Swapping string char with pointers

This is a discussion on Swapping string char with pointers within the C++ Programming forums, part of the General Programming Boards category; Hello again, Ran into a problem with the following, maybe someone can explain. char* str="abc"; *(str+0) = *(str+2); //Program Dumps ...

  1. #1
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    Swapping string char with pointers

    Hello again,

    Ran into a problem with the following, maybe someone can explain.

    char* str="abc";
    *(str+0) = *(str+2); //Program Dumps

    or

    char temp[]="abc";
    char* str = temp;
    *(str+0) = *(str+2); // no problem

    Above values mean nothing but I dont understand why the first one does'nt act the same as the second. Our book says either of these are valid declarations:

    char str[SIZE]="abc";
    or
    char* str="abc";

    The original problem was to output the reverse a string using pointers without having [ or ] anywhere in your program. The assignment is done as my instructor informed me that I did'nt have to "reconstruct" the string, but any help understanding the above would be greatly appreciated.

    BH

  2. #2
    End Of Line Hammer's Avatar
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    You can't safely modify a string literal as you are trying to do when the prog dumps. Do a search for previous threads on this topic to learn more.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  3. #3
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    Will try that, I did'nt have 'literal' in my earlier searches.

    Thanks,
    BH

  4. #4
    Cat
    Cat is offline
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    Re: Swapping string char with pointers

    Originally posted by Black-Hearted
    Hello again,

    Ran into a problem with the following, maybe someone can explain.

    char* str="abc";
    *(str+0) = *(str+2); //Program Dumps

    or

    char temp[]="abc";
    char* str = temp;
    *(str+0) = *(str+2); // no problem

    Above values mean nothing but I dont understand why the first one does'nt act the same as the second. Our book says either of these are valid declarations:

    char str[SIZE]="abc";
    or
    char* str="abc";

    The original problem was to output the reverse a string using pointers without having [ or ] anywhere in your program. The assignment is done as my instructor informed me that I did'nt have to "reconstruct" the string, but any help understanding the above would be greatly appreciated.

    BH
    The second one creates a new array, called temp, on the stack. Temp will have 4 bytes allocated to it, and the string literal "abc\0" will be copied to it. It exists on the stack, which has read/write access. When the variable goes out of scope, the 4 bytes are deallocated and removed. It creates a pointer, also on the stack, which points at the other stack variable.

    The first creates a pointer on the stack. This pointer is initialized with the address of a string literal (in whatever data segment the compiler puts literals in). Now, it's completely compiler-specific whether this memory is writeable. It will be readable, that much is assured. But writing to this is a bad idea; the compiler is allowed to make it read-only if it wants to (and often does). Further, the compiler is allowed to make strings overlap -- e.g. say you had this:

    char * cp1 = "abc";
    char * cp2 = "abc";
    char * cp3 = "bc";

    the compiler is allowed, but not required, to do something like this:

    Code:
    //Somewhere in a data segment
    // [Address] : Data
    [BASE_ADDR    ] : 'a'
    [BASE_ADDR + 1] : 'b'
    [BASE_ADDR + 2] : 'c'
    [BASE_ADDR + 3] : '\0'
    
    // On the stack
    // Variable : contents
    cp1 : BASE_ADDR
    cp2 : BASE_ADDR
    cp3 : BASE_ADDR + 1
    In that case, modifying one string could modify two others as well. It's never legal code to write to a string literal; whether it works or not is a matter of choice of a compiler manufacturer. Sometimes you can even get it to work in one compiler mode but not another (e.g. it may error in a debug build but not in a release).

    In any event, never write to a string literal. Bad karma.

    Technically, a string literal should be a const char *, but for compatibility it auto-converts to a char *, but you shouldn't use this. E.g. you should do things like:

    const char * filename = "myfile.txt"
    Last edited by Cat; 06-17-2003 at 06:54 PM.

  5. #5
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    Cat,

    Thanks for your reply as it added to the answers I had found yesterday searching this forum for string AND literal. I have a pretty good understanding of what is taking place there now.

    BH

    btw - I'm '98 compliant now

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