Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards


Compressed Pair


All of the contents of <boost/compressed_pair.hpp> are defined inside namespace boost.

The class compressed_pair is very similar to std::pair. However, if either of the template arguments are empty classes, then the empty base-class optimisation is applied to compress the size of the pair.

template <class T1, class T2>
class compressed_pair
    typedef T1                                                 first_type;
    typedef T2                                                 second_type;
    typedef typename call_traits<first_type>::param_type       first_param_type;
    typedef typename call_traits<second_type>::param_type      second_param_type;
    typedef typename call_traits<first_type>::reference        first_reference;
    typedef typename call_traits<second_type>::reference       second_reference;
    typedef typename call_traits<first_type>::const_reference  first_const_reference;
    typedef typename call_traits<second_type>::const_reference second_const_reference;

             compressed_pair() : base() {}
             compressed_pair(first_param_type x, second_param_type y);
    explicit compressed_pair(first_param_type x);
    explicit compressed_pair(second_param_type y);

    compressed_pair& operator=(const compressed_pair&);

    first_reference       first();
    first_const_reference first() const;

    second_reference       second();
    second_const_reference second() const;

    void swap(compressed_pair& y);

The two members of the pair can be accessed using the member functions first() and second(). Note that not all member functions can be instantiated for all template parameter types. In particular compressed_pair can be instantiated for reference and array types, however in these cases the range of constructors that can be used is limited. If types T1 and T2 are the same type, then there is only one version of the single-argument constructor, and this constructor initialises both values in the pair to the passed value.

Note that if either member is a POD type, then that member is not zero-initialized by the compressed_pair default constructor: it is up to you to supply an initial value for these types if you want them to have a default value.

Note that compressed_pair can not be instantiated if either of the template arguments is a union type, unless there is compiler support for boost::is_union, or if boost::is_union is specialised for the union type.

Finally, a word of caution for Visual C++ 6 users: if either argument is an empty type, then assigning to that member will produce memory corruption, unless the empty type has a "do nothing" assignment operator defined. This is due to a bug in the way VC6 generates implicit assignment operators.


namespace boost {
  template<typename T1, typename T2> class compressed_pair;

  template<typename T> class compressed_pair<T, T>;
  template<typename T1, typename T2> 
    void swap(compressed_pair< T1, T2 > & x, compressed_pair< T1, T2 > & y);

Based on contributions by Steve Cleary, Beman Dawes, Howard Hinnant and John Maddock.

Maintained by John Maddock.