that.name);
age = that.age;
}
return *this;
}
Note the difference between initialization and assignment:
we must tear down the old state before assigning it to name
to prevent memory leaks.
Also, we have to protect against the self-assignment of the form x = x
.
Without that check, delete[] name
would delete the array containing the source string,
because when you write x = x
, both this->name
and that.name
contain the same pointer.
Exception safety
Unfortunately, this solution will fail if new char[...]
throws an exception due to memory exhaustion.
One possible solution is to introduce a local variable and reorder the statements:
// 2. copy assignment operator
person& operator=(const person& that)
{
char* local_name = new char[strlen(that.name) + 1];
// If the above statement throws,
// the object is still in the same state as before.
// None of the following statements will throw an exception :)
strcpy(local_name, that.name);
delete[] name;
name = local_name;
age = that.age;
return *this;
}
This also takes care of self-assignment without an explicit check.
An even more robust solution to this problem is the copy-and-swap idiom,
but I will not go into the details of exception safety here.
I only mentioned exceptions to make the following point: Writing classes that manage resources is hard.
Noncopyable resources
Some resources cannot or should not be copied, such as file handles or mutexes.
In that case, simply declare the copy constructor and copy assignment operator as private
without giving a definition:
private:
person(const person& that);
person& operator=(const person& that);
Alternatively, you can inherit from boost::noncopyable
or declare them as deleted (in C++11 and above):
person(const person& that) = delete;
person& operator=(const person& that) = delete;
The rule of three
Sometimes you need to implement a class that manages a resource.
(Never manage multiple resources in a single class,
this will only lead to pain.)
In that case, remember the rule of three:
If you need to explicitly declare either the destructor,
copy constructor or copy assignment operator yourself,
you probably need to explicitly declare all three of them.
(Unfortunately, this "rule" is not enforced by the C++ standard or any compiler I am aware of.)
The rule of five
From C++11 on, an object has 2 extra special member functions: the move constructor and move assignment. The rule of five states to implement these functions as well.
An example with the signatures:
class person
{
std::string name;
int age;
public:
person(const std::string& name, int age); // Ctor
person(const person &) = default; // 1/5: Copy Ctor
person(person &&) noexcept = default; // 4/5: Move Ctor
person& operator=(const person &) = default; // 2/5: Copy Assignment
person& operator=(person &&) noexcept = default; // 5/5: Move Assignment
~person() noexcept = default; // 3/5: Dtor
};
The rule of zero
The rule of 3/5 is also referred to as the rule of 0/3/5. The zero part of the rule states that you are allowed to not write any of the special member functions when creating your class.
Advice
Most of the time, you do not need to manage a resource yourself,
because an existing class such as std::string
already does it for you.
Just compare the simple code using a std::string
member
to the convoluted and error-prone alternative using a char*
and you should be convinced.
As long as you stay away from raw pointer members, the rule of three is unlikely to concern your own code.
Best Answer
“The Rule of The Big Four (and a half)" states that if you implement one of
then you must have a policy about the others.
default constructor (which could be private)
copy constructor (deep copy of your resource. Here you have real code to handle your resource)
move constructor (using default constructor and swap) :
assignment operator (using constructor and swap)
destructor (release your resources)
friend swap (doesn't have default implementation :/ you should probably want to swap each member). This one is important contrary to the swap member method:
std::swap
uses move (or copy) constructor, which would lead to infinite recursion.From previous article:
so the
swap
method.The warning I already wrote is about to write the correct swap to avoid the infinite recursion.