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>
operator ()boost::hof::detail::tap_f69     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
84