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Understanding const Pointers
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The const qualifier was introduced in 2, where it was used to create variables that could not be changed (by the program) after they were created However, there is a second use of const that relates to pointers The const qualifier can be used to prevent the object pointed to by an argument to a function from being modified by that function That is, when a pointer is passed to a function, that function can modify the object pointed to by the pointer However, if the pointer is specified as const in the parameter declaration, the function code won t be able to modify the object For example, the sp_to_dash( ) function in the following program prints a dash for each space in its string argument That is, the string "this is a test" will be printed as "this-is-a-test" The use of const in the parameter declaration ensures that the code inside the function cannot modify the object pointed to by the parameter
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#include <stdioh> void sp_to_dash(const char *str); int main(void) { sp_to_dash("this is a test"); return 0; } void sp_to_dash(const char *str) { while(*str) { if(*str == ' ') printf("%c", '-'); else printf("%c", *str); str++; } }
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THE FOUNDATION OF C++
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If you had written sp_to_dash( ) in such a way that the string would be modified, it would not compile For example, if you had coded sp_to_dash( ) as follows, you would receive a compile-time error:
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/* This is wrong */ void sp_to_dash(const char *str)
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{ while(*str) { if(*str == ' ') *str = '-'; /* can't do this */ printf("%c", *str); str++; } }
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Many functions in the standard library use const in their parameter declarations Doing so ensures that no changes to the argument pointed to by a parameter will occur
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Pointers and Arrays
There is a close relationship between pointers and arrays Consider this fragment:
Here, p1 has been set to the address of the first array element in str If you wanted to access the fifth element in str, you could write
str[4]
*(p1+4)
Both statements return the fifth element Remember, arrays start at 0, so a 4 is used to index str You add 4 to the pointer p1 to get the fifth element because p1 currently points to the first element of str (Recall that an array name without an index returns the starting address of the array, which is the first element) In essence, C/C++ allows two methods of accessing array elements This is important because pointer arithmetic can be faster than array-indexing Since speed is often a consideration in programming, the use of pointers to access array elements is very common To see an example of how pointers can be used in place of array-indexing, consider these two simplified versions of the puts( ) standard library function one with array-indexing and one with pointers The puts( ) function writes a string to the standard output device
AM FL Y
char str[80], *p1; p1 = str;
6:
Pointers
/* Use array */ int puts(const char *s) { register int t; for(t=0; s[t]; ++t) putchar(s[t]); return 1; } /* Use pointer */ int puts(const char *s) { while(*s) putchar(*s++); return 1; }
THE FOUNDATION OF C++
Most professional C/C++ programmers would find the second version easier to read and understand In fact, the pointer version is the way routines of this sort are commonly written
Pointers to Character Arrays
String operations in C are usually performed by using pointers and pointer arithmetic because strings tend to be accessed in a sequential fashion For example, here is one version of the standard library function strcmp( ) that uses pointers:
/* Use pointers */ int strcmp(const char *s1, const char *s2) { while(*s1) if(*s1-*s2) return *s1-*s2; else { s1++; s2++; } return 0; /* equal */ }
Remember, strings in C are terminated by a null, which is a false value Therefore, a statement such as
while (*s1)
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