Thread: Dynamically Expanding an Array

  1. #1
    Registered User Vespasian's Avatar
    Join Date
    Aug 2011

    Dynamically Expanding an Array

    Consider the following:

    int *p;
    p = new int[5];
    for(int i=0;i<5;i++)
    Now suppose we want to add a 6th element (without using vector)

    One way is to copy it across to a larger array:

    int *p;
    p = new int[5];
    for(int i=0;i<5;i++)
    // realloc
    int* temp = new int[6];
    std::copy(p, p + 5, temp);
    delete [] p;
    p = temp;
    This looks like a very expensive operation and im looking for other ways.

    Three questions regarding the above:
    1. Not using vector, is this the best way to do this?
    2. What about using realloc? How would I use realloc in this situation?
    3. Any other ways apart from realloc or vector?
    Last edited by Vespasian; 05-28-2013 at 06:51 AM.

  2. #2
    Registered User
    Join Date
    Oct 2006
    if you were talking about objects that do a lot of work in their (copy) constructors, or that contain a lot of member data (large buffers, etc.), I'd agree that it would be pretty expensive, but int is not a type that is especially expensive to copy.

    to answer your questions:
    1. yes.
    2. don't use calloc/malloc/realloc/free in a C++ program, as there is no guarantee that it will cooperate properly with other things using new/delete.
    3. I can't think of any that are better than what you've got there.
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