What does this mean? Strange Konstruktor.

This is a discussion on What does this mean? Strange Konstruktor. within the C++ Programming forums, part of the General Programming Boards category; Hi! I'm a bit unsure about a konstruktor in a class deklaration. what does "hwColorType():r(),g(),b(){}" mean? Is it really needed? ...

  1. #1
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    What does this mean? Strange Konstruktor.

    Hi!

    I'm a bit unsure about a konstruktor in a class deklaration.

    what does "hwColorType():r(),g(),b(){}" mean?

    Is it really needed?

    Code:
    class hwColorType {
    public:  
    	GLubyte r,g,bl;
    	hwColorType():r(),g(),b() {}
    	hwColorType(GLubyte rr, GLubyte gg, GLubyte bb):r(rr),g(gg),b(bb) {}
    	
    };

  2. #2
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    it is an initializer list; at which it isn't doing anything.
    Code:
    hwColorType():r(),g(),b() {}
    the : means that it is an initializer list. what it does is assign values to data members, usually private.

    so for example:
    Code:
    hwColorType(GLubyte rr, GLubyte gg, GLubyte bb)
    :r(rr),g(gg),b(bb) 
    {
    }
    woud be the same thing as:
    Code:
    hwColorType(GLubyte rr, GLubyte gg, GLubyte bb) {
        r = rr;
        g = gg;
        b = bb;
    }
    the syntax does look a bit weird at first. hope this helps some.

  3. #3
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    Thanks, but I meant the other one with the empty initializers?

    hwColorType():r(),g(),b(){}

    Ah well...I suppose I can live with it

  4. #4
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    oh, okay. I don't know. It looks like it doesn't do anything, which would make it useless except for another way to construct an object.

  5. #5
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    The empty initialisers are default ctor's they are normally called automatically thus
    hwColorType():r(),g(),b(){}
    is almost certainly identical to
    hwColorType(){}
    the long version donsn't hurt nothin though.
    If you dont specify any ctor's, a default one is created for you that does nothing (except sometimes some magic that it's best not to think about)
    If you do specify a ctor, then the default ctor is not created.
    Thus you do need this trivial default ctor It lets you do things like

    hwColorType colors[16];


    Init lists are not the same as assignments in the body of the ctor.
    hwColorType(GLubyte rr, GLubyte gg, GLubyte bb) : r(rr), g(gg), b(bb) {}
    calls GLubyte's copy ctor for r,g, and b, then returns.
    Code:
    hwColorType(GLubyte rr, GLubyte gg, GLubyte bb) {
        r = rr;
        g = gg;
        b = bb;
    }
    calls the default ctor's for r,g, and b, then calls the assignment operators for each. For GLubyte this makes not the slightest difference but, you will want to avoid doing this for more complex objects.

  6. #6
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    Thanks for clearing this up grib I suspected it to be like this I had never seen it before though.

  7. #7
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    i knew it was a default, just thought it should assign data to something. i know it doesn't need to be done, but...

    essentially, for constructors and the initilizer list v. assignment; i have usually seen it used to assign data members values. I have yet to learn more and experience more programming, so...

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