使用boost pfr 遍历结构体

3rd/pfr/example/sample_printing.cpp

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+// Copyright 2016-2023 Antony Polukhin
+
+// Distributed under the Boost Software License, Version 1.0.
+// (See the accompanying file LICENSE_1_0.txt
+// or a copy at <http://www.boost.org/LICENSE_1_0.txt>.)
+
+//[pfr_sample_printing
+/*`
+    The following example shows how to write your own io-manipulator for printing:
+*/
+#include <boost/pfr/ops.hpp>
+#include <ostream>
+
+namespace my_ns {
+
+/// Usage:
+///     struct foo {std::uint8_t a, b;};
+///     ...
+///     std::cout << my_ns::my_io(foo{42, 22});
+///
+/// Output: 42, 22
+template <class T>
+auto my_io(const T& value);
+
+namespace detail {
+    // Helpers to print individual values
+    template <class T>
+    void print_each(std::ostream& out, const T& v) { out << v; }
+    void print_each(std::ostream& out, std::uint8_t v) { out << static_cast<unsigned>(v); }
+    void print_each(std::ostream& out, std::int8_t v) { out << static_cast<int>(v); }
+
+    // Structure to keep a reference to value, that will be ostreamed lower
+    template <class T>
+    struct io_reference {
+        const T& value;
+    };
+
+    // Output each field of io_reference::value
+    template <class T>
+    std::ostream& operator<<(std::ostream& out, io_reference<T>&& x) {
+        const char* sep = "";
+
+        boost::pfr::for_each_field(x.value, [&](const auto& v) {
+            out << std::exchange(sep, ", ");
+            detail::print_each(out, v);
+        });
+        return out;
+    }
+}
+
+// Definition:
+template <class T>
+auto my_io(const T& value) {
+    return detail::io_reference<T>{value};
+}
+
+} // namespace my_ns
+//] [/pfr_sample_printing]
+
+
+#include <iostream>
+#include <sstream>
+
+int main() {
+    struct foo {std::uint8_t a, b;};
+
+    std::ostringstream oss;
+    oss << my_ns::my_io(foo{42, 22});
+
+    if (oss.str() != "42, 22") {
+        return 1;
+    }
+
+    struct two_big_strings {
+        std::string first;
+        std::string second;
+    };
+
+#if BOOST_PFR_USE_CPP17 || BOOST_PFR_USE_LOOPHOLE
+    const char* huge_string = "Some huge string that should not fit into std::string SSO."
+        "And by 'huge' I mean really HUGE string with multiple statements and a lot of periods........."
+    ;
+
+    oss.str({});
+    oss << my_ns::my_io(two_big_strings{
+        huge_string, huge_string
+    });
+
+    if (oss.str() != huge_string + std::string(", ") + huge_string) {
+        return 2;
+    }
+#endif
+
+    return 0;
+}