[boost] Re: copy_if, composition, and proper predicate

8 Mar 2005

      "Michael Goldshteyn" <mgoldshteyn@earthlink.net> wrote in message 
news:d0l8qu$qfq$1@sea.gmane.org...
...
Given:
// Definitions
class Element
{
public:
 const std::string &GetTag() const;
};
typedef std::vector<Element> ElementContainer;
// Now here is what is required
ElementContainer elements;
...
ElementContainer matchingElements;
std::string matchingTag("abc");
copy_if(elements.begin(),elements.end(),back_inserter(matchingElements)),pred);
--
What is pred? Assuming that iElements represents the iterator inside 
copy_if, pred should be something like: iElements->GetTag()==matchingTag
I could go about this by creating a new function object class/struct, 
which takes matchingTag as its sole constructor argument and does the 
comparison inside operator(). But, I want to use in place composition 
using binders, etc... to create the pred, so as not to have to create a 
function object class/struct. The question is, "What piece of code should 
pred be?"
// Almost does the right thing, but it's only *iElements==matchingTag, I 
need iElements->GetTag()==matchingTag
bind_2nd(equal_to,matchingTag)
This seems like a very common operation. How can I compose the predicate, 
in place to do this? If there are multiple solutions, I would like to hear 
them as well!
Thanks,
Michael
The first way that comes to mind:

copy_if(
      elements.begin()
    , elements.end()
    , back_inserter(matchingElements)
    , boost::bind<bool>(
          equal_to<string>()
        , boost::bind<string>(
              boost::mem_fn(&Element::GetTag)
            , _1
        )
        , matchingTag
    )
);

Note that I use the boost::bind<return_type> form, 'cuz I'm used to working 
with VC6 which can't deduce template arguments correctly.

HTH

Pablo Aguilar