Bind operator== in standard algorithms

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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