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1134 Destructor
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We must still provide one more operation, which defines what happens when a Vec object is destroyed An object that is created in a local scope is destroyed as soon as it goes out of scope; a dynamically allocated object is destroyed when we delete a pointer to the object For example, consider the split function from 611/103:
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vector<string> split(const string& str) { vector<string> ret; // split str into words and store in ret return ret; }
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When we return from split, the local variable ret goes out of scope and is destroyed Just as with copy and assignment, it is up to the class to say what happens when objects are destroyed Like constructors, which say how to create objects, there is a special member function, called a destructor, that controls what happens when objects of the type are destroyed Destructors have the same name as the name of the class prefixed by a tilde(~) Destructors take no arguments and have no return value The work of the destructor is to do any cleanup that should be done whenever an object goes away Typically, this cleanup involves releasing resources, such as memory, that the constructor has allocated:
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template <class T> class Vec { public:
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For Vecs, we allocate memory in the constructors, and so we must free it in the destructor This job is similar to what the assignment operator does to obliterate the old left-hand side Not surprisingly, we can call the same utility function from the destructor, with the aim of destroying the elements and freeing the space that they occupied
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1135 Default operations
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Some classes, such as the Student_info types that we defined in s 4 and 9, do not explicitly define a copy constructor, assignment operator, or destructor A logical question is: What happens when objects of such types are created, copied, assigned, and destroyed The answer is that if the class author does not specify these operations, the compiler synthesizes default versions of the unspecified operations The default versions are defined to operate recursively copying, assigning, or destroying each data element according to the appropriate rules for the type of that element Members that are of class type are copied, assigned, or destroyed by calling the copy constructor, assignment operator, or destructor for the data element Members that are of built-in type are copied or assigned by copying or assigning their value The destructor for built-in types has no work to do even if the type is a pointer In particular, destroying a pointer through the default destructor does not free the space at which the pointer points Now we can understand how the default Student_info operations execute For example, the copy constructor copies four data elements To do so, it invokes the string and vector copy constructors to copy the name and homework members respectively It copies the two double values, midterm and final, directly Finally, as we saw in 95/164, there is a default for the default constructor If the class defines no constructors at all, then the compiler will synthesize the default constructor, which is the constructor that has no parameters The synthesized default constructor recursively initializes each data member in the same way as the object itself is initialized: If the context requires default-initialization, it will default-initialize the data members; if the context requires value-initialization, it will value-initialize the data members It is important to note that if a class defines any constructor explicitly, even a copy constructor, then the compiler will not synthesize a default constructor for that class Default constructors are essential in several contexts: One such context is in the synthesized default constructor itself In order to be used as a data member of a class that relies on the synthesized default constructor, the data type must itself provide a default constructor Therefore, it is usually a good idea to give a class a default constructor, either explicitly, as we did in 9, or implicitly, as we did in 4
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