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THE FOUNDATION OF C++
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All pointer arithmetic is relative to its base type (Assume 2-byte short integers)
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Pointer Comparisons
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You can compare two pointers in a relational expression For instance, given the pointers p and q, the following statement is perfectly valid:
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if(p<q) printf("p points to lower memory than q\n");
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Generally, pointer comparisons are useful only when two or more pointers are pointing to a common object As an example, imagine that you are constructing a stack routine to hold integer values A stack is a list that uses first in, last out accessing It
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is often compared to a stack of plates on a table the first one set down is the last one to be used Stacks are used frequently in compilers, interpreters, spreadsheets, and other system-related software To create a stack, you need two routines: push( ) and pop( ) The push( ) function puts values on the stack, and pop( ) takes them off In the following expample, the stack is held in the array stack, which is STCKSIZE elements long The variable tos holds the memory address of the top of the stack and is used to prevent stack overflows and underflows Once the stack has been initialized, push( ) and pop( ) can be used to access the stack These routines are shown here with a simple main( ) function to drive them:
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#include <stdioh> #include <stdlibh> #define STCKSIZE 50 void push(int i); int pop(void); int *p1, *tos, stack[STCKSIZE]; int main(void) { int value; p1 = stack; /* assign p1 the start of stack */ tos = p1; /* let tos hold top of stack */ do { printf("Enter a number (-1 to quit, 0 to pop): "); scanf("%d", &value); if(value!=0) push(value); else printf("this is it %d\n", pop()); } while(value!=-1); return 0; } void push(int i) { p1++; if(p1==(tos + STCKSIZE)) { printf("stack overflow"); exit(1); }
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*p1 = i; THE FOUNDATION OF C++ } int pop(void) { if(p1==tos) { printf("stack underflow"); exit(1); } p1--; return *(p1+1); }
Both the push( ) and pop( ) functions perform a relational test on the pointer p1 to detect limit errors In push( ), p1 is tested against the end of stack by adding STCKSIZE (the size of the stack) to tos In pop( ), p1 is checked against tos to be sure that a stack underflow has not occurred In pop( ), the parentheses are necessary in the return statement Without them, the statement would look like
return *p1 + 1;
which would return the value at location p1 plus 1, not the value of the location p1+1
Dynamic Allocation and Pointers
Once compiled, all C/C++ programs organize the computer s memory into four regions: program code, global data, the stack, and the heap The heap is an area of free memory that is managed by the dynamic allocation functions malloc( ) and free( ) These functions were introduced in 5 in conjunction with arrays Here we will examine them further, beginning with a review of their of their basic operation Although C++ still supports C s dynamic allocation functions, it also defines its own approach, which is based upon dynamic allocation operators These are described in Part Three The malloc( ) function allocates memory and returns a pointer to the start of it free( ) returns previously allocated memory to the heap for possible reuse The prototypes for malloc( ) and free( ) are void *malloc(size_t num_bytes); void free(void *p);
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Both functions use the <stdlibh> header Here, num_bytes is the number of bytes requested If there is not enough free memory to fill the request, malloc( ) returns a null The type size_t is defined in <stdlibh> and specifies an unsigned integer type that is capable of holding the largest amount of memory that may be allocated with a single call to malloc( ) It is important that free( ) be called only with a valid, previously allocated pointer; otherwise, the organization of the heap could be damaged, which might cause a program crash The code fragment shown here allocates 25 bytes of memory:
char *p; p = (char *) malloc(25);
After the assignment, p points to the first of 25 bytes of free memory The cast to char * is not needed for C but is required for C++ programs In C, if no type cast is used with malloc( ), the pointer type is converted automatically to the same type as the pointer variable on the left side of the assignment In C++, such implicit pointer conversions are disallowed Although not needed by C, the use of the type cast allows your C code to be compatible with C++ As another example, this fragment allocates space for 50 integers It uses sizeof to ensure portability
int *p; p = (int *) malloc(50*sizeof(int));
Since the heap is not infinite, whenever you allocate memory it is imperative to check the value returned by malloc( ) to make sure that it is not null before using the pointer Using a null pointer may crash the computer The proper way to allocate memory and test for a valid pointer is illustrated in this code fragment:
int *p; if((p = (int *) malloc(100))==NULL) { printf("Out of memory\n"); exit(1); }
The macro NULL is defined in <stdlibh> Of course, you can substitute some sort of error handler in place of exit( ) The point is that you do not want the pointer p to be used if it is null You should include the header <stdlibh> at the top of any file that uses malloc( ) and free( ) because it contains their prototypes
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