[/
  Copyright 2000 Beman Dawes & John Maddock.
  Copyright (c) 2021, Alan Freitas

  Distributed under the Boost Software License, Version 1.0.

  See accompanying file LICENSE_1_0.txt
  or copy at http://boost.org/LICENSE_1_0.txt
]

[section Compressed Pair]

[section Introduction]

All of the contents of [@../../../../boost/compressed_pair.hpp `<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
[@https://en.cppreference.com/w/cpp/language/ebo ['empty base-class optimisation]]
is applied to compress the size of the pair.

[endsect]

[section Synopsis]

```cpp
template <class T1, class T2>
class __compressed_pair__
{
public:
    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 [@https://en.cppreference.com/w/cpp/named_req/PODType 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:/libs/type_traits/index.html `boost::is_union`], or
if [@boost:/libs/type_traits/index.html `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.

[endsect]

[/===============]
[xinclude tmp/compressed_pair_reference.xml]
[/===============]

[section Acknowledgments]

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

Maintained by [@mailto:john@johnmaddock.co.uk John Maddock].

[endsect]

[endsect]
