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Wednesday, June 2, 2010

C++ Copy constructor

OOP: Copy Constructors

When copies of objects are made
A copy constructor is called whenever a new variable is created from an object. This happens in the following cases (but not in assignment).

A variable is declared which is initialized from another object, eg,
Person q("Mickey"); // constructor is used to build q.
Person r(p);        // copy constructor is used to build r.
Person p = q;       // copy constructor is used to initialize in declaration.
p = q;              // Assignment operator, no constructor or copy constructor.
A value parameter is initialized from its corresponding argument.
f(p);               // copy constructor initializes formal value parameter.
An object is returned by a function.
C++ calls a copy constructor to make a copy of an object in each of the above cases. If there is no copy constructor defined for the class, C++ uses the default copy constructor which copies each field, ie, makes a shallow copy.

Don't write a copy constructor if shallow copies are ok
If the object has no pointers to dynamically allocated memory, a shallow copy is probably sufficient. Therefore the default copy constructor, default assignment operator, and default destructor are ok and you don't need to write your own.

If you need a copy constructor, you also need a destructor and operator=
If you need a copy constructor, it's because you need something like a deep copy, or some other management of resources. Thus is is almost certain that you will need a destructor and override the assignment operator.

Copy constructor syntax
The copy constructor takes a reference to a const parameter. It is const to guarantee that the copy constructor doesn't change it, and it is a reference because a value parameter would require making a copy, which would invoke the copy constructor, which would make a copy of its parameter, which would invoke the copy constructor, which ...

Here is an example of a copy constructor for the Point class, which doesn't really need one because the default copy constructor's action of copying fields would work fine, but it shows how it works.

//=== file Point.h =============================================
class Point {
        . . .
        Point(const Point& p);   // copy constructor
        . . .
//=== file Point.cpp ==========================================
. . .
Point::Point(const Point& p) {
    x = p.x;
    y = p.y;
    . . .
//=== file my_program.cpp ====================================
. . .
Point p;            // calls default constructor
Point s = p;        // calls copy constructor.
p = s;              // assignment, not copy constructor.
Difference between copy constructor and assignment
A copy constructor is used to initialize a newly declared variable from an existing variable. This makes a deep copy like assignment, but it is somewhat simpler:

There is no need to test to see if it is being initialized from itself.
There is no need to clean up (eg, delete) an existing value (there is none).
A reference to itself is not returned.
Unaddressed issue
[Question: When is the base (parent) class copy constructor called? Is it like Java and the parent constructor is called at the beginning of each constructor? Does it have to be explicit?]

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