C.Plus.Plus.Primer.4th.Edition [Electronic resources] نسخه متنی

Chapter 13. Copy Control

CONTENTS

Section 13.1 The Copy Constructor

476

Section 13.2 The Assignment Operator

482

Section 13.3 The Destructor

484

Section 13.4 A Message-Handling Example

486

Section 13.5 Managing Pointer Members

492

Chapter Summary

502

Defined Terms

502

Each type, whether a built-in or class type, defines the meaning of a (possibly empty) set of operations on objects of that type. We can add two int values, run size on a vector, and so on. These operations define what can be done with objects of the given type.

Each type also defines what happens when objects of the type are created. Initialization of objects of class type is defined by constructors. Types also control what happens when objects of the type are copied, assigned, or destroyed. Classes control these actions through special member functions: the copy constructor, the assignment operator, and the destructor. This chapter covers these operations.

copy constructor is a special constructor that has a single parameter that is a (usually const) reference to the class type. The copy constructor is used explicitly when we define a new object and initialize it from an object of the same type. It is used implicitly when we pass or return objects of that type to or from functions.

The destructor is complementary to the constructors: It is applied automatically when an object goes out of scope or when a dynamically allocated object is deleted. The destructor is used to free resources acquired when the object was constructed or during the lifetime of the object. Regardless of whether a class defines its own destructor, the compiler automatically executes the destructors for the nonstatic data members of the class.

We'll learn more about operator overloading in the next chapter, but in this chapter we cover the assignment operator. Like constructors, the assignment operator may be overloaded by specifying different types for the right-hand operand. The version whose right-hand operand is of the class type is special: If we do not write one, the compiler will synthesize one for us.

Collectively, the copy constructor, assignment operator, and destructor are referred to as copy control. The compiler automatically implements these operations, but the class may define its own versions.

Copy control is an essential part of defining any C++ class. Programmers new to C++ are often confused by having to define what happens when objects are copied, assigned, or destroyed. This confusion is compounded because if we do not explicitly define these operations, the compiler defines them for usalthough they might not behave as we intend.

Often the compiler-synthesized copy-control functions are finethey do exactly the work that needs to be done. But for some classes, relying on the default definitions leads to disaster. Frequently, the most difficult part of implementing the copy-control operations is recognizing when we need to override the default versions. One especially common case that requires the class to define its own the copy-control members is if the class has a pointer member.