Is there a standard C++ function object for taking apart a std::pair?

Question

Does anyone know if there's a de-facto standard (i.e., TR1 or Boost) C++ function object for accessing the elements of a std::pair? Twice in the past 24 hours I've wished I had something like the keys function for Perl hashes. For example, it would be nice to run std::transform on a std::map object and dump all the keys (or values) to another container. I could certainly write such a function object but I'd prefer to reuse something that's had a lot of eyeballs on it.

Answer 1

boost::bind is what you look for.

boost::bind(&std::pair::second, _1); // returns the value of a pair

Example:

typedef std::map<std::string, int> map_type;

std::vector<int> values; // will contain all values
map_type map;
std::transform(map.begin(), 
               map.end(), 
               std::back_inserter(values), 
               boost::bind(&map_type::value_type::second, _1));

Answer 2

From the way you worded your question, I'm not sure this is a proper response, but try boost::tie (part of the Boost::tuple library). It works on std::pairs too.

Answer 3

boost::bind is often used to adapt std::map containers for use with algorithms. Here is an example:

void print_string(const std::string& s) {
  std::cout << s << '\n';
}


std::map<int,std::string> my_map;
my_map[0]="Boost";
my_map[1]="Bind";


std::for_each(my_map.begin(), my_map.end(),
              boost::bind(&print_string, boost::bind(
              &std::map<int,std::string>::value_type::second,_1)));

Answer 4

What about using combinations of different containers.

For example when I wanted to partition a vector into items contained in a supplemental map and items that where not contained in the supplemental map I used the following:

typedef int DWORD; 
typedef std::pair<std::string, bool> user_info; 
typedef std::map<DWORD, user_info> USER_MAP; 
typedef std::vector<DWORD> VEC_STAFF; 

VEC_STAFF::iterator it = std::partition(Staff.begin(), Staff.end(), (bind(&USER_MAP::find, m_Users, _1) != m_Users.end()));

Now I have a second problem - during the running of the application the status bool of user_info can change, and later on I want to re-partition the vector with items that have a status bool of true rather than just being contained in the supplemental map.

However I seem to have a problem accessing the second item of a nested pair.

I tried the following but I cannot seem to access the nested pair!

CActiveUsers::VEC_STAFF::const_iterator itCurEnd = partition(Staff.begin(), Staff.end(), bind(&USER_MAP::value_type::second::second, bind(&USER_MAP::find, &m_Users, _1)) == true); 

Answer 5

Take a look at boost::adaptors. There are predefined adaptors for iterating over map keys or values without copying them to an intermediate container.

Answer 6

One option that wasn't suggested is std::tr1::get. See sections 6.1.2 and 6.1.4 of n1745.

std::pair< std::string, int > p( "foo", 1729 );

int hr = std::tr1::get< 1 >( p );

Definitely not as easy to use as bind in the map extraction case you mentioned but still worth knowing about. Adapting Johannes' code:

typedef std::map<std::string, int> map_type;

std::vector<int> values; // will contain all values
map_type map;

// std::tr1::get is overloaded so we need to help the compiler choose
const map_type::value_type::second_type & (*get)( const map_type::value_type & ) =
  &std::tr1::get< 1, map_type::value_type::first_type, map_type::value_type::second_type >;

std::transform(map.begin(), 
               map.end(), 
               std::back_inserter(values), 
               get);