David Abrahams wrote:

Careful; vector iterators are not neccessarily pointers.

Hm yes, that's true. But does this matter? I have a vector of some type, and a vector of pointers to some type. I don't even deal with iterators when working on the pointer-vector.

struct indirect_less { template <class It> bool operator()(It i1, It i2) const { typedef typename std::iterator_traits<It>::value_type value_type; return std::less()(i1,i2); } };

sort(ptrcoll.begin(), ptrcoll.end, indirect_less());

Ah I see. But what, if I want to abstract from the predicate? I want to create a class which is as generic as possible, so I don't have to rewrite it for each and every possible predicate. What if I want to pass the predicate to the constructor of this template. For one, passing a pointer to a function returning bool and taking on argument must be possible, and also passing a function object, maybe of type std::unary_function. template<typename It> class indirect { const std::unary_function& fctor_; public: indirect( const std::unary_function& fctor ): fctor_(fctor) {} bool operator()(It i1, It i2) const { typedef typename std::iterator_traits<It>::value_type value_type; return fctor_()(i1,i2); } }; Would that work? -- Matthias Kaeppler

Matthias Kaeppler writes:

David Abrahams wrote:

...

Careful; vector iterators are not neccessarily pointers.

Hm yes, that's true. But does this matter? I have a vector of some type, and a vector of pointers to some type. I don't even deal with iterators when working on the pointer-vector.

Sorry, I read your post too quickly and jumped to the wrong conclusion.

...

struct indirect_less { template <class It> bool operator()(It i1, It i2) const { typedef typename std::iterator_traits<It>::value_type value_type; return std::less()(i1,i2);

Whoops; I meant: return std::less()(*i1,*i2);

...

} }; sort(ptrcoll.begin(), ptrcoll.end, indirect_less());

Ah I see. But what, if I want to abstract from the predicate? I want to create a class which is as generic as possible, so I don't have to rewrite it for each and every possible predicate.

I take it from reading this message and your other one that you mean you want to write an adapter for any binary predicate on T that will instead take arguments that can be dereferenced to get Ts and forward those Ts on to the underlying binary predicate (but that's not really clear)? template <class Pred> struct indirector { indirector(Pred const& p) : p(p) {} template <class It> bool operator()(It i1, It i2) const { typedef typename std::iterator_traits<It>::value_type value_type; return p(i1,i2); } Pred p; }; template <class Pred> indirector<Pred> indirect(Pred const& p) { return indirector(p); } sort(ptrcoll.begin(), ptrcoll.end, indirect(less<int>())); HTH -- Dave Abrahams Boost Consulting www.boost-consulting.com