I don't know of any easy way to do this using the bits provided with the STL. There is probably some clever boost way, using iterator adapters, or boost::lambda, but personally I wouldn't go that way Obviously C++0x lambdas will make all this easy Your problem is attempting to cast a function like this: const std::string&(Atom::*)() into a function like this: std::string (Atom::*)() If you replace your typedef R (U::*fn_t)() with typedef const R& (U::*fn_t)() const then it should work The following avoids this problem and also provides type inference so that you can just write mem_fun_eq(&Atom::Name, atomname) It compiles for me, although I haven't tested it template class mem_fun_eq_t : public std::unary_function { private: R (U::*fn_)() const; S val_; public: mem_fun_eq_t(R (U::*fn )() const, S val) : fn_(fn), val_(val){} bool operator()(U * u) { return (u->*fn_)() == val_; } }; template mem_fun_eq_t mem_fun_eq(R (U::*fn)() const, S val) { return mem_fun_eq_t(fn, val); }.
I don't know of any easy way to do this using the bits provided with the STL. There is probably some clever boost way, using iterator adapters, or boost::lambda, but personally I wouldn't go that way. Obviously C++0x lambdas will make all this easy.
Your problem is attempting to cast a function like this: const std::string&(Atom::*)() into a function like this: std::string (Atom::*)() If you replace your typedef R (U::*fn_t)(); with typedef const R& (U::*fn_t)() const; then it should work. The following avoids this problem and also provides type inference so that you can just write mem_fun_eq(&Atom::Name, atomname). It compiles for me, although I haven't tested it.
Template class mem_fun_eq_t : public std::unary_function { private: R (U::*fn_)() const; S val_; public: mem_fun_eq_t(R (U::*fn )() const, S val) : fn_(fn), val_(val){} bool operator()(U * u) { return (u->*fn_)() == val_; } }; template mem_fun_eq_t mem_fun_eq(R (U::*fn)() const, S val) { return mem_fun_eq_t(fn, val); }.
Basically, you call on mem_fun to create an object that accepts two arguments T* and a template argument to the function A if it has one (or void if it doesn't). Hence you combine it like so: template struct MyPredicate{ MyPredicate(MemFunc _functionObj, CompareType _value) : m_Value(_value), m_Function(_functionObj){} bool operator()(const T &_input){ return m_Value == m_Function(_input); } private: MemFunc m_Function; CompareType m_Value; }; Edit: Ok, that's not completely working so why not have: struct NamePred: binary_function{ bool operator()(Atom *_obj, string _val){ return _obj->Name() == _val; }; }; then use bind2nd find_if( atoms.begin(), atoms.end(), bind2nd( NamePred, "yo" ) ).
Yes, just having the type of the function be a template parameter makes most of the pain go away. You should do this. – Richard Wolf Jul 27 '10 at 0:22 Can the MemFunc type be deduced?
– Shamster Jul 27 '10 at 0:24 @Shamster: good point. A wrapper function would be required to achieve the required type deduction, similar to the mem_fun and mem_fun_ref wrapper functions. – Richard Wolf Jul 27 '10 at 0:29 I'm actually trying to get it to compile with VS2008 but that's exactly the problem I'm running into.It's having issues with resolving the mem_fun return type.
Damn, thought this would help. I'll keep working at it. – wheaties Jul 27 '10 at 0:30.
I'm building a series of predicates that duplicate lots of code, and so are being changed into a single template function class based on the std::unary_function. Creates a different error! Firstly, am I reinventing the wheel with this predicate?
Is there something in the STL that I've missed that does the same job in a single class? I can always break the predicate down into several more specific ones, but I'm trying to avoid that. Secondly, can you help me with the compiler errors?
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