The latest version of this topic can be found at Copy Constructors and Copy Assignment Operators (C++).
Starting in C++11, two kinds of assignment are supported in the language: copy assignment and move assignment. In this article "assignment" means copy assignment unless explicitly stated otherwise. For information about move assignment, see Move Constructors and Move Assignment Operators (C++).
Both the assignment operation and the initialization operation cause objects to be copied.
Assignment: When one object's value is assigned to another object, the first object is copied to the second object. Therefore,
causes the value of to be copied to .
Initialization: Initialization occurs when a new object is declared, when arguments are passed to functions by value, or when values are returned from functions by value.
You can define the semantics of "copy" for objects of class type. For example, consider this code:
The preceding code could mean "copy the contents of FILE1.DAT to FILE2.DAT" or it could mean "ignore FILE2.DAT and make a second handle to FILE1.DAT." You must attach appropriate copying semantics to each class, as follows.
By using the assignment operator together with a reference to the class type as the return type and the parameter that is passed by reference—for example .
By using the copy constructor. For more information about the copy constructor, see Rules for Declaring Constructors.
If you do not declare a copy constructor, the compiler generates a member-wise copy constructor for you. If you do not declare a copy assignment operator, the compiler generates a member-wise copy assignment operator for you. Declaring a copy constructor does not suppress the compiler-generated copy assignment operator, nor vice versa. If you implement either one, we recommend that you also implement the other one so that the meaning of the code is clear.
Member-wise assignment is covered in more detail in (NOTINBUILD) Memberwise Assignment and Initialization.
The copy constructor takes an argument of type class-name&, where class-name is the name of the class for which the constructor is defined. For example:
Make the type of the copy constructor's argument const class-name& whenever possible. This prevents the copy constructor from accidentally changing the object from which it is copying. It also enables copying from const objects.
Compiler-generated copy constructors, like user-defined copy constructors, have a single argument of type "reference to class-name." An exception is when all base classes and member classes have copy constructors declared as taking a single argument of type constclass-name&. In such a case, the compiler-generated copy constructor's argument is also const.
When the argument type to the copy constructor is not const, initialization by copying a const object generates an error. The reverse is not true: If the argument is const, you can initialize by copying an object that is not const.
Compiler-generated assignment operators follow the same pattern with regard to const. They take a single argument of type class-name& unless the assignment operators in all base and member classes take arguments of type constclass-name&. In this case, the class's generated assignment operator takes a const argument.
When virtual base classes are initialized by copy constructors, compiler-generated or user-defined, they are initialized only once: at the point when they are constructed.
The implications are similar to those of the copy constructor. When the argument type is not const, assignment from a const object generates an error. The reverse is not true: If a const value is assigned to a value that is not const, the assignment succeeds.
For more information about overloaded assignment operators, see Assignment.
Special Member Functions
In the C++programming language, a copy constructor is a special constructor for creating a new objectas a copy of an existing object. Copy constructors are the standard way of copying objects in C++, as opposed to cloning, and have C++-specific nuances.
The first argument of such a constructor is a reference to an object of the same type as is being constructed (const or non-const), which might be followed by parameters of any type (all having default values).
Normally the compiler automatically creates a copy constructor for each class (known as an implicit copy constructor) but for special cases the programmer creates the copy constructor, known as a user-defined copy constructor. In such cases, the compiler does not create one. Hence, there is always one copy constructor that is either defined by the user or by the system.
A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written (see Rule of three).
Copying of objects is achieved by the use of a copy constructor and an assignment operator. A copy constructor has as its first parameter a (possibly const or volatile) reference to its own class type. It can have more arguments, but the rest must have default values associated with them. The following would be valid copy constructors for class :
The first one should be used unless there is a good reason to use one of the others. One of the differences between the first and the second is that temporaries can be copied with the first. For example:
Another difference between them is the obvious:
The form of the copy constructor is used when it is necessary to modify the copied object. This is very rare but it can be seen used in the standard library's . A reference must be provided:
The following are invalid copy constructors (Reason - is not passed as reference) :
because the call to those constructors would require a copy as well, which would result in an infinitely recursive call.
The following cases may result in a call to a copy constructor:
- When an object is returned by value
- When an object is passed (to a function) by value as an argument
- When an object is thrown
- When an object is caught
- When an object is placed in a brace-enclosed initializer list
These cases are collectively called copy-initialization and are equivalent to:
It is however, not guaranteed that a copy constructor will be called in these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example being the return value optimization (sometimes referred to as RVO).
An object can be assigned value using one of the two techniques:
- Explicit assignment in an expression
Explicit assignment in an expression
An object can be initialized by any one of the following ways.
a. Through declaration
b. Through function arguments
c. Through function return value
The copy constructor is used only for initializations, and does not apply to assignments where the assignment operator is used instead.
The implicit copy constructor of a class calls base copy constructors and copies its members by means appropriate to their type. If it is a class type, the copy constructor is called. If it is a scalar type, the built-in assignment operator is used. Finally, if it is an array, each element is copied in the manner appropriate to its type.
By using a user-defined copy constructor the programmer can define the behavior to be performed when an object is copied.
These examples illustrate how copy constructors work and why they are required sometimes.
Implicit copy constructor
Let us consider the following example:
Output10 15 10 23 15 10
As expected, timmy has been copied to the new object, timmy_clone. While timmy's age was changed, timmy_clone's age remained the same. This is because they are totally different objects.
The compiler has generated a copy constructor for us, and it could be written like this:
So, when do we really need a user-defined copy constructor? The next section will explore that question.
User-defined copy constructor
Now, consider a very simple dynamic array class like the following:
Output25 25 Segmentation fault
Since we did not specify a copy constructor, the compiler generated one for us. The generated constructor would look something like:
The problem with this constructor is that it performs a shallow copy of the data pointer. It only copies the address of the original data member; this means they both share a pointer to the same chunk of memory, which is not what we want. When the program reaches line (1), copy's destructor gets called (because objects on the stack are destroyed automatically when their scope ends). Array's destructor deletes the data array of the original, therefore when it deleted copy's data, because they share the same pointer, it also deleted first's data. Line (2) now accesses invalid data and writes to it! This produces the infamous segmentation fault.
If we write our own copy constructor that performs a deep copy then this problem goes away.
Here, we are creating a new int array and copying the contents to it. Now, other's destructor deletes only its data, and not first's data. Line (2) will not produce a segmentation fault anymore.
Instead of doing a deep copy right away, there are some optimization strategies that can be used. These allow you to safely share the same data between several objects, thus saving space. The copy-on-write strategy makes a copy of the data only when it is written to. Reference counting keeps the count of how many objects are referencing the data, and will delete it only when this count reaches zero (e.g. boost::shared_ptr).
Copy constructors and templates
Contrary to expectations, a template copy constructor is not a user-defined copy constructor. Thus it is not enough to just have:
(Note that the type of can be .) A user-defined, non-template copy constructor must also be provided for construction of Array from Array.
Bitwise copy constructor
There is no such thing as "bitwise copy constructor" in C++. However, the default generated copy constructor copies by invoking copy constructors on members, and for a raw pointer member this will copy the raw pointer (i.e. not a deep copy).
Logical copy constructor
A logical copy constructor makes a true copy of the structure as well as its dynamic structures. Logical copy constructors come into the picture mainly when there are pointers or complex objects within the object being copied.
Explicit copy constructor
An explicit copy constructor is one that is declared explicit by using the explicit keyword. For example:
It is used to prevent copying of objects at function calls or with the copy-initialization syntax.