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Dynamically Allocated Objects
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The lifetime of global objects and local objects is strictly defined The programmer cannot change their lifetime in any way However, it is sometimes necessary to create objects whose lifetime can be controlled by the programmer, whose allocation and deallocation can happen or be avoided depending on the results of the operations within the execution of the program For example, one may want to allocate a string to contain the text of an error message only if the error is actually encountered during the execution of the program If the program can generate more than one error message, the size of the string allocated will vary according to the size of the text for the error encountered One does not know in advance the size of the string that should be allocated, because it depends on the kind of error encountered during the execution of the program A third kind of object allows the programmer to completely control when its allocation and deallocation take place Such an object is called a dynamically allocated object A dynamically allocated object is allocated on a pool of available memory referred to as the program's free store A programmer creates a dynamically allocated object with a new expression and terminates the lifetime of such an object with a delete expression A dynamically allocated object is either a single object or an array of objects The size of an array allocated on the free store can be a value evaluated at run-time In this section on dynamically allocated objects, we will look at three forms of new expressions: one supporting the dynamic allocation of single objects, another supporting the dynamic allocation of arrays, and a third form, called the placement new expression When the free store is exhausted, a new expression throws an exception; exceptions are examined further in 11 In 15, we discuss in detail the use of new and delete expressions with class types Dynamic Allocation and Deallocation of Single Objects A new expression consists of the keyword new followed by a type specifier This type specifier can refer to a built-in type or a class type For example,
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new int;
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allocates one object of type int from the free store Similarly,
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new iStack;
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allocates one iStack class object By itself, a new expression is not terribly useful How can we actually use the allocated object One aspect of free store memory is that the objects allocated from it are unnamed The new expression does not return the actual allocated object but instead returns a pointer to the object All manipulation of the object is done indirectly through pointers For example:
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int *pi = new int;
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The new expression creates one object of type int, to which pi refers Allocating memory at run-time on the free store, such as through the preceding new expressions, is referred to as dynamic memory allocation We say that the memory addressed by pi is allocated dynamically
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A second aspect of the free store is that the allocated memory is uninitialized Free store memory contains random bit patterns left from previous uses of that memory prior to the execution of our program The test
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if ( *pi == 0 )
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is always likely to fail, because the object addressed by pi contains random bits We therefore recommend that objects created with a new expression be initialized A programmer can initialize the object of type int in the preceding example as follows:
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int *pi = new int( 0 );
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The constant within the parentheses provides an initial value with which the object created by the new expression is initialized pi therefore refers to an object of type int that has a value of 0 The expression within the parentheses is spoken of as the initializer This initializer need not be a constant value Any expression with a result that can be converted to type int is a valid initializer The sequence of operations in a new expression is as follows: the object is allocated from the free store, and then the object is initialized with the value within the parentheses To allocate the object on the free store, the new expression calls the library operator new() The preceding new expression is roughly equivalent to the following code sequence
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int ival = 0; // creates an int object initialized to 0 int *pi = &ival; // the pointer now addresses the object
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except, of course, that the object pointed to by pi is allocated by the library operator new() and resides on the program's free store Similarly,
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iStack *ps = new iStack( 512 );
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creates an object of type iStack with a size of 512 elements In the case of a class object, the value or values in parentheses are passed to the associated constructor of the class, which is invoked following the successful allocation of the object (Dynamic allocation of class objects is discussed in more detail in Section 158 The remainder of this section focuses on the built-in types) There is one problem with the new expressions presented thus far The free store, unfortunately, represents a finite resource: at some point during program execution, we might in practice exhaust the free store, resulting in the failure of a new expression If the operator new() called by the new expression cannot acquire the requested memory, in general it throws an exception called bad_alloc (Exception handling in general is discussed in 11) The lifetime of the object to which pi refers ends when the memory in which the object resides is deallocated The memory is deallocated when pi is the operand of a delete expression For example,
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delete pi;
deallocates the memory to which pi refers, ending the lifetime of the object of type int The programmer controls when the lifetime
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