ceph/src/boost/libs/pfr/example/sample_printing.cpp

   1 // Copyright 2016-2020 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>
  28     void print_each(std::ostream& out, const T& v) { out << v; }
  29     void print_each(std::ostream& out, std::uint8_t v) { out << static_cast<unsigned>(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>
  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>
  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
  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 }