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The first two of these lines can be combined, thereby initializing q as it is declared: float* q=new float; Note that using the new operator to initialize q only initializes the pointer itself, not the memory to which it points. It is possible to do both in the same statement that declares the pointer: float* q=new float(3.14159); cout <<*q; // OK: both q and *q have been initialized In the unlikely event that there is not enough free memory to allocate a block of the required size, the new operator will return 0 (the NULL pointer): double* p=new double; if (p == 0) abort ( ); // insufficient memory else *p=3.141592658979324; This prudent code calls an abort ( ) function to prevent dereferencing the NULL pointer. Consider again the two alternatives to allocating memory: float x=3.14159; // allocates named memory float* p=new float (3.14159); // allocates unnamed memory In the first case, memory is allocated at compile time to the named variable x. In the second case, memory is allocated at run time to an unnamed object that is accessible through *p. The delete Operator The delete operator reverses the action of the new operator, returning allocated memory to the free store. It should only be applied to pointers that have been allocated explicitly by the new operator: float* q=new float(3.14159); delete q; // deallocates q *q=2.71828; // ERROR: q has been deallocated Deallocating q returns the block of sizeof (float) bytes to the free store, making it available for allocation to other objects. Once q has been deallocated, it should not be used again until after it has been reallocated.
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Note! A deallocated pointer, also called a dangling pointer, is like an uninitialized pointer: it doesn't point to anything.
A pointer to a constant cannot be deleted: const int * p=new int; delete p; // ERROR: cannot delete pointer to const This restriction is consistent with the general principle that constants cannot be changed. Using the delete operator for fundamental types (char, int, float, double, etc.) is generally not recommended because little is gained at the risk of a potentially disastrous error: float x=3.14159; // x has value 3.14159 float* p=&x; // p references x delete p; // RISKY: p not allocated by new This would deallocate x, a mistake that can be very difficult to debug. Dynamic Arrays An array name is just a constant pointer allocated at compile time: float a[20]; // a is a const pointer 20 floats float* const p=new float[20]; // so is p Here, both a and p are constant pointers to blocks of 20 floats. The declaration of a is called static binding because it is allocated at compile time; the symbol is bound to the allocated memory even if the array is never used while the program is running. In contrast, we can use a non-constant pointer to postpone the allocation of memory until the program is running. This is generally called run-time binding or dynamic binding. An array that is declared this way is called a dynamic array. Compare the two ways of defining an array: float a[20]; float *p=new float [20]; // static array // dynamic array
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The static array a is created at compile time; its memory remains allocated thoughout the run of the program. The dynamic array p is created at run time; its memory allocated only when its declaration executes. Furthermore, the memory allocated to the array p is deallocated as soon as the delete operator is invoked on it: delete [] p; // deallocates the array p The subscript operator [ ] must be included, because p is an array. Example 6.9 Using Dynamic Arrays The get() function here creates a dynamic array void get(double*& a, int& n) { cout <<"Enter number of items: "; cin >>n; a=new double [n]; cout <<"Enter " <<n <<" items: "; for (int i=0; i<n; i++) cin >>a[i]; } void print (double* a, int n) { for (int i=0; i < n; i++) cout <<a[i] <<" "; cout <<endl; } void main () { double* a; // a is now an unallocated pointer int n; // allocate it use it destroy it get(a, n); print(a, n); delete [] a; get(a, n); print(a, n); delete [] a; }
Enter number of items: 4 Enter 4 items: 1.1 2.2 3.3 7.7 1.1 2.2 3.3 7.7 Enter number of items: 2 Enter 2 items: 1.23 9.87 1.23 9.87
Inside the get() function, n is obtained and the new operator allocates storage for n doubles. So the array is created "on the fly" while the program is running. Before get() is used to create another array for a, the current array has to be deallocated with the delete operator. Note that the subscript operator [] must be specified when deleting an array. Note that the a is a pointer that is passed by reference: void get(double*& a, int& n)
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