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We can visualize these variables like this: The assignment ***pppc = 'w' refers to the contents of the address pc that is pointed to by the address ppc that is pointed to by the address pppc. 7.13 POINTERS TO FUNCTIONS
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Like an array name, a function name is actually a constant pointer. We can think of its value as the address of the code that implements the function.
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CHAP. 7]
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A pointer to a function is simply a pointer whose value is the address of the function name. Since that name is itself a pointer, a pointer to a function is just a pointer to a constant pointer. For example,
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int f(int); // declares function f int (*pf)(int); // declares function pointer pf pf = &f; // assigns address of f to pf
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We can visualize the function pointer like this: The value of function pointers is that they allow us to define functions of functions. This is done by passing a function pointer as a parameter to another function. EXAMPLE 7.18 The Sum of a Function
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The sum() function has two parameters: the function pointer pf and the integer n: int sum(int (*)(int), int); int square(int); int cube(int);
int f(int n) { . . . }
int main() { cout << sum(square,4) << endl; // 1 + 4 + 9 + 16 cout << sum(cube,4) << endl; // 1 + 8 + 27 + 64 } The call sum(square,4) computes and returns the sum square(1) + square(2) + square(3) + square(4). Since square(k) computes and returns k*k, the sum() function returns 1 + 4 + 9 + 16 = 30. Here are the function definitions and the output: int sum(int (*pf)(int k), int n) { // returns the sum f(0) + f(1) + f(2) + . . . + f(n-1): int s = 0; for (int i = 1; i <= n; i++) s += (*pf)(i); return s; } int square(int k) { return k*k; } int cube(int k) { return k*k*k; } 30 100 The sum() function evaluates the function to which pf points, at each of the integers 1 through n, and returns the sum of these n values. Note that the declaration of the function pointer parameter pf in the sum() function s parameter list requires the dummy variable k .
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[CHAP. 7
7.14 NUL, NULL, AND void The constant 0 (zero) has type int. Nevertheless, this symbol can be assigned to all the fundamental types:
char '\0' short int 0 int 0 unsigned int 0 float 0.0 double 0.0 In each case, the object is initialized to the number 0. In the case of type char, the character c becomes the null character; denoted by '\0' or NUL, it is the character whose ASCII code is 0. char c = 0; short d = 0; int n = 0; unsigned u = 0; float x = 0; double z = 0; // // // // // // initializes initializes initializes initializes initializes initializes c d n u x z to to to to to to the the the the the the
The values of pointers are memory addresses. These addresses must remain within that part of memory allocated to the executing process, with the exception of the address 0x0. This is called the NULL pointer. The same constant applies to pointers derived from any type:
char* pc = 0; // initializes pc to NULL short* pd = 0; // initializes pd to NULL int* pn = 0; // initializes pn to NULL unsigned* pu = 0; // initializes pu to NULL float* px = 0; // initializes px to NULL double* pz = 0; // initializes pz to NULL The NULL pointer cannot be dereferenced. This is a common but fatal error: int* p = 0; *p = 22; // ERROR: cannot dereference the NULL pointer
A reasonable precaution is to test a pointer before attempting to dereference it:
// ok This tests the condition (p != NULL) because that condition is true precisely when p is if (p) *p = 22;
nonzero. The name void denotes a special fundamental type. Unlike all the other fundamental types, void can only be used in a derived type:
// ERROR: no object can have type void // OK The most common use of the type void is to specify that a function does not return a value: void swap(double&, double&); Another, different use of void is to declare a pointer to an object of unknown type: void* p = q; void x; void* p;
This use is most common in low-level C programs designed to manipulate hardware resources. Review Questions
7.1 7.2 7.3 How do you access the memory address of a variable How do you access the contents of the memory location whose address is stored in a pointer variable Explain the difference between the following two declarations:
int n1=n; int& n2=n;
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