new/delete vs array on stack

This is a discussion on new/delete vs array on stack within the C++ Programming forums, part of the General Programming Boards category; suppose I need a quick and dirty array for a calculation. Will it be better to use new/delete as opposed ...

  1. #1
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    new/delete vs array on stack

    suppose I need a quick and dirty array for a calculation. Will it be better to use new/delete as opposed to just allocating them on stack? For example:

    Code:
    int dostuff(){
       char * x = new char[2048];
       // do stuff
       delete x;
    }
    as opposed to:

    Code:
    int dostuff(){
       char x[2048];
       // do stuff with x
    }
    I have a feeling the second one isn't very common.. thanks in advance.

  2. #2
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    Quote Originally Posted by underthesun View Post
    Will it be better to use new/delete as opposed to just allocating them on stack?
    it wont be better to do that unless you want the size to change based off of some conditions
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  3. #3
    Guest Sebastiani's Avatar
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    >> I have a feeling the second one isn't very common.. thanks in advance.

    It usually depends on how large (in bytes) the buffer is going to be, and if the function is recursive or not. You generally have more heap space available to you than stack, so if the buffer is large or the recursion deep you might get into some trouble allocating off the stack.
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  4. #4
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    Quote Originally Posted by underthesun View Post
    suppose I need a quick and dirty array for a calculation. Will it be better to use new/delete as opposed to just allocating them on stack? For example:

    Code:
    int dostuff(){
       char * x = new char[2048];
       // do stuff
       delete x;
    }
    Well I certainly wouldn't use the code you've got, since it has a big bug in it. You use the array version of new, but the single version of delete. You need to use:
    delete [] x; if x is allocated as an array instead of a single char.

    If you don't know how many chars you'll need, why bother with new/delete anyways? Just use std::vector<char> or std::string.
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

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    Whoops, nice catch there. I actually do know how much space I'll need, it's just that it might overflow from the stack.. I guess I'll switch to new[] and delete[].

    Thanks

  6. #6
    Guest Sebastiani's Avatar
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    >> I guess I'll switch to new[] and delete[].

    And if an exception is thrown from the function, who cleans up the memory? Nobody! Pointers don't have destructors. That's why their so unsafe for managing memory. As cpjust recommended, use std::vector, std::string, or at the very least some user-defined type to ensure that the resources are cleaned up properly.
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

  7. #7
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    good point, but the "operation" I have right now merely consists of memcpy's, and is used for an opengl function (mashing and interleaving vertex & color data from two arrays). I guess I'll switch to something else when the function grows more complicated. thanks

    p.s I believe opengl never throws exceptions, but instead just have their own error logging functionality.

  8. #8
    Cat without Hat CornedBee's Avatar
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    OpenGL doesn't throw, indeed, but who will go through all the code if you switch to a wrapper that does?

    Be exception-safe now, because it's not going to happen later. There is simply no good reason to use a local pointer over a vector. The vector is simpler to write, less things to remember, safer to use - it simply has no drawbacks.
    All the buzzt!
    CornedBee

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  9. #9
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    Hmm, good point. Here comes vector<GLfloat>..

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