C Tutorial/Pointer/const pointer
Содержание
A non-constant pointer to constant data
#include <stdio.h>
void printCharacters( const char *sPtr );
int main()
{
char string[] = "print characters of a string";
printf( "The string is:\n" );
printCharacters( string );
printf( "\n" );
return 0;
}
void printCharacters( const char *sPtr )
{
for ( ; *sPtr != "\0"; sPtr++ ) {
printf( "%c", *sPtr );
}
}The string is: print characters of a string
Attempting to modify a constant pointer to non-constant data
- ptr is a constant pointer to an integer that can be modified through ptr.
- ptr always points to the same memory location.
#include <stdio.h>
int main()
{
int x;
int y;
int * const ptr = &x;
*ptr = 7; /* allowed: *ptr is not const */
// ptr = &y; /* error: ptr is const; cannot assign new address */
return 0;
}Attempting to modify data through a non-constant pointer to constant data.
xPtr cannot be used to modify the value of the variable to which it points.
#include <stdio.h>
void f( const int *xPtr );
int main()
{
int y;
f( &y );
return 0;
}
void f( const int *xPtr )
{
//*xPtr = 100; /* error: cannot modify a const object */
}Using a non-constant pointer to non-constant data
#include <stdio.h>
#include <ctype.h>
void convertToUppercase( char *sPtr ); /* prototype */
int main()
{
char string[] = "characters and abcde";
printf( "The string before conversion is: %s", string );
convertToUppercase( string );
printf( "\nThe string after conversion is: %s\n", string );
return 0;
}
void convertToUppercase( char *sPtr )
{
while ( *sPtr != "\0" ) {
if ( islower( *sPtr ) ) {
*sPtr = toupper( *sPtr );
}
++sPtr;
}
}The string before conversion is: characters and abcde The string after conversion is: CHARACTERS AND ABCDE
