130 lines
3.1 KiB
C++
130 lines
3.1 KiB
C++
// Copyright Louis Dionne 2013-2017
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// Distributed under the Boost Software License, Version 1.0.
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// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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#include <boost/hana/assert.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/at_key.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/concept/struct.hpp>
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#include <boost/hana/define_struct.hpp>
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#include <boost/hana/for_each.hpp>
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#include <boost/hana/keys.hpp>
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#include <boost/hana/length.hpp>
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#include <boost/hana/not_equal.hpp>
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#include <boost/hana/tuple.hpp>
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#include <iostream>
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#include <limits>
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#include <sstream>
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#include <string>
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#include <type_traits>
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namespace hana = boost::hana;
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struct Car {
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BOOST_HANA_DEFINE_STRUCT(Car,
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(std::string, brand),
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(std::string, model)
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);
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};
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struct Person {
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BOOST_HANA_DEFINE_STRUCT(Person,
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(std::string, name),
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(std::string, last_name),
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(int, age)
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);
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};
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template <typename T>
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std::enable_if_t<std::is_same<T, int>::value,
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T> from_json(std::istream& in) {
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T result;
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in >> result;
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return result;
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}
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template <typename T>
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std::enable_if_t<std::is_same<T, std::string>::value,
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T> from_json(std::istream& in) {
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char quote;
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in >> quote;
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T result;
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char c;
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while (in.get(c) && c != '"') {
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result += c;
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}
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return result;
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}
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template <typename T>
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std::enable_if_t<hana::Struct<T>::value,
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T> from_json(std::istream& in) {
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T result;
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char brace;
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in >> brace;
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// CAREFUL: This only works if the input JSON contains the fields in the
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// same order as the struct declares them.
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hana::for_each(hana::keys(result), [&](auto key) {
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in.ignore(std::numeric_limits<std::streamsize>::max(), ':');
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auto& member = hana::at_key(result, key);
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using Member = std::remove_reference_t<decltype(member)>;
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member = from_json<Member>(in);
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});
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in >> brace;
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return result;
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}
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template <typename Xs>
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std::enable_if_t<hana::Sequence<Xs>::value,
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Xs> from_json(std::istream& in) {
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Xs result;
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char bracket;
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in >> bracket;
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hana::length(result).times.with_index([&](auto i) {
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if (i != 0u) {
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char comma;
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in >> comma;
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}
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auto& element = hana::at(result, i);
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using Element = std::remove_reference_t<decltype(element)>;
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element = from_json<Element>(in);
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});
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in >> bracket;
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return result;
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}
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int main() {
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std::istringstream json(R"EOS(
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[
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{
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"name": "John",
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"last_name": "Doe",
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"age": 30
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},
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{
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"brand": "BMW",
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"model": "Z3"
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},
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{
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"brand": "Audi",
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"model": "A4"
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}
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]
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)EOS");
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auto actual = from_json<hana::tuple<Person, Car, Car>>(json);
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auto expected = hana::make_tuple(Person{"John", "Doe", 30},
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Car{"BMW", "Z3"},
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Car{"Audi", "A4"});
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BOOST_HANA_RUNTIME_CHECK(actual == expected);
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}
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