How to Write a Copy Constructor

In C++, a copy constructor is a special method created within a class that returns an exact duplicate of the object that calls it. For the simplest classes, the C++ language handles the creation of copy constructors automatically. However, if your class contains a pointer to a dynamically allocated range of data, you need to write a copy constructor. This example assumes you are writing a copy constructor for a class named \"Circle.\"

Things You'll Need

• Computer<br />C++ class that uses dynamically allocated memory or pointers

Instructions

1. Add the following to the circle.h file to declare the copy constructor function:<br /><br />Circle(const Circle& c)<br /><br />It should, of course, be declared in the public section of the file, not the private.

2. Add the following to the circle.c file to add the copy constructor programming:<br /><br />Circle::Circle(const Circle& c) {<br /> // The rest of the code will go here.<br />}

3. Write the code to copy all the static data from the first class to the second. <br /><br />x = c.x<br />GO<br />y = c.y<br />GO<br />r = c.r<br />GO<br /><br />This information would have been copied, even by the default copy constructor.

4. Write the code to copy dynamic memory and pointers. Consider a variable that is part of the circle called \"Color *c.\" Because this is a pointer, attempting to simply copy the variable will only copy a memory address, and this will lead to trouble further down the line if one of the two circles is deleted. So, use the \"de-reference\" operator: <br /><br />color = c.*color<br /><br />This ensures that the 'color' variable within the new circle is a copy of the value of the original's color, and not just a copy of a shared memory location.

Tips & Warnings

• The copy constructor takes itself as argument with the 'const' keyword. This ensures that no modifications, intentional or otherwise, are made to the original in the course of the copying process. It is also passed with the \"&\" symbol, to ensure that the source copy is passed into the function by reference. This prevents a cyclic redundancy error: in order to pass it in its vanilla form 'by value', a copy must be made. However, it is the function for creating copies that is being written.

• The copy constructor will never need to be called directly. Instead, it will be called whenever a copy needs to be made with an '=' sign.

References

• Cplusplus: Pointers
• Fredosaurus: Copy Constructors