Thread: using class as template variable

  1. #16
    Senior Member joshdick's Avatar
    Join Date
    Nov 2002
    Phildelphia, PA
    You would be to if you had as much downtime as I do at work

  2. #17
    Registered User Micko's Avatar
    Join Date
    Nov 2003
    Ok, I can't sleep, so I'm going to explaine shallow vs. deep copy in a little bit detail, because it's seems there are
    people who don't understand this very important issue. And for those who do understand...well, it can't hurt anyway.
    The default copy constructor simply copies each member variable from the object passed as a parameter to the member
    variables of the new object. This is called a member-wise (or shallow) copy, and although this is fine for most member
    variables, it breaks pretty quickly for member variables that are pointers to objects on the free store.
    Member-wise copy copies the exact values of one object's member variables into another object.
    Pointers in both objects end up pointing to the same memory.
    A deep copy copies the values allocated on the heap to newly allocated memory.
    If the object1 includes a pointer member variable, that points to an object on the free store, the default
    copy constructor will produce soething like this:

                   Object 1                  the heap                   Object 2
              |---------------|        |------------------|        |---------------|
              |    pointer    |------->|       var        |<-------|   pointer     |
              |---------------|        |------------------|        |---------------|
              |               |        |                  |        |      ...      |
              |---------------|        |------------------|        |---------------|
    Now try to understand what will happen if either object1 or object2 goes out of scope.
    Assume Object 1 goes out of scope (for example at the end of some function), then it's destructor is called.
    If destructor frees this memory (which is very likely, because you don't want to create a memory leak)
    and the Object 2 is still pointing to the memory, a stray pointer has been created and it is a possible disater in your program.

    Solution is to write your own copy constructor and overload assignment operator (according to the BigThree rule) to avoid shallow copy and create deep copy.
    Deep copy will produce something like this:
                          the heap                   Object 2
                     |------------------|        |---------------|
                     |       var        |<-------|   pointer     |
                     |------------------|        |---------------|
                     |                  |        |      ...      |
                     |------------------|        |---------------|
                          the heap                   Object 1
                     |------------------|        |---------------|
                     |       var        |<-------|   pointer     |
                     |------------------|        |---------------|
                     |                  |        |      ...      |
                     |------------------|        |---------------|
    When you implement for instance, operator= then implementation of copy constructor becoming trivial.
    One thing you should always do when implementing assignment operator is to check for self assignment (or
    write clever code in which you do that implicitely) because it can be a very big problem. Why?
    Well, I'll leavet that to you. There's plenty answers on the Internet and also, do a bord search.

    P.S. I don't know how to do formatting like Prelude did in her tutorials, so I used code tags. Moderators are free to change it to appropriate style if they want.

    - Micko
    Last edited by Micko; 04-18-2005 at 04:53 PM.
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  3. #18
    Registered User
    Join Date
    Apr 2005
    i finally managed to solve my problem =) thanks for all your help!

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