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const int * p = new int; delete p; // ERROR: cannot delete pointer to const
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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:
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float x = 3.14159; float* p = &x; delete p; // x contains the value 3.14159 // p contains the address of x // RISKY: p was not allocated by new
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This would deallocate the variable X, a mistake that can be very difficult to debug.
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6.10 DYNAMIC ARRAYS
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An array name is really just a constant pointer that is allocated at compile time:
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float a[20]; // a is a const pointer to a block of 20 floats float* const p = new float[20]; // so is p
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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.
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In contrast, we can use a non-constant pointer to postpone the allocation of memory until the program is runnning. This is generally called run-time binding or dynamic binding:
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float* p = new float[20];
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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 .
POINTERS AND REFERENCES
[CHAP. 6
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
Note that the subscript operator [ 1 must be included this way, because p is an array.
EXAMPLE 6.12 Using Dynamic Arrays
The get ( ) function here creates a dynamic array void get(double*& a, int& n) { tout << "Enter number of items: I'; tin >> n; a = new double[n]; tout -C-C "Enter 'I CC n C-C I items, one per line:\n"; for (int i = 0; i < n; i++) { tout << "\t" << i-t1 << I': II; tin >> a[i]; 1 1 void print(double* a, int n) 1 for (int i = 0; i c n; i++) tout << a[i] << 'I 1'; tout -CC endl; 1 main0 1 double* a; int n; get& n) ; print(a, n); delete [] a; getb, n); print(a, n);
// a is simply an unallocated pointer // now a is an array of n doubles // now a is simply an unallocated pointer again // now a is an array of n doubles
CHAP. 61
POINTERS AND REFERENCES
Inside the get ( ) function, the new operator allocates storage for n doubles after the value of n is obtained interactively. 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 de 1 e t e operator. Note that the subscript operator [ ] must be specified when deleting an array. Note that the array parameter a is a pointer that is passed by reference: void get(double*& a, int& n) This is necessary because the new operator will change the value of a which is the address of the first element of the newly allocated array.
6.11 USING const WITH POINTERS A pointer to a constant is different from a constant pointer. This distinction is illustrated in the following example.
EXAMPLE 6.13 Constant Pointers, Pointer Constants, and Constant Pointer Constants This fragment declares four variables: a pointer p, a constant pointer cp, a pointer pc to a constant, and a constant pointer cpc to a constant: int * p; // a pointer to an int // ok: increments int *p ++(*P); // ok: increments pointer p ++p; int * const cp; // a constant pointer to an int // ok: increments int *cp ++ ("cp) ; ++cp; // illegal: pointer cp is constant const int * pc; // a pointer to a constant int // illegal: int *pc is constant .++ (*PC> ; ++pc; // ok: increments pointer pc const int * const cpc; // a constant pointer to a constant int ++(*cpc>; // illegal: int *cpc is constant ++cpc; // illegal: pointer cpc is constant
Note that the reference operator * may be used in a declaration with or without a space on either side. Thus, the following three declarations are equivalent:
int* p; int * p; int *p; // indicates that p has type int* (pointer to int) // style sometimes used for clarity // old C style
6.12 ARRAYS OF POINTERS AND POINTERS TO ARRAYS The elements of an array may be pointers. Here is an array of 4 pointers to type double:
double* p[4];
Its elements can allocated like any other pointer: pm = new double(3.141592653589793); We can visualize this array like this:
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