# `tie`

### [This version of tie has been removed from the utility.hpp header. There is a new, more general version of tie in the Boost Tuples Library. The more general version handles an (almost) arbitrary number of arguments, instead of just two. The version in utility.hpp had to be removed to avoid name clashes.]

    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;
    }

The output is:

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

Copyright © 2000 | Jeremy Siek, Univ.of Notre Dame (jsiek@lsc.nd.edu) Lie-Quan Lee, Univ.of Notre Dame (llee1@lsc.nd.edu) Andrew Lumsdaine, Univ.of Notre Dame (lums@lsc.nd.edu)