Creating double dimension array.

This is a discussion on Creating double dimension array. within the C++ Programming forums, part of the General Programming Boards category; Hi, I would like to creat dynamically a double dimension array. I did something like that: Code: #include <cstdio> int ...

  1. #1
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    Creating double dimension array.

    Hi,
    I would like to creat dynamically a double dimension array. I did something like that:
    Code:
    #include <cstdio>
    
    int main()
    {
            int **p;
            p=new int[100];
            for(int i=0;i<100;i++)
                    p[i]=new int[30];
            return 0;
    }
    But i don't know if it's smart. Maybe there is any smarter way to create such array? And if i create array like that i have to use 'delete' first for each p[i] and then for generally p, right ?

    Best regards,
    apacz

  2. #2
    and the hat of wrongness Salem's Avatar
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    I hope that first assignment got a warning from your compiler.

    Code:
    int main()
    {
            int **p;
            p=new int*[100]; // 100 pointers here, not 100 ints
            for(int i=0;i<100;i++)
                    p[i]=new int[30];
            return 0;
    }
    > And if i create array like that i have to use 'delete' first for each p[i] and then for generally p, right ?
    Yes, nested allocations like this need to be deleted in inside-out order.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    Thx for answer.
    But in such situation:
    Code:
    graph::graph(int i)
    {
            if(i<=0)
                    throw bad_size(i);
            else
            {
                    vertex_number=i;
                    w=new int*[i];
                    for(int y=0;y<i;y++)
                    {
                            w[y]=new int[i];
                            for(int ii=0;ii<y;ii++)
                                    w[y][ii]=-1;
                    }
            }
    }
    Do i have to code a destructor for graph? And if yes do i have to destruct in this destructor all variables from that class or only **w and the rest leave alone ?
    Regards,
    apacz

  4. #4
    and the hat of wrongness Salem's Avatar
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    new/deletes always have to balance, so new in a constructor should be balanced by a delete in a destructor.

    Consider using vectors instead.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  5. #5
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    Using a vector makes things much easier (no destructor code necessary to clean it up).

    Otherwise, if you stick with dynamic allocation, you should consider disabling or implementing the copy constructor and copy assignment operator for your graph class. Otherwise you will get strange crashes if a copy of your object is made.

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