c++ - 序列化地图?
问题描述
我正在使用谷物库将我的类序列化为文件,但遇到了麻烦std::map
- 特别是使用std::filesystem::path
.
假设我有一个Object
只包含 a 的类,map<fs::path, fs::path>
以及谷物所需的序列化函数:
struct Object
{
map<fs::path, fs::path> _map;
template<class Archive>
void serialize(Archive & archive)
{
archive(_map);
}
friend ostream& operator<<(ostream& outs, const Object c)
{
for (auto const &pair: c._map)
std::cout << "{" << pair.first << ": " << pair.second << "}\n";
return outs;
}
};
在我的主要功能中,我有:
int main()
{
// create Object
cout << "Creating object..." << endl;
Object o;
fs::path a = "bye.txt";
fs::path b = "hello.txt";
o._map[a] = b;
o._map[b] = a;
cout << "Object created: " << endl;
cout << o;
// serialize
cout << "Serializing object...." << endl;
stringstream ss;
cereal::BinaryOutputArchive oarchive(ss);
oarchive(o);
cout << "Object serialized." << endl;
// write to file
cout << "Writing serialized object to file...." << endl;
ofstream file("serialized_object");
file << ss.str();
file.close();
cout << "Object written to file." << endl;
// read from file
cout << "Reading from file..." << endl;
stringstream ss2;
fs::path ins = "serialized_object";
ifstream file_stream(ins, ios::binary);
ss2 << file_stream.rdbuf();
cereal::BinaryInputArchive iarchive(ss2);
Object out;
iarchive(out);
cout << "Object read from file." << endl;
cout << out;
}
在我的输出中,当它从序列化文件中读取时,我看到了错误:
Creating object...
Object created:
{"bye.txt": "hello.txt"}
{"hello.txt": "bye.txt"}
Serializing object....
Object serialized.
Writing serialized object to file....
Object written to file.
Reading from file...
terminate called after throwing an instance of 'cereal::Exception'
what(): Failed to read 2573 bytes from input stream! Read 28
我的包括:
#include <iostream>
#include <filesystem>
#include <fstream>
#include <string.h>
#include <map>
#include "cereal/types/map.hpp"
#include "cereal/archives/binary.hpp"
#include "cereal/types/string.hpp"
为了能够序列化,我在开头包含了以下代码fs::path
:
namespace std
{
namespace filesystem
{
template<class Archive>
void CEREAL_LOAD_MINIMAL_FUNCTION_NAME(const Archive&, path& out, const string& in)
{
out = in;
}
template<class Archive>
string CEREAL_SAVE_MINIMAL_FUNCTION_NAME(const Archive& ar, const path& p)
{
return p.string();
}
}
}
我确定我错过了一些明显的东西,因为这是我第一次使用谷物。有没有人知道我为什么会遇到这个问题?
解决方案
根据谷物文档,ios::binary
在使用 BinaryArchive 时必须始终使用该标志。这是他们文档中此页面的特定部分:
当使用二进制存档和文件流 (std::fstream) 时,请记住在构造流时指定二进制标志 (std::ios::binary)。这可以防止流将您的数据解释为 ASCII 字符并更改它们。
另一个问题是基于 Cereal 如何保证序列化已完成。根据下面的此链接,在尝试读取存档之前,输出存档对象应超出范围(调用析构函数)。他们使用RAIIifstream
方法,就像这里ofstream
提到的一样。
您可以通过允许 Cereal 库在内部执行写入和读取来大大简化您的代码。这是您的主要功能的修改版本:
int main()
{
// create Object
cout << "Creating object..." << endl;
Object o;
fs::path a = "bye.txt";
fs::path b = "hello.txt";
o._map[a] = b;
o._map[b] = a;
cout << "Object created: " << endl;
cout << o;
cout << "Serializing object...." << endl;
// scope boundary (a function call would be cleaner)
{
ofstream ofstm("serialized_object", ios::binary);
cereal::BinaryOutputArchive oarchive(ofstm);
// serialize
oarchive(o);
}
cout << "Object serialized and written." << endl;
// deserialize the object by reading from the archive
Object out;
// scope boundary (a function would be cleaner)
{
ifstream istm("serialized_object", ios::binary);
cereal::BinaryInputArchive iarchive(istm);
//deserialize
iarchive(out);
}
cout << "Object read from file." << endl;
cout << out;
}
我希望这有帮助。请在使用前对这一切进行测试。
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