boolean evaluating objects

This is a discussion on boolean evaluating objects within the C++ Programming forums, part of the General Programming Boards category; Hi, I was wondering how is something like fail-bit evaluation of cin implemented. I am trying to write a class: ...

  1. #1
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    boolean evaluating objects

    Hi,
    I was wondering how is something like fail-bit evaluation of cin implemented. I am trying to write a class:
    Code:
    class A {
         public:
              bool b();
    };
    such that -
    Code:
    int main() {
         A a_inst;
         if (a_inst) {
              //blah blah
         }
    }
    is equivalent to -
    Code:
    int main() {
         A a_inst;
         if (a_inst.b()) {
              //blah blah
         }
    }
    is it possible?

    Thank you very much

  2. #2
    C++まいる!Cをこわせ!
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    I'm supposing that your code checks if the object is "true?"
    If so, you could overload operator bool and call member function b.
    If not, then I discourage you from doing it because it creates hard to read code, not to mention it's a bad habit.

  3. #3
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    Thank you for your reply.

    I'm supposing that your code checks if the object is "true?"
    Yes, that is what I am trying to do.

    If so, you could overload operator bool and call member function b.
    That sounds like what I am looking for, can you please give an example of how that is done?

    Thank you

  4. #4
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    cin is a little more complicated than a simple operator bool. Stream classes overload operator void* which can be evaluated as a bool.

    Either way, make sure that it makes sense for your class. It can lead to weird consequences and often just the regular function will work well enough.

  5. #5
    C++まいる!Cをこわせ!
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    Make sure it makes sense before doing it, first.
    Code:
    class my_class
    {
    	operator bool () const;
    };
    
    my_class::operator bool() const
    {
    	return (expression_here);
    }

  6. #6
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    Thank you for your replies.

    I have not actually came up with a case where I intend to use this, but was just wondering about how is it done.

  7. #7
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    I believe streams use operator void* to stop the user from accidentally passing a stream to a function that takes a bool. If you use operator bool, then this would work:
    Code:
    void myclass::output(bool showHeader)
    {
      if (showHeader)
        std::cout << header;
      std::cout << data;
    }
    
    int main()
    {
      outputclass oc;
    
      std::ofstream fout("file.txt");
      oc.output(fout); // Doesn't compile
    
      my_class mc;
      oc.output(mc); // compiles, but maybe doesn't do what you want
    }
    That first try won't compile because the stream overloads operator void* and you can't do two implicit conversions. The second try will compile, which only makes sense if your my_class works like a bool.

  8. #8
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    I see what you mean now.

    Thank you very much.

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