Thread: question about constant variable in c/c++

  1. #1
    Registered User
    Join Date
    Dec 2010

    question about constant variable in c/c++

    i have a question about constant variable in c/c++.The following is my code:

    int main()
            const int a = 10;
            int * p = (int*)&a;
            *p = 11;
            printf("the address of p = %p\n",p);
            printf("the address of a = %p\n",&a);
            printf("value of  a  =  %d\n",a);
            printf("value of *p = %d\n",*p);
            return 0;
    And the output is :

    the address of p = 0xbfecca1c
    the address of a = 0xbfecca1c
    value of  a  =  10
    value of *p = 11
    I don't know why variable "a" and "p" have the same address but their value are different?

  2. #2
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    Because you're a naughty boy right here:
    int * p = (int*)&a;
    At that point the compiler can do what it wants. What it apparently wanted to do (and which it is allowed to do) is to go through and turn all the a's into 10's (sort of like pre-processing) without bothering to refer to the actual memory location.

  3. #3
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    Quote Originally Posted by fatshaw View Post
            printf("the address of p = %p\n",p);
    That's not going to print the address of p. You'd need to use &p to print its address.

    Your code has undefined behaviour, any outcome is valid.
    My homepage
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