Re: [Boost-users] [lambda] Defining a predicate for std::find_if if the key to a std::map is std::pair

Subject: Re: [Boost-users] [lambda] Defining a predicate for std::find_if if the key to a std::map is std::pair
From: Nat Goodspeed (nat_at_[hidden])
Date: 2009-09-24 08:51:07

Roman Perepelitsa wrote:

> 2009/9/24 Ryan McConnehey <mccorywork_at_[hidden] 
> <mailto:mccorywork_at_[hidden]>>
> 
>     I have the following std::map signature.
> 
>     typedef std::pair<std::string, int>  cPair;
>     typedef std::map<cPair, std::vector<classA> >  cMap;
>     cMap  myMap;
> 
>     When entering values into the map, the std::string is the only part
>     of the key that needs to be unique.  Upon retrieving map values both
>     values of the key need to be compared.  I've tried to define my
>     insert comparison as follows, but am uncertain how to define the
>     predicate with boost::lambda.
> 
>     std::string givenName = "some_text_value";
>     int givenInt = 3;
>     cMap::const_iterator itor = std::find_if( myMap.begin(),
>     myMap.end(), boost::lambda::bind( ??? , _1) == givenName );
> 
> 
> cMap::const_iterator itor =
>     std::find_if(myMap.begin(), myMap.end(),
>                  boost::lambda::bind(&cMap::value_type::first, _1) == 
> givenName);
> 
> Note that you can use std::map::find instead:
> 
> cMap::const_iterator itor = myMap.find(givenName);

I'd endorse Roman's suggestion, maybe something like:

cMap::const_iterator itor =
     myMap.find(cMap::key_type(givenName, properInt));

You said "my insert comparison," as though perhaps if itor == 
myMap.end() you intend to follow up with an insertion. For such cases I 
typically just code the insert() call:

std::pair<cMap::iterator, bool> inserted =
     myMap.insert(cMap::value_type(cMap::key_type(givenName, properInt),
                                   cmap::mapped_type()));
if (! inserted.second)  // oh, it already existed?
{
     ...
}

The latter form traverses the map structure just once, whether or not 
there's an existing entry with this key value.