How to evaluate an object directly in a boolean context?

Question

I want to evaluate an instance of some class in a boolean context. Or to be clearer, i want to define how the object reacts if its used directly in a boolean context.
Here an example:

class Foo 
{
  int state;
  Foo(): state(1) {}
  bool checkState()
  {
    return (state >= 0);
  }
  void doWork() 
  { 
    /*blah with state*/
  }
};

int main()
{
  Foo obj;
//while(obj.checkState())  //this works perfectly, and thats what i indent to do!
  while(obj)               //this is what want to write
    obj.doWork();
  return 0;
}

Ok, its just a nice to have :-), but is this possible at all? If yes, how?

Thanks!

score 13 · Accepted Answer · answered Mar 22 '13 at 14:35

13

Use an explicit conversion operator to bool:

explicit operator bool() const { return (state >= 0); }

This does exactly what you want: define what happens when the object is evaluated in a boolean context.

If you have an older compiler, you cannot use explicit, and that is bad because operator bool() (without explicit) can end up used unwantingly in non-boolean contexts. In that case, use the safe bool idiom instead.

answered Mar 22 '13 at 14:35

R. Martinho Fernandes

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ok i tried this and got the error "only declarations of constructors can be 'explicit'". I assume to have an "old" compiler with GCC 4.1.2? I am reading your "safe bool idiom" right now. – Philipp Michalski Mar 22 '13 at 14:44
Yes, GCC 4.1.2 does not support this. IIRC you need at least 4.5 (and the `-std=c++0x` flag) – R. Martinho Fernandes Mar 22 '13 at 14:47
OMG, this idiom is just a pain in the a** for new OO programmers :-D . I actually have an abstract base class, so i went for the way of reusable testable_with_virtual method, but i got stuck with the problem, that within the code i copied, there is the error "safe_bool_base::this_type_does_not_support_comparisons() const' is protected" although its used in a derived class. – Philipp Michalski Mar 22 '13 at 15:32

score 2 · Answer 2 · edited May 23 '17 at 10:30

2

You can use operator bool():

explicit operator bool() const
{
    return (state >=0) ;
}

As pointed out you want to use explicit to prevent this being used in an integer context. Also main should return an int.

edited May 23 '17 at 10:30

Community

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1

answered Mar 22 '13 at 14:36

Shafik Yaghmour

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No, this is a bad idea. This operator will be used in integer contexts, for example. – R. Martinho Fernandes Mar 22 '13 at 14:37
Yeah thanks for the hint with main, i knew that, but it wasnt that important for me at that moment. I ll edit my post though, to reflect this! – Philipp Michalski Mar 22 '13 at 15:45
@PhilippMichalski It is important to think user who will come later on and see your post and try to learn something from it. So having clean code in the post is important. – Shafik Yaghmour Mar 22 '13 at 15:50

How to evaluate an object directly in a boolean context?

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