boost equivalents for select1st and select2nd
It is often the case that one is dealing with key value pairs when using the STL containers. Sometimes, I have found the need to extract either the key or the value from these pairs as part of a call to a standard C++ algorithm. Unfortunatelly, unless one is using the SGI implementation of STL, function objects such as select1st and select2nd are not available. So, I started to look for a boost equivalent and finally found it in the bind library. Here are the mechanisms, with the std prefix omitted for brevity: Given: MyMap mymap; // A map of some key type to some value type MyVector myvector; // A vector of elements either of key type or value type, depending on requirements --- // Copy all keys from the map to the vector, which is assumed to containe elements of key type: // Implementation using select1st, when available (e.g. SGI STL extensions) transform(mymap.begin(),mymap.end(),back_inserter(myvector),select1st<MyMap::value_type>()); // Implementation using boost::bind transform(mymap.begin(),mymap.end(),back_inserter(myvector),boost::bind(&MyMap::value_type::first,_1)); // Copy all keys from the map to the vector, which, in this case, is assumed to containe elements of value type: // Implementation using select2nd, when available transform(mymap.begin(),mymap.end(),back_inserter(myvector),select2nd<MyMap::value_type>()); // Implementation using boost::bind transform(mymap.begin(),mymap.end(),back_inserter(myvector),boost::bind(&MyMap::value_type::second,_1)); --- I hope this helps anyone else that needs similar functionality and doesn't want to reinvent the wheel. Thanks, Michael Goldshteyn
It is often the case that one is dealing with key value pairs when using the STL containers. Sometimes, I have found the need to extract either the key or the value from these pairs as part of a call to a standard C++ algorithm. Unfortunatelly, unless one is using the SGI implementation of STL, function objects such as select1st and select2nd are not available. So, I started to look for a boost equivalent and finally found it in the bind library. Here are the mechanisms, with the std prefix omitted for brevity:
MyMap mymap; // A map of some key type to some value type MyVector myvector; // A vector of elements either of key type or value
(...)
// Implementation using boost::bind transform(mymap.begin(),mymap.end(),back_inserter (myvector),boost::bind(&MyMap::value_type::second,_1));
And here's the most straighforward of them, but overlooked, implementation using BOOST_FOREACH, no std-omitting was necessary at all: BOOST_FOREACH(const MyMap::value_type &v, mymap) myvector.push_back(v.first); cheers, Marcin
"Marcin Kalicinski" <kalita@poczta.onet.pl> wrote in message news:drmai0$4k0$1@sea.gmane.org...
MyMap mymap; // A map of some key type to some value type MyVector myvector; // A vector of elements either of key type or value
(...)
// Implementation using boost::bind transform(mymap.begin(),mymap.end(),back_inserter (myvector),boost::bind(&MyMap::value_type::second,_1));
And here's the most straighforward of them, but overlooked, implementation using BOOST_FOREACH, no std-omitting was necessary at all:
BOOST_FOREACH(const MyMap::value_type &v, mymap) myvector.push_back(v.first);
cheers, Marcin
An interesting alternate solution, but I was mainly looking for a way that didn't involve directly using a for loop or a derivative thereof in my code. That is to say, I was trying to just use a standard algorithm (i.e. transform), rather than coding a loop, which despite appearances is what BOOST_FOREACH is. Note that I am not arguing which way is better, only presenting a solution to those who like for_each and transform. I have to say, though, that BOOST_FOREACH does have its uses and certainly allows for a more concise and less error prone way to do something with each object in a container. The best solution to the problem is to have standard copy_keys and copy_values algorithms which, similar to transform takes begin and end iterators to the source and destination ranges, performing the necessary .first/.second accesses and then just a straight copy. In fact, I think this whole ordeal has inspired me enough to perhaps add these to the boost wiki site. Michael Goldshteyn
participants (2)
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Marcin Kalicinski -
Michael Goldshteyn