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[Page 499 (continued)]

9.7. Destructors

A destructor is another type of special member function. The name of the destructor for a class is the tilde character (~) followed by the class name. This naming convention has intuitive appeal, because as we will see in a later chapter, the tilde operator is the bitwise complement operator, and, in a sense, the destructor is the complement of the constructor. Note that a destructor is often referred to with the abbreviation "dtor" in the literature. We prefer not to use this abbreviation.

A class's destructor is called implicitly when an object is destroyed. This occurs, for example, as an automatic object is destroyed when program execution leaves the scope in which that object was instantiated. The destructor itself does not actually release the object's memoryit performs termination housekeeping before the system reclaims the object's memory, so the memory may be reused to hold new objects.

A destructor receives no parameters and returns no value. A destructor may not specify a return typenot even void. A class may have only one destructordestructor overloading is not allowed.

Common Programming Error 9.3

It is a syntax error to attempt to pass arguments to a destructor, to specify a return type for a destructor (even void cannot be specified), to return values from a destructor or to overload a destructor.

Even though destructors have not been provided for the classes presented so far, every class has a destructor. If the programmer does not explicitly provide a destructor, the compiler creates an "empty" destructor. [Note: We will see that such an implicitly created destructor does, in fact, perform important operations on objects that are created through composition (Chapter 10) and inheritance (Chapter 12).] In Chapter 11, we will build destructors appropriate for classes whose objects contain dynamically allocated memory (e.g., for arrays and strings) or use other system resources (e.g., files on disk, which we study in Chapter 17). We discuss how to dynamically allocate and deallocate memory in Chapter 10.

[Page 500]

Software Engineering Observation 9.11

As we will see in the remainder of the book, constructors and destructors have much greater prominence in C++ and object-oriented programming than is possible to convey after only our brief introduction here.

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