Pointer alloc size question

This is a discussion on Pointer alloc size question within the C++ Programming forums, part of the General Programming Boards category; I realized something that I can't figure out: how do we obtain the length of some dynamic array allocated with ...

  1. #1
    Registered User mikahell's Avatar
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    Question Pointer alloc size question

    I realized something that I can't figure out: how do we obtain the length of some dynamic array allocated with a pointer? Considering that any pointer gives a "sizeof()" of 4 bytes, and that these 4 bytes represent an address, then where is the allocation's length stored?

    As with the following, operators "new" and "delete" will know how much space they have to allocate, and how much space they will have to deallocate:

    Code:
    int *ptr;
    ptr = new int[100];
    delete[] ptr; //The function will know that "ptr" points to an array of 100 ints...
    So where can we find out the information that actually tells the allocated size? I know that before "new-ing" it would be a good idea to store the length in a variable if we later want to use that length, but where is the info? Could it be possible that only the OS knows how much space that ptr uses?

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    Devil's Advocate SlyMaelstrom's Avatar
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    You can't. Hence the reason you can go out of the bounds of an array. It's up to the programmer to mark the size of a particular array. As for the delete[] operator. I believe it stores the amount allocated somewhere when a call to new[] is made, or perhaps it allocates an extra null byte so delete[] knows where to stop. I honestly couldn't tell you, you could probably tell by looking at the assembly.
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    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by mikahell
    I know that before "new-ing" it would be a good idea to store the length in a variable if we later want to use that length, but where is the info?
    Yes. Remember how much you ask for. Implementation specifics are then irrelevant.
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    Cat
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    new[] and delete[] can store that size information however they choose. SOME (and note, this is just SOME) store the size as 4 bytes before the beginning of the array.

    However, you should really not be passing arrays to functions anyway. Use std::vector instead, and use .size().
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