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float* p; // p is a pointer to a float
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it only allocates memory for the pointer itself. The value of the pointer will be some memory address, but the memory at that address is not yet allocated. This means that storage could already be in use by some other variable. In this case, p is uninitialized: it is not pointing to any allocated memory. Any attempt to access the memory to which it points will be an error:
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*p = 3.14159; // ERROR: no storage has been allocated for *P // x contains the value 3.14159 // p contains the address of x // OK: *p has been allocated
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A good way to avoid this problem is to initialize pointers when they are declared:
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float x = 3.14159; float* p = &x; cout << *p; In this case, accessing *p is
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no problem because the memory needed to store the float 3.14159 was automatically allocated when x was declared; p points to the same allocated memory. Another way to avoid the problem of a dangling pointer is to allocate memory explicitly for the pointer itself. This is done with the new operator:
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float* q; q = new float; *q = 3.14159; // allocates storage for 1 float // OK: *q has been allocated
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CHAP. 7]
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The new operator returns the address of a block of s unallocated bytes in memory, where s is the size of a float. (Typically, sizeof(float) is 4 bytes.) Assigning that address to q guarantees that *q is not currently in use by any other variables. 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(); // allocator failed: 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; float* p = new float(3.14159); // allocates named memory // 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. EXAMPLE 7.14 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. 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; float* p = &x; delete p; // x contains the value 3.14159 // p contains the address of x // RISKY: p was not allocated by new
This would deallocate the variable x, a mistake that can be very difficult to debug.
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[CHAP. 7
7.9 DYNAMIC ARRAYS An array name is really just a constant pointer that is allocated at compile time:
float a[20]; // a is a const pointer to a block of 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:
float* p = new float[20];
An array that is declared this way is called a dynamic array. Compare the two ways of defining an array:
float a[20]; // static array float* p = new float[20]; // dynamic array The static array a is created at compile time; its memory remains allocated throughout 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 [] must be included this way, because p is an array.
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