Re: [boost] [range] [extensions] Default values for range access adaptors?

6 Jun 2013


      2013/6/6 Akira Takahashi 
...
Hi Michel,
...
I see. `optional_front` usage is following:
if (optional<T> x = r | optional_front()) {
  std::cout << x.get() << std::endl;
}
If `front` return signle range, the usage is following:
for (T x : x | front) {
  std::cout << x << std::endl;
}
If `front` is error, the return value is empty range.
I think single range approach is better. However, I think now `front` and
`optional_front` is useful.
I would like to add `optional_front` and `front_range`.
I implemented `optional_front`, `optional_back`, and `optional_at`.
https://github.com/faithandbrave/OvenToBoost/commit/3c828848be96de1c7d5d17a9...

usage:

#include <iostream>
#include <vector>
#include 
#include 
#include 

bool is_even(int x) { return x % 2 == 0; }

int main()
{
    using namespace boost::adaptors;
    using namespace boost::range::access;

    std::vector<int> v = boost::assign::list_of(1)(2)(3);

    if (boost::optional x = v | filtered(is_even) | optional_front) {
        std::cout << x.get() << std::endl;
    }
    else {
        std::cout << "not found" << std::endl;
    }
}

result:
2


Thanks,
Akira
...
...
========================
Akira Takahashi
mailto:faithandbrave@gmail.com
site : https://sites.google.com/site/faithandbrave/about/en