C++ std::transform vector of pairs->first to new vector

Question

Sorry for a little bit beginner question. There are vector and vector of pairs

typedef std::vector  TItems;
typedef std::vector < std::pair <         


        

Answer 1

I really want you to use std::get as the functor, because it's already provided as a library function!!

Wouldn't it be great if we could write this line!?

std::transform(pairs.begin(), pairs.end(), std::back_inserter(items), std::get<0>);

... But it's a bit more terrible than that. You need to disambiguate which get to use:

int main() {
  std::vector items;
  std::vector> pairs;

  pairs.push_back(std::make_pair(1, 3));
  pairs.push_back(std::make_pair(5, 7));

  std::transform(pairs.begin(), pairs.end(), std::back_inserter(items),
                 (const int& (*)(const std::pair&))std::get<0>);

  return 0;
}

The problem is, std::get is overloaded to take 1. pair&, 2. const pair&, and 3. pair&& as the parameters, so that it will work for any sort of pair as input. Unfortunately, the overloads get in the way of the template type deduction for std::transform, so our original line

std::transform(pairs.begin(), pairs.end(), std::back_inserter(items), std::get<0>);

yields

 error: no matching function for call to ‘transform(std::vector >::iterator, std::vector >::iterator, std::back_insert_iterator >, )’
   std::transform(pairs.begin(), pairs.end(), std::back_inserter(items), std::get<0>);
                                                                                    ^
...

/usr/include/c++/4.8/bits/stl_algo.h:4915:5: note:   template argument deduction/substitution failed:
 note:   couldn't deduce template parameter ‘_UnaryOperation’
   std::transform(pairs.begin(), pairs.end(), std::back_inserter(items), std::get<0>);

It doesn't know which overload of std::get you are asking for when deducing the template for std::transform, so you have to specify it manually. Casting the function pointer to the right type tells the compiler, "Hey, please use the overload where get takes a const& and returns a const&!"

But at least we're using standard library components (yay)?

And in terms of number of lines, it's no worse than the other options: http://ideone.com/6dfzxz