Re: [Boost-users] Bind operator== in standard algorithms

From: Alan M. Carroll (amc_at_[hidden])
Date: 2005-08-18 14:35:39

It's all about types. In the stand alone expression, the bind() result is evaluated by passing it x, yielding an int which is compared to the int y, so operator== is called on two ints. In the find_if call, the bind() result is *not* evaluated so operator== is called on the bind type and an int.

The bottom line is that you can only use functions and functors with bind, not operators like "==". If you want to do that kind of thing, you should look in to the Boost.lambda library.

As a side note, is there a standard/Boost library with the functor equivalent of std::equal_to for the unary operators & and * ? Or do I need to write those myself?

At 03:26 AM 8/18/2005, you wrote:
> Ok, I've just discovered boost::bind, and it is a marvelous discovery
>:-) Now, I do seem to have a problem: the operator overloading for
>logical operators seems to work on its own, but not when I apply it
>within standard algorithms such as find_if. Still, the bind
>documentation states that this should be possible, and gives examples
>such as:
>std::remove_if( first, last, !bind( &X::visible, _1 ) ); 
>std::find_if( first, last, bind( &X::name, _1 ) == "peter" );
>
> If we look at some simple test code I wrote:
>
> int add3( int i ){  return i + 3; };
>
> int main()
> {
>    int x = 2; int y = 5;
>    bind( add3, _1)(x) == y;  // Compiles
>    find_if( l.begin(), l.end(), bind<int>( equal_to<int>(),
>bind(add3, _1), y) ); // Compiles
>    find_if( l.begin(), l.end(), bind(add3, _1) == y ); // Doesn't compile
> }
>
> From the last line I get the following error message:
> error: no match for 'operator==' in 'boost::bind [with R = int, B1 =
>int, A1 = boost::arg<1>](add3, (<unnamed>::_1, boost::arg<1>())) == y'
>
> I'm puzzled why the standalone bind expression containing == does
>compile, whereas the one inside find_if doesn't. Of course, I do have
>a functional equivalent I can work with in the first find_if line, but
>it does seem neat to be able to write more readable code, as in the
>examples. Anyone care to explain this to me?
> Ok, standard disclaimers: I use the g++ 3.4.4 compiler on Fedora, I
>did include all necessary boost:: and std:: namespace stuff in my
>code, but left it out in this post, and I have no clue whatsoever
>which version of boost I have installed ( Can I find it in the code
>somewhere? Otherwise I'll ask the sysadmin which version he installed
>when I see him.).
>
>-- 
>Alex Borghgraef
>
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