[boost] Boost Iterator Library - small extension proposal

Subject: [boost] Boost Iterator Library - small extension proposal
From: Detlef Wilkening (detlef_at_[hidden])
Date: 2011-11-30 15:00:47

Hello Boostler,

at first, excuse my english - I am not a native speaker.

I used sometimes the Boost.Iterator library - especially the filter 
iterators. I like them much, but I don't like the work to create the 
iterator objects.

---
Example:
struct IsEven
{
    inline bool operator()(int n) const
    {
       return n%2==0;
    }
};
vector<int> v;
IsEven iseven;
filter_iterator<IsEven, vector<int>::iterator>
    filter_iter_begin(iseven, v.begin(), v.end());
filter_iterator<IsEven, vector<int>::iterator>
    filter_iter_end(iseven, v.end(), v.end());
---
My first problem is, that I need 3 times "v.end()" and 2 times "iseven". 
And the type is very long.
I don't think, that the "make" functions makes it much better. I use 
"auto" from C++11 in this example, to make it a little bit shorter.
---
Example:
struct IsEven
{
    inline bool operator()(int n) const
    {
       return n%2==0;
    }
};
vector<int> v;
IsEven iseven;
auto filter_iter_begin2 =
    make_filter_iterator<IsEven>(v.begin(), v.end());
auto filter_iter_end2 =
    make_filter_iterator<IsEven>(v.end(), v.end());
---
Again I need 3 times "v.end()" and 2 times the type "IsEven". And the 
type is again very long - thanks to "auto" in C++11.
My second problem is, that both use cases don't work well with lambda 
expressions in C++11. For "make" I need the type of the lambda, which I 
don't have - and I need it 2 times. And in the first solution I must 
write the lambda expression 2 times, which is not good.
Perhaps there are some other solutions, wich I don't see - than please 
explain them to mee.
-----
My proposal is a new "make" function, which is easer to use (only 1 time 
"v.end()") and works well with lambda expressions, but has furthermore a 
stupid type - but in C++11 with "auto" that doen't matter.
---
template<class Iter, class Fct>
    pair<filter_iterator<Fct, Iter>, filter_iterator<Fct, Iter>>
    make_filter_iterators(Iter begin, Iter end, Fct fct)
{
    filter_iterator<Fct, Iter> filter_iter_begin(fct, begin, end);
    filter_iterator<Fct, Iter> filter_iter_end(fct, end, end);
    return
       pair<filter_iterator<Fct, Iter>, filter_iterator<Fct, Iter>>
      (filter_iter_begin, filter_iter_end);
}
Usage:
vector<int> v;
auto iters = make_filter_iterators(
    v.begin(), v.end(), [](int n){ return n%2==0; });
copy(iters.first, iters.second, ostream_iterator<int>(cout, " "));
---
You see, with "auto" easy to use, and work well with lambda expressions. 
My proposal is to add such a "make" function to all iterator adaptors in 
Boost.Iterator.
Thank you for reading my idea.
Greetings
Detlef Wilkening