[英]c++ Read/Write class from/to binary file
我需要将一个类写入二进制文件,然后再将其读回。
我有Triangle
和BinaryFile
类,以及其他一些类。 我不确定我是写错还是阅读不正确。 读取时发生错误。 调试后,我认为它为我的私有变量获取了不合适的数据。 如果有人能给我一些有关如何使其正常工作的建议,我将感到非常高兴。
我不确定是否应该粘贴整个代码,所以我将为您提供一小段代码。 以防万一,这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,英语说得不太好,所以我为自己的错误事先表示歉意。
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;
}
};
答案取决于您是否只是为了了解文件的工作方式而做,还是保存到文件中只是偶然的事,而又不在乎文件的工作方式。
如果您只想保存和还原内容,而又不关心它的工作方式,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,那么您将需要使用自己的读写功能。 我制作了一个示例程序,将解释其工作方式:
#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;
}
由于您的源代码很大且缺少数据文件,因此重现该错误并不容易。 但是快速检查显示您使用bloc操作读取和写入二进制数据:
Triangle trig;
...
File.read((char*)&trig, sizeof(Triangle));
不幸的是,这种方法仅在要保存/加载的对象属于可微复制的类时才有效,如以下代码所示:
if (is_trivially_copyable<Triangle>::value)
cout << "Triangle is trivially copyable" << endl;
else cout << "Triangle is not trivially copyable" << endl;
因此,您必须逐字段序列化对象内容写入字段,而不是使用bloc操作。 此序列化常见问题解答应帮助您考虑替代方案。
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