Re: [boost] bind and lambda don't work together?

From: Richard Smith (richard_at_[hidden])
Date: 2003-08-08 05:16:37

Victor A. Wagner, Jr. wrote:

> I finally had a chance to go back and look at an attempt to use lambda in
> one of my commercial endeavors.  It turns out that I was getting "ambiguous
> _1" with my compiler (VC.net2003) between lambda and bind.

Boost.lambda places _1 in the boost::lambda namespace.
Boost.bind places _1 in the global namespace.  This is fine
so long as you avoid using directives and are careful with
using declarations.

As soon as you have a using directive for the boost::lambda
namespace, boost::lambda::_1 will be injected into the
global namespace -- even if the using directive is at block
scope -- and any use of _1 will be ambiguous.

If you have translation units that use both Boost.bind and
Boost.lambda, the safest advice is (i) avoid all using
directives for namespace boost::lambda; (ii) put using
declarations at block scope.  For example,

  int main()
  {
    boost::function< int (int, int) > times;
    {
      using boost::lambda::_1;
      using boost::lambda::_2;
      times = _1 * _2;
    }

    boost::function< int (int) > square
      ( boost::bind( times, _1, _1 ) );

    std::cout << times(10,20) << std::endl;
    std::cout << square(9) << std::endl;
  }

--
Richard Smith