Returning a pointer to a vector

This is a discussion on Returning a pointer to a vector within the C++ Programming forums, part of the General Programming Boards category; Hello, I have a class which contains a vector. This vector is created in a function called "CreateVector". So far, ...

  1. #1
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    Returning a pointer to a vector

    Hello,

    I have a class which contains a vector. This vector is created in a function called "CreateVector". So far, I have created this vector in this function, and returned the vector as a whole. However, because the vector is large, ideally I'd like to be able to return a pointer to the vector. How can I do this? Can you create a pointer to a vector?

    Here is what I have so far:

    Code:
    #include <vector>
    
    using namespace std;
    
    class My_Class
    {
    private:
        vector<int*> x;
        vector<int*> CreateVector()
        {
            vector<int*> y(100);
            for (int i = 0; i < 100; i ++)
            {
                y[i] = i * i;
            }
            return y;
        };
    public:
        void SetX()
        {
            x = CreateVector();
        };
    };
    
    in main()
    {
        My_Class my_object;
        my_object.SetX();
    
        return 0;
    }
    (This is just an example piece of code to demonstrate my problem, the actual CreateVector() function does much more than is shown)

    What do I need to do so that I can return a pointer to this vector, in order to speed up the program?

    Thanks!

  2. #2
    C++ Witch laserlight's Avatar
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    You would need to use dynamic memory allocation to return a pointer to a vector whose lifetime exceeds that of the function call. (Or use a static local variable, but that is a bad solution here.)

    I suggest that you change the function signature to add a reference parameter to the vector. This way the call will pass an empty vector that the function will fill.

    That said, a compiler optimisation known as return value optimisation can make it such that there is no extra cost of copying in returning the vector by value... but then this optimisation is not always applied.
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  3. #3
    C++まいる!Cをこわせ! Elysia's Avatar
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    But they also say: don't optimize prematurely.
    Return the vector by value and if you find that it is running slowly, then you can change the design.
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    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
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  4. #4
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    Code:
    vector<int*> y(100);
    for (int i = 0; i < 100; i ++)
    {
      y[i] = i * i;
    }
    return y;
    y is a vector of int pointers so doing y[i] = i*i; is wrong. You need vector<int>

    As for your question, in the current code it looks to me that you can just operate on x. Both the function and x are prive anyway.

    Code:
    x.resize(100);
    for()
    x[i] = i*i;
    x should also be vector<int> instead of vector<int *> of course

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