Copy Constructors in VS .NET

Creator QR in VS .NET Copy Constructors

One of the more important forms of an overloaded constructor is the copy constructor Defining a copy constructor can help you prevent problems that might occur when one object is used to initialize another Let's begin by restating the problem that the copy constructor is designed to solve By default, when one object is used to initialize another, C++ performs a bitwise copy That is, an identical copy of the initializing object is created in the target object Although this is perfectly adequate for many cases and generally exactly what you want to happen there are situations in which a bitwise copy should not be used One of the most common is when an object allocates memory when it is created For example, assume a class called MyClass that allocates memory for each object when it is created, and an object A of that class This means that A has already allocated its memory Further, assume that A is used to initialize B, as shown here: MyClass B = A; If a bitwise copy is performed, then B will be an exact copy of A This means that B will be using the same piece of allocated memory that A is using, instead of allocating its own Clearly, this is not the desired outcome For example, if MyClass includes a destructor that frees the memory, then the same piece of memory will be freed twice when A and B are destroyed! The same type of problem can occur in two additional ways: first, when a copy of an object is made when it is passed as an argument to a function; second, when a temporary object is created as a return value from a function Remember, temporary

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objects are automatically created to hold the return value of a function and they may also be created in certain other circumstances To solve the type of problem just described, C++ allows you to create a copy constructor, which the compiler uses when one object initializes another Thus, your copy constructor bypasses the default bitwise copy The most common general form of a copy constructor is classname (const classname &o) { // body of constructor } Here, o is a reference to the object on the right side of the initialization It is permissible for a copy constructor to have additional parameters as long as they have default arguments defined for them However, in all cases the first parameter must be a reference to the object doing the initializing It is important to understand that C++ defines two distinct types of situations in which the value of one object is given to another The first is assignment The second is initialization, which can occur any of three ways: When one object explicitly initializes another, such as in a declaration When a copy of an object is made to be passed to a function When a temporary object is generated (most commonly, as a return value) The copy constructor applies only to initializations For example, assuming a class called myclass, and that y is an object of type myclass, each of the following statements involves initialization

myclass x = y; // y explicitly initializing x func(y); // y passed as a parameter y = func(); // y receiving a temporary, return object

Following is an example where an explicit copy constructor is needed This program creates a very limited "safe" integer array type that prevents array boundaries from being overrun ( 15 shows a better way to create a safe array that uses overloaded operators) Storage for each array is allocated by the use of new, and a pointer to the memory is maintained within each array object

/* This program creates a "safe" array class Since space for the array is allocated using new, a copy constructor is provided to allocate memory when one array object is used to initialize another */