ceph/src/boost/libs/mpl/doc/src/refmanual/empty.rst

   1 .. Sequences/Intrinsic Metafunctions//empty
   2
   3 empty
   4 =====
   5
   6 Synopsis
   7 --------
   8
   9 .. parsed-literal::
  10
  11     template<
  12           typename Sequence
  13         >
  14     struct empty
  15     {
  16         typedef |unspecified| type;
  17     };
  18
  19
  20
  21 Description
  22 -----------
  23
  24 Returns an |Integral Constant| ``c`` such that ``c::value == true`` if
  25 and only if the sequence is empty.
  26
  27
  28 Header
  29 ------
  30
  31 .. parsed-literal::
  32
  33     #include <boost/mpl/empty.hpp>
  34
  35
  36 Model of
  37 --------
  38
  39 |Tag Dispatched Metafunction|
  40
  41
  42 Parameters
  43 ----------
  44
  45 +---------------+-----------------------+-----------------------------------+
  46 | Parameter     | Requirement           | Description                       |
  47 +===============+=======================+===================================+
  48 | ``Sequence``  | |Forward Sequence|    | A sequence to test.               |
  49 +---------------+-----------------------+-----------------------------------+
  50
  51
  52 Expression semantics
  53 --------------------
  54
  55 For any |Forward Sequence| ``s``:
  56
  57
  58 .. parsed-literal::
  59
  60     typedef empty<s>::type c;
  61
  62 :Return type:
  63     Boolean |Integral Constant|.
  64
  65 :Semantics:
  66     Equivalent to ``typedef is_same< begin<s>::type,end<s>::type >::type c;``.
  67
  68 :Postcondition:
  69     ``empty<s>::value == ( size<s>::value == 0 )``.
  70
  71
  72
  73 Complexity
  74 ----------
  75
  76 Amortized constant time.
  77
  78
  79 Example
  80 -------
  81
  82 .. parsed-literal::
  83
  84     typedef range_c<int,0,0> empty_range;
  85     typedef vector<long,float,double> types;
  86
  87     BOOST_MPL_ASSERT( empty<empty_range> );
  88     BOOST_MPL_ASSERT_NOT( empty<types> );
  89
  90
  91 See also
  92 --------
  93
  94 |Forward Sequence|, |Integral Constant|, |size|, |begin| / |end|
  95
  96
  97 .. copyright:: Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams
  98    Distributed under the Boost Software License, Version 1.0. (See accompanying
  99    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)