C++ have a constructor call another constructor

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  1. #1
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    C++ have a constructor call another constructor

    So... I understand why this doesn't work, I just can't think a way to do it (besides copy and pasting from the first constructor to the second, which I want to avoid).
    Code:
    class Foo {
       // The REAL constructor
       Foo (Bar const & B) { 
          // Complicated stuff
       }
    
       // A constructor that turns i into a Bar so we can call the real constructor.
       Foo (int i) {
          Bar converted_i;
          // Complicated stuff that turns i into a Bar.
          Foo (converted_i);
       }
    
       // ...
    };
    This doesn't work as I had hoped. Instead of having the int constructor 'switch over' to the Bar constructor, it just constructs a temporary Foo object, which does nothing.

    I hope the comments make it clear what I'm trying to do... but for the life of me, I don't know how.
    Callou collei we'll code the way
    Of prime numbers and pings!

  2. #2
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    Yeah, I think I get it. First of all, from a design standpoint, Foo's constructor shouldn't know how to convert an integer to a Bar. One of Bar's constructors/methods should do this. What I would do is create a protected method(non-constructor) that turns this instance of Foo into the Bar given, however you do that. Then just call that in the two constructors. Hope this helps.

  3. #3
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    Quote Originally Posted by UMR_Student View Post
    Yeah, I think I get it. First of all, from a design standpoint, Foo's constructor shouldn't know how to convert an integer to a Bar. One of Bar's constructors/methods should do this. What I would do is create a protected method(non-constructor) that turns this instance of Foo into the Bar given, however you do that. Then just call that in the two constructors. Hope this helps.
    I understand what you are saying, and the more I think about it, the more that's the solution. I just wished for something a bit more elegant.
    Callou collei we'll code the way
    Of prime numbers and pings!

  4. #4
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by QuestionC View Post
    I understand what you are saying, and the more I think about it, the more that's the solution. I just wished for something a bit more elegant.
    Code:
    class Bar
    {
    public:
        Bar(int x);
    };
    
    class Foo {
    public:
       // The REAL constructor
       Foo (Bar const & B) { 
          // Complicated stuff
       }
    
       // ...
    };
    
    int main()
    {
        Foo x(10);
    }
    This works for me... Foo takes a Bar reference, and a Bar can be constructed from an int, so the compiler automatically constructs a Bar from the value 10.

  5. #5
    Cat without Hat CornedBee's Avatar
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    Constructor chaining is not supported in C++03. You'll have to wait for C++09 for this.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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