Feature request: add select1st and select2nd functors

[evantill]

What do you think of adding select1st and select2nd functors in ETL library ?

(I can submit a PR)

[jwellbelove]

Sounds like a nice and simple addition. If you do a PR, don't forget to use the ETL_OR_STD::pair type.

[evantill]

I have found two implementations :

• Option A:

    template <class FirstType>
    struct select1st
    {
      typedef FirstType type;

      template <class T>
      inline const type& operator()(const T& x) const
      {
        return x.first;
      }

      template <class T>
      inline type& operator()(T& x)
      {
        return const_cast<type&>(x.first);
      }
    };

usage example :

typedef etl::pair<int, std::string> EtlPair;
EtlPair ep1(1, "Hello");

auto selector = select1st<EtlPair::first_type>();

CHECK_EQUAL(1, selector(ep1));

• Option B:

    template<typename T>
    struct Select1st {
      typedef typename T::first_type type;

      inline type& operator()(T& x) const
      {
        return x.first;
      }

      inline const type& operator()(const T& x) const
      {
        return x.first;
      }
    };

usage example :

typedef etl::pair<int, std::string> EtlPair;
EtlPair ep1(1, "Hello");

auto selector = Select1st<EtlPair>();

CHECK_EQUAL(1, selector(ep1));

Which one to use in ETL ? And which naming convention to use ?

[jwellbelove]

I prefer option B. Option A looks odd as it seems to require type information that was already implied in the function name. BTW You don't need the inline for templates.

[evantill]

Perfect (I also prefer option B).

[evantill]

@jwellbelove can we close this issue ?

[jwellbelove]

To be included in next release.