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Dynamic Allocation and Deallocation of const Objects A programmer may want to create an object on the free store but prevent the program from changing the value of the object once it has been initialized You can do this by creating the object on the free store as a const object A programmer wanting to create a const object on the free store can use a new expression as follows:
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const int *pci = new const int(1024);
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A const object created on the free store has a few special properties First, the const object must be initialized If the parentheses containing the initializer are omitted, a compiler error is generated (except that, for an object of a class type with a default constructor, the initializer can be omitted) Second, the pointer initialized with the value returned from the new expression must be a pointer to const type In the preceding example, pci is of type pointer to const int The pointer refers to the const int object allocated by the new expression What does it mean for an object on the free store to be const It means that once the object has been initialized, the value of the object cannot be changed Although the value of the object cannot be modified, its lifetime is ended with a delete expression For example:
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delete pci;
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Even though the operand of the delete expression is a pointer to const int, the delete expression is valid and causes the memory to which pci refers to be deallocated It is not possible to create a const array of elements of built-in type on the free store for the simple reason that it is not possible to initialize the elements of an array of built-in type created with a new expression All objects created const on the free store must be initialized, and because a const array cannot be initialized (except for array of classes), attempting to create a const array of builtin type with a new expression results in a compile-time error:
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const int *pci = new const int[100]; // error
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Placement New Expressions There is a third form of new expression in which the programmer can request that the object be created in memory that is already allocated This form of new expression is called a placement new expression The programmer specifies the address of the memory where the object is to be created in the new expression The form of this new expression is as follows:
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new (place_address) type-specifier
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place_address must be a pointer To use this form of new expression, the header file <new> must be included This facility allows the programmer to preallocate a large amount of memory that later will contain objects created by this form of new expression For example:
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#include <iostream> #include <new> const int chunk = 16; class Foo { public: int val() { return _val; } Foo() { _val = 0; } private: int _val; }; // preallocate the memory, but no Foo objects char *buf = new char[ sizeof(Foo) * chunk ]; int main() { // create a Foo object in buf Foo *pb = new (buf) Foo; // check that an object was placed in buf if ( pb->val() == 0 ) cout "new expression worked!" endl; // cannot use pb here delete[] buf; return 0; }
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When compiled and executed, this program generates the following output:
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new expression worked!
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There is no delete expression to match a placement new expression Such a delete expression is not needed, because placement new expressions do not allocate memory In the preceding example, it is not the memory addressed by the pointer pb that must be deleted but rather the memory addressed by buf This memory is deleted at the end of the program when the character buffer is no longer needed Because buf refers to a character array, the delete expression has the form
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delete[] buf;
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When the character buffer is deleted, the lifetime of any object it contains ends In our example, pb does not refer to a valid object of class type Foo anymore
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