c++ - 将 write_xml 提升到包含在 back_insert_device 中的向量会产生 0 大小的向量

Question

在下面的代码中，为什么向量 'v' 的大小为 0？如果将 write_xml 传递给 stringstream，则代码可以正常工作。但我宁愿在最终转储文本 XML 时不必支付字符串分配的成本。

#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>
#include <vector>
#include <iostream>

using namespace boost::iostreams;

int main()
{
  boost::property_tree::ptree pt;
  pt.put("file.name", "testf");
  pt.put("file.type", "xml");
  pt.put("file.size", 0);
  std::vector<char> v;
  back_insert_device<std::vector<char>> sink{v};
  stream<back_insert_device<std::vector<char>>> os{sink};
  boost::property_tree::write_xml(os, pt);
  std::cout << v.size() << std::endl;
}

标签： c++boost

Answer 1

您忘记刷新流。您可以os.flush()或只是让析构函数处理它：

住在科利鲁

#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/xml_parser.hpp>
#include <iostream>
#include <vector>

using namespace boost::iostreams;

int main()
{
    boost::property_tree::ptree pt;
    pt.put("file.name", "testf");
    pt.put("file.type", "xml");
    pt.put("file.size", 0);

    std::vector<char> v;
    {
        back_insert_device<std::vector<char>>         sink{v};
        stream<back_insert_device<std::vector<char>>> os{sink};

        boost::property_tree::write_xml(os, pt);
    }
    std::cout << v.size() << std::endl;
}

印刷

100

---

奖金

一如既往，使用 JSON 库！

住在科利鲁

#include <boost/json.hpp>
#include <boost/json/src.hpp>
#include <iostream>

namespace json = boost::json;

int main() {
    json::object file{
        {"name", "testf"},
        {"type", "xml"},
        {"size", 0},
    };

    auto v = serialize(json::object{{"file", file}});
    std::cout << v.size() << ": " << v << std::endl;
}

印刷

47: {"file":{"name":"testf","type":"xml","size":0}}