I was able to answer my own question after some additional reading. This: find_if(a_vector.begin(), a_vector.end(), bind(equal_to<string>(),bind(&dref,_1),string("with "))); is equivalent to: (?) find_if(a_vector.begin(), a_vector.end(), (bind(&dref, _1) == string("with "))); Please correct me if I am missing anything. There's a steep learning curve and I'm still on it. Thanks, Graham ----- Original Message ---- From: Forever Kid To: boost-users@lists.boost.org Sent: Thursday, March 29, 2007 11:30:03 AM Subject: boost::bind syntax with a vector of shared_ptrs and find_if usage What is the syntax for the line below without using bind's overloaded '==' operator and instead using equal_to<>? an_iterator = find_if(a_vector.begin(), a_vector.end(), (bind(&dref, _1) == string("with "))); I am doing these as exercises in an attempt to wrap my head around boost::bind. What has me puzzled, at the moment, is how find_if 'knows' (above) to automatically compare what the share_ptrs are pointing to (the strings). Doesn't a_vector.begin() return an iterator to a shared_ptr and not a string? Thanks again, Graham Current working example: const string& dref(const shared_ptr<string> &sp) { return *sp; } int main (int argc, char *argv[]) { vector a_vector; shared_ptr<string> sp1(new string("Play ")); shared_ptr<string> sp2(new string("with ")); shared_ptr<string> sp3(new string("C++!")); a_vector.push_back(sp1); a_vector.push_back(sp2); a_vector.push_back(sp3); vector::iterator an_iterator; an_iterator = find_if(a_vector.begin(), a_vector.end(), (bind(&dref, _1) == string("with "))); if (an_iterator != a_vector.end()) { shared_ptr<string> sp(*an_iterator); cout << "Found " << *sp << endl; } return 0; } ----- Original Message ---- From: Jeff F To: boost-users@lists.boost.org Sent: Thursday, March 29, 2007 8:11:56 AM Subject: Re: [Boost-users] boost::bind syntax with a vector of weak_ptrs Forever Kid wrote:

What is the correct boost::bind syntax to use with a standard algorithm such as find_if with an array of weak_ptrs or shared_ptrs to an object such as a string?

My weak attempt to figure this out is below:

shared_ptr<string> sp1(new string("Play ")); shared_ptr<string> sp2(new string("with ")); shared_ptr<string> sp3(new string("C++!"));

vector a_vector;

a_vector.push_back(weak_ptr<string>(sp1)); a_vector.push_back(weak_ptr<string>(sp2)); a_vector.push_back(weak_ptr<string>(sp3));

// What is the correct syntax here? find_if(a_vector.begin(), a_vector.end(), (bind(&equal_to<string>(), _1))(string("Play "));

The above has many problems unrelated to weak_ptrs. (bind(&equal_to<string>(),_1))(string("Play ")); |----------------------------||---------------| a b Part "a" above is not a properly formed bind expression: 1 - you are passing the address of an equal_to<...> instance. 2 - equal_to<...> expects two arguments you supply one. Part "b" attempts to invoke the function object resulting from Part "a", and passes the result(if part "a" were well formed); a bool to find_if. find_if then tries to apply operator() to the bool, which again is ill formed. The proper bind expression for comparing a string _value_ is: bind( equal_to<string>(), _1, string("Play ") ) To use this you'd need an indirect or transform iterator adaptor that derefs the weak ptrs. See http://www.boost.org/libs/iterator/doc/indirect_iterator.html http://www.boost.org/libs/iterator/doc/transform_iterator.html I'm not sure of the details in using an indirect iterator with weak_ptr. A transform iterator would require you to provide a deref function, and use it with the iterator adaptor library. This is untested code: std::string deref_wps( weak_ptr<string> wps ){ return /* derefed wps */; } find_if( make_transform_iterator(a_vector.begin(),&deref_wps) , make_transform_iterator(a_vector. end(),&deref_wps) , bind( equal_to<string>(), _1, string("Play ") ) ); Or using some of the nice operator overloading provided by bind, you can forego the iterator adaptors: find_if( a_vector.begin() , a_vector. end() , bind( &deref_wps, _1 ) == string("Play ") ); Jeff _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users ____________________________________________________________________________________ Looking for earth-friendly autos? Browse Top Cars by "Green Rating" at Yahoo! Autos' Green Center. http://autos.yahoo.com/green_center/ ____________________________________________________________________________________ Be a PS3 game guru. Get your game face on with the latest PS3 news and previews at Yahoo! Games. http://videogames.yahoo.com/platform?platform=120121