Luis De la Parra wrote:
hello, I don't have much experience with boost, so I'm having problems getting boost lambda to work...hope someone here can point me in the right direction.
I have: class Song {public: string artist( ) const { return m_artist } string title( ) const { return m_title } private: string m_artist, m_title; } typedef boost::shared_ptr<Song> pSong; vector<pSong> c;
and I am trying to use lambda to find a song in c which matches a certain artist and title...something like
iterator it = find_if( c.begin(), c.end(), something )
but I havn't figured out what "something" should look like
I've tried find_if(begin(),end(), (_1->artist() == "a1" && _1->title = "t1" ) ); but the compiler -- or I =) -- doesn't seem to know the type of _1 --> error: base operand of `->' has non-pointer type `const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >'
If I try it = find_if(begin(),end(), boost::bind(&Song::artist,_1) == "a1" ); it fails because "_1" is ambiguos (defined as lambda/placeholder and boost/arg )
using boost::lambda::bind (and not including the "normal" bind ) gives a bunch cryptic errors...
You're almost there. The problem is that lambda::bind (proudly) does not support shared_ptr as a first 'this' argument, as boost::bind does. So you have to dereference it first: it = find_if(c.begin(), c.end(), bind(&Song::artist, *_1) == "a1"); Alas, this doesn't work either, because Lambda cannot dereference a shared_ptr. To make it work, you will have to patch your <boost/lambda/detail/operator_return_type_traits.hpp> header a bit. First, find the portion that says // A is a nonreference type template <class A> struct contentsof_type { typedef typename std::iterator_traits<A>::reference type; }; and change the typedef line to read typedef typename boost::indirect_reference<A>::type type; Now add #include "boost/indirect_reference.hpp" somewhere at the top, and now this program should compile: #include <boost/shared_ptr.hpp> #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace boost::lambda; #include <string> #include <vector> #include <algorithm> class Song { public: std::string artist( ) const { return m_artist; } std::string title( ) const { return m_title; } private: std::string m_artist, m_title; }; typedef boost::shared_ptr<Song> pSong; std::vector<pSong> c; int main() { std::find_if( c.begin(), c.end(), bind(&Song::artist, *_1) == "a1" && bind(&Song::artist, *_1) == "t1" ); } Well, it does for me. :-)