1 // Copyright 2016-2021 Antony Polukhin
2
3 // Distributed under the Boost Software License, Version 1.0.
4 // (See the accompanying file LICENSE_1_0.txt
5 // or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)
6
7 //[pfr_sample_printing
8 /*`
9 The following example shows how to write your own io-manipulator for printing:
10 */
11 #include <boost/pfr/ops.hpp>
12 #include <ostream>
13
14 namespace my_ns {
15
16 /// Usage:
17 /// struct foo {std::uint8_t a, b;};
18 /// ...
19 /// std::cout << my_ns::my_io(foo{42, 22});
20 ///
21 /// Output: 42, 22
22 template <class T>
23 auto my_io(const T& value);
24
25 namespace detail {
26 // Helpers to print individual values
27 template <class T>
print_each(std::ostream & out,const T & v)28 void print_each(std::ostream& out, const T& v) { out << v; }
print_each(std::ostream & out,std::uint8_t v)29 void print_each(std::ostream& out, std::uint8_t v) { out << static_cast<unsigned>(v); }
print_each(std::ostream & out,std::int8_t v)30 void print_each(std::ostream& out, std::int8_t v) { out << static_cast<int>(v); }
31
32 // Structure to keep a reference to value, that will be ostreamed lower
33 template <class T>
34 struct io_reference {
35 const T& value;
36 };
37
38 // Output each field of io_reference::value
39 template <class T>
operator <<(std::ostream & out,io_reference<T> && x)40 std::ostream& operator<<(std::ostream& out, io_reference<T>&& x) {
41 const char* sep = "";
42
43 boost::pfr::for_each_field(x.value, [&](const auto& v) {
44 out << std::exchange(sep, ", ");
45 detail::print_each(out, v);
46 });
47 return out;
48 }
49 }
50
51 // Definition:
52 template <class T>
my_io(const T & value)53 auto my_io(const T& value) {
54 return detail::io_reference<T>{value};
55 }
56
57 } // namespace my_ns
58 //] [/pfr_sample_printing]
59
60
61 #include <iostream>
62 #include <sstream>
63
main()64 int main() {
65 struct foo {std::uint8_t a, b;};
66
67 std::ostringstream oss;
68 oss << my_ns::my_io(foo{42, 22});
69
70 if (oss.str() != "42, 22") {
71 return 1;
72 }
73
74 struct two_big_strings {
75 std::string first;
76 std::string second;
77 };
78
79 #if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_LOOPHOLE
80 const char* huge_string = "Some huge string that should not fit into std::string SSO."
81 "And by 'huge' I mean really HUGE string with multiple statements and a lot of periods........."
82 ;
83
84 oss.str({});
85 oss << my_ns::my_io(two_big_strings{
86 huge_string, huge_string
87 });
88
89 if (oss.str() != huge_string + std::string(", ") + huge_string) {
90 return 2;
91 }
92 #endif
93
94 return 0;
95 }
96