C++ function object to return `p->first` and `p->second`

Question

Is there a builtin function object that returns p->first and p->second, so that I can happily write

transform(m.begin(),m.end         


        

Answer 1

We can easily write a select1st and select2nd:

struct select1st
{
   template< typename K, typename V >
   const K& operator()( std::pair const& p ) const
   {
       return p.first;
   }
};

struct select2nd
{
   template< typename K, typename V >
   const V& operator()( std::pair const& p ) const
   {
       return p.second;
   }
};

Here is an alternative, actually more flexible version:

struct select1st
{
   template< typename P >
   typename P::first_type const& operator()( P const& p ) const
   {
       return p.first;
   }
};

struct select2nd
{
   template< typename P >
   typename P::second_type const& operator()( P const& p ) const
   {
       return p.second;
   }
};

subsequently:

transform(m.begin(),m.end(),back_inserter(keys), select1st());
transform(m.begin(),m.end(),back_inserter(vals), select2nd());