ceph/src/boost/boost/hof/tap.hpp

   1 /*=============================================================================
   2     Copyright (c) 2014 Paul Fultz II
   3     tap.h
   4     Distributed under the Boost Software License, Version 1.0. (See accompanying
   5     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   6 ==============================================================================*/
   7
   8 #ifndef BOOST_HOF_GUARD_FUNCTION_TAP_H
   9 #define BOOST_HOF_GUARD_FUNCTION_TAP_H
  10
  11 /// tap
  12 /// ===
  13 ///
  14 /// Description
  15 /// -----------
  16 ///
  17 /// The `tap` function invokes a function on the first argument passed in and
  18 /// then returns the first argument. This is useful in a chain of pipable
  19 /// function to perform operations on intermediate results. As a result, this
  20 /// function is [`pipable`](/include/boost/hof/pipable).
  21 ///
  22 /// Synopsis
  23 /// --------
  24 ///
  25 ///     template<class T, class F>
  26 ///     pipable constexpr T tap(T&& x, const F& f);
  27 ///
  28 /// Requirements
  29 /// ------------
  30 ///
  31 /// F must be:
  32 ///
  33 /// * [UnaryInvocable](UnaryInvocable)
  34 ///
  35 /// Example
  36 /// -------
  37 ///
  38 ///     #include <boost/hof.hpp>
  39 ///     #include <cassert>
  40 ///     #include <iostream>
  41 ///     using namespace boost::hof;
  42 ///
  43 ///     struct sum_f
  44 ///     {
  45 ///         template<class T, class U>
  46 ///         T operator()(T x, U y) const
  47 ///         {
  48 ///             return x+y;
  49 ///         }
  50 ///     };
  51 ///
  52 ///     const pipable_adaptor<sum_f> sum = {};
  53 ///     int main() {
  54 ///         // Prints 3
  55 ///         int r = 1 | sum(2) | tap([](int i) { std::cout << i; }) | sum(2);
  56 ///         assert(r == 5);
  57 ///     }
  58 ///
  59
  60 #include <boost/hof/pipable.hpp>
  61 #include <boost/hof/apply.hpp>
  62 #include <boost/hof/detail/static_const_var.hpp>
  63
  64 namespace boost { namespace hof { namespace detail {
  65
  66 struct tap_f
  67 {
  68     template<class T, class F>
  69     constexpr T operator()(T&& x, const F& f) const
  70     BOOST_HOF_RETURNS_DEDUCE_NOEXCEPT((boost::hof::apply(f, x), BOOST_HOF_FORWARD(T)(x)))
  71     {
  72         return boost::hof::apply(f, x), BOOST_HOF_FORWARD(T)(x);
  73     }
  74 };
  75
  76 }
  77
  78 BOOST_HOF_DECLARE_STATIC_VAR(tap, pipable_adaptor<detail::tap_f>);
  79
  80
  81 }} // namespace boost::hof
  82
  83 #endif