c++ - 当我尝试使用读取和写入时，为什么 fstream 无法正常工作？

Question

我正在尝试使用 fstream 和 map 创建本地文件数据库。我正在使用 fstream 类进行读写。但它正在创建一个空文件，我尝试使用运算符 >>、<< 和函数 read()、write()。而且它也不起作用。

这里是代码:

#include <iostream>
#include <map>
#include <fstream>
#include <string>
using str = std::string;
class MapDB{
    public:
        MapDB(str path){
            this->dbfile.open(path, std::fstream::in | std::fstream::out | std::fstream::trunc);
        }
        void PushValue(str key, str value){
            this->Load();
            this->db[key] = value;
            this->Save();
        }
        str GetValue(str key){
            this->Load();
            return this->db[key];
        }
        void RemoveKey(str key){
            this->db.erase(key);
            this->Save();
        }
        ~MapDB(){
            this->dbfile.close();
        }
    private:
        void Load(){
            //this->dbfile.read((char*)&this->db, sizeof(MapDB));
            this->dbfile >> (char*)&this->db;
        }
        void Save(){
            this->dbfile.clear();
            //this->dbfile.write((char*)&this->db, sizeof(MapDB));
            this->dbfile << (char*)&this->db;
        }
        std::fstream dbfile;
        std::map<str, str> db;
};
int main(int argc, char *argv[]){
    std::map<str, str> mydb;
    MapDB mydata("/path/to/file/data.data");
    mydata.PushValue("key", "value");
    std::cout << mydata.GetValue("key") << std::endl;
}

有什么想法吗？

Answer 1

您的代码中存在多个问题:

1. std::map<>不是微不足道的，无法按照您尝试的方式序列化去做吧。您必须自己逐项序列化它。
2. dbfile.clear();只清除流的错误标志可能不是您想要做的。
3. Load() 应该定位读取光标，Save() 应该定位读取光标写入光标。
4. 您的构造函数会截断文件。所以没有机会读书由 MapDB 的另一个实例编写的内容。 (也许这是故意的)

我不敢冒险说这个列表已经接近完整。希望这个例子能给您一些提示:

class MyDB
{
public:
    // constructor.
    // creates or opens the file in binary mode (because we store binary data like the number of items and, the length of the strings).
    // truncates the file on open if the flag 'truncateOnOpen' is set.
    MyDB(const std::string& filename, bool truncateOnOpen)
        : dbfile(filename, std::fstream::binary | std::fstream::in | std::fstream::out | (truncateOnOpen ? std::fstream::trunc : 0))
    {}

    void Load()
    {
        // drop old database content
        db.clear();

        // position read cursor to the beginning of the file
        dbfile.seekg(0);

        // read the number of entries
        size_t entries = 0;
        dbfile.read(reinterpret_cast<char*>(&entries), sizeof(entries));

        // read key and value for each entry
        std::string key, value;
        for (size_t i = 0; i < entries; ++i)
        {
            readString(&key);
            readString(&value);
            db[key] = value;
        }
    }

    void Save()
    {
        // position the write cursor to the beginning of the file
        dbfile.seekp(0);

        // write thenumber of entries
        size_t entries = db.size();
        dbfile.write(reinterpret_cast<const char*>(&entries), sizeof(entries));

        // write key and value for each entry
        for (auto& it : db)
        {
            writeString(it.first);
            writeString(it.second);
        }
    }

private:
    // reads a single string from the file
    bool readString(std::string* target)
    {
        // read the length of the string stored in the file
        size_t len;
        if (dbfile.read(reinterpret_cast<char*>(&len), sizeof(len)).fail())
            return false;

        // preallocate memory for the string
        target->resize(len);

        // read the string from the file
        return dbfile.read(&target->front(), target->size()).good();
    }

    // writes a single string to the file
    void writeString(const std::string& source)
    {
        // write the length of the string to the file
        size_t len = source.size();
        dbfile.write(reinterpret_cast<char*>(&len), sizeof(len));
        
        // write the string itself to the file
        dbfile.write(source.data(), source.size());
    }

private:
    std::fstream dbfile;

public:
    std::map<std::string, std::string> db;
};