RE: [Boost-users] [BLL] How to search in a list for an object comparing a data member?
Gianluca Silvestri <gianluca.silvestri@maior.it> wrote:
Hi all, What I want to achieve is a code similar to this:
struct Foo { int fDataMember;}
int main() { std::vector<Foo> foos; Foo fo; std::find_if(foos.begin(), foos.end(), _1->fDataMember == fo.fDataMember); //<-- won't compile; }
Well, apart from the missing semi-colon, missing includes and missing closing brace, I imagine you're having trouble with "_1->fDataMember". This is a documented limitation of the lambda library - there is no way to defer lookup of member names (overloading -> just changes the type and value of the pointer used) so member access can't be done on a placeholder.
I know I could use a functor object but this is just an example for a more general need.
Sorry, you'll have to define functors. These template functor classes might be useful: template<typename C, typename V, const V C::* P> struct lookup_member_const { const V & operator()(const C & c) const { return c.*P; } }; template<typename C, typename V, V C::* P> struct lookup_member { V & operator()(C & c) const { return c.*P; } }; Mind you, there might be similar things in Boost already.
struct Foo { int fDataMember;}
int main() { std::vector<Foo> foos; Foo fo; std::find_if(foos.begin(), foos.end(), _1->fDataMember == fo.fDataMember); //<-- won't compile; }
As your vector is of Foo, not Foo*, did you mean "_1.fDataMember" ? If so, you can do this: std::find_if(foos.begin(), foos.end(), bind(&Foo::fDataMember,_1) == fo.fDataMember); That assumes: #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace boost::lambda;
I imagine you're having trouble with "_1->fDataMember".
I didn't realize there was a limitation when using "->". Surely there must already be something in the lambda library to deal with this? It seems like something that must come up a lot. Darren
-----Messaggio originale----- Da: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] Per conto di Darren Cook Inviato: venerdì 31 ottobre 2003 14.31 A: Boost Users mailing list Oggetto: Re: [Boost-users] [BLL] How to search in a list for an objectcomparinga data member?
struct Foo { int fDataMember;}
int main() { std::vector<Foo> foos; Foo fo; std::find_if(foos.begin(), foos.end(), _1->fDataMember == fo.fDataMember); //<-- won't compile; }
As your vector is of Foo, not Foo*, did you mean "_1.fDataMember" ?
If so, you can do this: std::find_if(foos.begin(), foos.end(), bind(&Foo::fDataMember,_1) == fo.fDataMember);
That assumes: #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace boost::lambda;
I imagine you're having trouble with "_1->fDataMember".
I didn't realize there was a limitation when using "->". Surely there must already be something in the lambda library to deal with this? It seems like something that must come up a lot.
Darren
Thanks Gianluca
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Darren Cook <darren@dcook.org> writes:
struct Foo { int fDataMember;}
int main() { std::vector<Foo> foos; Foo fo; std::find_if(foos.begin(), foos.end(), _1->fDataMember == fo.fDataMember); //<-- won't compile; }
As your vector is of Foo, not Foo*, did you mean "_1.fDataMember" ?
If so, you can do this: std::find_if(foos.begin(), foos.end(), bind(&Foo::fDataMember,_1) == fo.fDataMember);
That assumes: #include <boost/lambda/lambda.hpp> #include <boost/lambda/bind.hpp> using namespace boost::lambda;
I imagine you're having trouble with "_1->fDataMember".
I didn't realize there was a limitation when using "->". Surely there must already be something in the lambda library to deal with this? It seems like something that must come up a lot.
Have you tried something along the lines of: (&_1)->*&Foo::fDataMember == var(fo.fDataMember) or bind(&Foo::fDataMember,_1) == var(fo.fDataMember) ?? -- Dave Abrahams Boost Consulting www.boost-consulting.com
participants (4)
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Ben Hutchings -
Darren Cook -
David Abrahams -
Gianluca Silvestri