constant string

[jerico]

Hi.I have some confusion regarding a constant string and constant pointer to a string.

char *str="hello";
Here I think "hello" is constant so we cant modify it.Then what about
const char *str="hello" and
char *const str="hello"?

Please explain it with examples.thanks.

Jerico

[horace1]

have a look at
http://duramecho.com/ComputerInformation/WhyHowCppConst.html

[jerico]

I would like to learn it from you.In my previous post,you explained it very thoroughly.Please.

Jerico

[horace1]

hope this helps!

// simple examples of const - invalid statements are commented out
#include <stdio.h>
 
int main()
{
    char *s = "hello S";                     // "hello S" is a constant, s is pointed at it   
    char a[30] = "hello A";                  // define an array, initialise it with characters "hello A"
    printf("s = %s a = %s\n", s, a);
    //*(s+2) = 'X';                          // error! change third element of char* pointed to by s 
                                             //   would give run time segmentation error
    a[2] = 'X';                              // change third element of array a 
    printf("s = %s a = %s\n", s, a);
    s=a;                                     // point s at array a
    *(s+2) = 'Z';                            // change third element of char* pointed to by , i.e. a[2]
    printf("s = %s a = %s\n", s, a);
 
    // define pointer s1 to a constant char*, we cannot change the char* s1 points at
    const char *s1 = "hello";                // pointer to a constant  char* 
    //*s1='X';                               // compile time error! - assignment of read-only location
    s1=a;                                    // but we can point s1 at array a
    //*s1='X';                               // compile time error! - although s1 points at a (which can be written)
                                             //    compiler still gives assignment of read-only location
 
    // define pointer s2 a constant pointer to char*
    char *const s2 = a;                      // constant pointer to char *, initialised to point to a
    *s2='Q';                                 // change a[0]
    printf("s2 = %s a = %s\n", s2, a);
    //s2=a;                                  // compile time error! - assignment of read-only location
                                             //  cannot change what s2 points at
 
    // define pointer s3 to a constant pointer to constant char*
    const char *const s3 = a;                // constant pointer to const char *, initialised to point to a
    printf("s3 = %s a = %s\n", s3, a);
 
getchar();
}
 
 

try compiling and running it. then one by one uncomment some of the lines which should cause errors and see what you get.