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

Roman Perepelitsa wrote:

2009/9/24 Ryan McConnehey <mccorywork@gmail.com <mailto:mccorywork@gmail.com>>

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.