Pointer Arithmetic and Typecasting

This is a discussion on Pointer Arithmetic and Typecasting within the C Programming forums, part of the General Programming Boards category; Hi... Ok, there's no real purpose to this, I'm just bored... Anyway, basically I want to be able to manipulate ...

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    Pointer Arithmetic and Typecasting

    Hi... Ok, there's no real purpose to this, I'm just bored... Anyway, basically I want to be able to manipulate a void pointer by assigning it the address of an integer, and try and manipulate the value of that integer via dereferencing and typecasting the void pointer and assigning it a value.

    I'm trying to do it like so:

    Code:
    int y = 0;
    void * x = &y;
    (int *)(*x) = 5;
    My take on the above is:

    declare a void pointer and give it the address of the 'y' integer.
    derference the pointer and typecast to a pointer to an integer.

    I get the following compile errors:

    Code:
    error C2100: illegal indirection	
    warning C4047: '=' : 'int *' differs in levels of indirection from 'int'
    Could someone please explain to me what I'm doing wrong, how I can accomplish this, why, and how, etc.

    Trying to get a better understanding of the above.

    Thanks!
    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.

  2. #2
    C++ Witch laserlight's Avatar
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    What you are doing here is dereferencing a void pointer and then casting the result to int*, and then assigning 5 to the result:
    Code:
    (int *)(*x) = 5;
    What you probably want to do is to cast the void pointer to int*, then dereferencing the result and assigning 5 to the final result:
    Code:
    *(int *)x = 5;
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Quote Originally Posted by laserlight View Post
    What you are doing here is dereferencing a void pointer and then casting the result to int*, and then assigning 5 to the result:
    Code:
    (int *)(*x) = 5;
    What you probably want to do is to cast the void pointer to int*, then dereferencing the result and assigning 5 to the final result:
    Code:
    *(int *)x = 5;
    That worked, thanks a lot, I understand
    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.

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