c ++从/到二进制文件读/写类

Question

我需要将一个类写入一个二进制文件，然后我需要读取它。

我有Triangle和BinaryFile班级，还有一些其他班级。我不确定我是写错了还是读错了。读取时出现错误。调试后，我认为它为我的私有变量获取了不适当的数据。如果有人能给我一些有关如何使其正常工作的建议，我将非常高兴。

我不确定是否应该粘贴整个代码，所以我会给你一小段代码。以防万一，这里是我的源代码的下载链接：

https://my.pcloud.com/publink/show?code=XZJ7CYZbsLWLglqV5p83csijcEUTFqqpM3k

我是编程新手，我的英语不太好，所以我提前为我的错误道歉。

class Point
{
private:
    int x;
    int y;
};

class Figure
{
private:
    string name;
    string type;
};

class Triangle: public Figure
{
private:
    Point p1, p2, p3;
};

class BinaryFile
{
private:
    string FileName;
    fstream File;
public:
    //...
    void AddNewFigure(istream& stream)
    {       
        File.open(this->FileName, ios::binary | ios::app);
        if(!this->File)
        {
            cerr<<"File error <"<<this->FileName<<">\n";
            exit(1);
        }
        Triangle fig;
        fig.MakeNewFigure(stream);
        File.write((char*)&fig, sizeof(Triangle));
        File.close();
    }

    Triangle GetTriangle()
    {
        Triangle trig;
        Point p;
        string str(""); int x(0);
        File.open(this->FileName, ios::binary | ios::in);
        if(!this->File)
        {
            cerr<<"File error <"<<this->FileName<<">\n";
            exit(1);
        }
        File.read((char*)&trig, sizeof(Triangle));
        File.close();
        return trig;
    }
};

Answer 1

答案取决于您这样做是为了了解文件的工作原理，还是保存到文件只是偶然的，您并不关心它是如何工作的。

如果您只想保存和恢复这些内容，而您不关心它是如何工作的，那么请使用第三方库。有很多很多。

如果您想学习如何对文件进行读写，那么您将需要创建自己的读写函数。我制作了一个示例程序来解释它是如何工作的：

#include <string>
#include <fstream>
#include <iostream>

class Point
{
private:
    int x;
    int y;
public:
    Point():x(0),y(0){}
    Point(int x,int y):x(x),y(y){}

    void write(std::ostream& f)
    {
        // We can just write out the bytes for x and y because
        // they are primitive types stored in the class
        f.write( (char*)&x, sizeof(x) );
        f.write( (char*)&y, sizeof(y) );
    }
    void read(std::istream& f)
    {
        // We can just read the bytes directly into x and y because
        // they are primitive types stored in the class
        f.read( (char*)&x, sizeof(x) );
        f.read( (char*)&y, sizeof(y) );
    }
};

class Figure
{
private:
    std::string name;
    std::string type;
public:
    Figure(){}
    Figure(std::string name,std::string type):name(name),type(type){}

    void write(std::ostream& f)
    {
        size_t size;

        // we need to store the data from the string along with the size
        // because to restore it we need to temporarily read it somewhere
        // before storing it in the std::string (istream::read() doesn't
        // read directly to std::string)

        size = name.size();
        f.write( (char*)&size, sizeof(size_t) );
        f.write( (char*)name.c_str(), size );

        size = type.size();
        f.write( (char*)&size, sizeof(size_t) );
        f.write( (char*)type.c_str(), size );
    }
    void read(std::istream& f)
    {
        size_t size;
        char *data;

        // when we read the string data we need somewhere to store it
        // because we std::string isn't a primitive type.  So we read
        // the size, allocate an array, read the data into the array,
        // load the std::string, and delete the array

        f.read( (char*)&size, sizeof(size) );
        data = new char[size+1];
        f.read( data, size );
        data[size]='\0';
        name = data;
        delete data;

        f.read( (char*)&size, sizeof(size) );
        data = new char[size+1];
        f.read( data, size );
        data[size]='\0';
        type = data;
        delete data;
    }
};

class Triangle: public Figure
{
private:
    Point p1, p2, p3;
public:
    Triangle(){}
    Triangle(Point x,Point y,Point z,Figure f):p1(x),p2(y),p3(z),Figure(f){}


    void write(std::ostream& f)
    {
        // First write the base class then write the members of this class
        Figure::write(f);
        p1.write(f);
        p2.write(f);
        p3.write(f);
    }
    void read(std::istream& f)
    {
        // First read the base class then read the members of this class
        Figure::read(f);
        p1.read(f);
        p2.read(f);
        p3.read(f);
    }
};

class BinaryFile
{
private:
    std::string FileName;
    std::fstream File;
public:
    BinaryFile(std::string FileName) : FileName(FileName) {};
    void WriteTriangle()
    {
        File.open(FileName, std::ios::binary | std::ios::out);
        if(!File)
        {
            std::cerr<<"File error <"<<FileName<<">\n";
            exit(1);
        }
        Triangle trig({1,2},{3,4},{5,6},{"name","type"}); // something new
        trig.write(File);
        File.close();
    }

    Triangle ReadTriangle()
    {
        File.open(FileName, std::ios::binary | std::ios::in);
        if(!File)
        {
            std::cerr<<"File error <"<<FileName<<">\n";
            exit(1);
        }
        Triangle trig; // default values
        trig.read(File);
        File.close();
        return trig;
    }
};

main()
{
    BinaryFile bin("file.bin");
    bin.WriteTriangle();
    Triangle trig = bin.ReadTriangle();
    // at this point trig has the values we stored
    return 0;
}

Answer 2

您正在寻找的是序列化应保存到文件的类/数据。有几个库已针对时间和内存消耗进行了优化。您介意使用第三方库吗？

如果没有，请看看boost serialization、谷物甚至 Google 的ProtoBuf。如果您使用 C++11，我认为 Cereal 是一个好的开始。

如果您想编写自己的序列化，则必须考虑到对于每个具有动态大小的对象（例如字符串），您还需要将对象的大小保存到文件中。欲了解更多信息，请查看这里：

/sf/answers/770251331/