Problem converting std::vector into a reference

This is a discussion on Problem converting std::vector into a reference within the C++ Programming forums, part of the General Programming Boards category; Mario F., your suggestion worked perfectly. God knows why, but it did. Actually, since I've had many, many error related ...

  1. #16
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    Mario F., your suggestion worked perfectly. God knows why, but it did.

    Actually, since I've had many, many error related to references based on temporary variables, I don't think I'd be that suprised about the compiler complaining about the 'stmt' reference being an error but, like you say, I have no idea why MSVC++ thought the first parameter was the error...

    Thanks for your help though; 'twas wrecking my head

  2. #17
    (?<!re)tired Mario F.'s Avatar
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    > God knows why, but it did.

    Because you were calling your function like this:

    Code:
    mDBH.query( data, "asdosah" )
    "asdosah" is a string literal. String literals are of type const char[]. This means that there is the need for a conversion to take place. And according to the explanation of my previous post, a non const reference can only be initialized with an object of the exact same type. A const, on the other hand, doesn't need to.
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    Ahhhh. That makes perfect sense. I'm incredibly suprised that I hadn't noticed this before now, since I've used function calls like this before and seemingly 'missed' the very same error.

    The help is hugely welcome. Thanks.

  4. #19
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    Quote Originally Posted by drrngrvy
    Ahhhh. That makes perfect sense. I'm incredibly suprised that I hadn't noticed this before now, since I've used function calls like this before and seemingly 'missed' the very same error.

    The help is hugely welcome. Thanks.
    For future reference, it's an lvalue/rvalue thing. An lvalue is any expression that can appear on the left side of an assignment operator. For example:

    int x;
    x = 5;

    In the second statement, x is a valid lvalue, because it can appear on the left side of =. 5, on the other hand, is not a valid lvalue, as it can't:

    5 = x; // compiler error!

    A non-const reference MUST be a reference to an lvalue. A string literal is not an lvalue. Also, a non-const reference will never do type conversion -- so a reference to double can't take an int, for example.

    A const reference will accept anything, lvalue or rvalue, and it will do the same type conversions that would occur if you weren't using references.
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