Thread: subscipt overload (returning reference to array?)

  1. #1
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    subscipt overload (returning reference to array?)

    Hi all,

    I'm a bit stumped about the idea of returning a reference to an array inside a struct. For example,

    Code:
    struct MyStruct
    {
         float x, y;
    
         float& operator [](int i)
         {
              return ((&x)[i]);  // this confuses me
         }
    I can use the overload to access x and y by using their corresponding subscripts. But how does that work? I understand incrementing a pointer to x will give me y if I dereference it. The overall syntax of that return confuses me.

    Hope someone can help me

  2. #2
    C++ Witch laserlight's Avatar
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    This:
    Code:
    (&x)[i]
    is equivalent to:
    Code:
    *(&x + i)
    Suppose i == 0, then we end up with:
    Code:
    *(&x + 0)
    which is equivalent to:
    Code:
    *&x
    which is equivalent to:
    Code:
    x
    Great, except that suppose i == 1, then we end up with:
    Code:
    *(&x + 1)
    Essentially we're treating x as if it were an array of 1 float object. Hence, &x + 1 is a pointer one-past-the-end of this "array". This is okay, but then we try to dereference this pointer, resulting in undefined behaviour.

    So, what you have here is an operator[] overload that only ever works if the argument is 0.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    Thank you for your response. Let's say though I initialize a MyStruct object in main(), assuming I have a constructor,

    Code:
    int main()
    {
         MyStruct ms(0.1, 0.2);
         ms.[0] // == 0.1
         ms.[1] // == 0.2
    Those are the values I get for each subscript operator overload argument. Is that purely because when I initialize the constructor, each value is adjacent in memory?

  4. #4
    C++ Witch laserlight's Avatar
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    Yes, and of the same type.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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