tie

[tie has been deprecated.  Its functionality is supplied by the Boost Tuples Library.]

template <class A, class B>
tied<A,B> tie(A& a, B& b);

This is a utility function that makes it more convenient to work with a function which returns a std::pair<>. The effect of the tie() function is to allow the assignment of the two values of the pair to two separate variables. The idea for this comes from Jaakko Järvi's Binders [1].

Where Defined

boost/utility.hpp

Example

An example of using the tie() function with the vertices() function, which returns a pair of type std::pair<vertex_iterator,vertex_iterator>. The pair of iterators is assigned to the iterator variables i and end.

  graph_traits< adjacency_list<> >::vertex_iterator i, end;
  for(tie(i,end) = vertices(G); i != end; ++i)
    // ...

Here is another example that uses tie() for handling operations with std::set.

#include <set>
#include <algorithm>
#include <iostream>
#include <boost/utility.hpp>

int
main(int, char*[])
{
  {
    typedef std::set<int> SetT;
    SetT::iterator i, end;
    bool inserted;
    
    int vals[5] = { 5, 2, 4, 9, 1 };
    SetT s(vals, vals + 5);
    
    // Using tie() with a return value of pair<iterator,bool>

    int new_vals[2] = { 3, 9 };

    for (int k = 0; k < 2; ++k) {
      boost::tie(i,inserted) = s.insert(new_vals[k]);
      if (!inserted)
        std::cout << *i << " was already in the set." << std::endl;
      else
        std::cout << *i << " successfully inserted." << std::endl;    
    }
  }    
  {
    int* i, *end;
    int vals[6] = { 5, 2, 4, 4, 9, 1 };
    std::sort(vals, vals + 6);

    // Using tie() with a return value of pair<iterator,iterator>

    boost::tie(i,end) = std::equal_range(vals, vals + 6, 4);
    std::cout << "There were " << std::distance(i,end)
              << " occurrences of " << *i << "." << std::endl;
    // Footnote: of course one would normally just use std::count()
    // to get this information, but that would spoil the example :)
  }
  return 0;
}

  3 successfully inserted.
  9 was already in the set.
  There were 2 occurrences of 4.