阅读随机访问文件

Question

我开发了一个C++应用程序，用于在随机访问文件上读取和写入数据。 （我使用Visual C++ 2010）

这里是我的程序：

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


using namespace std; 
class A 
{ 
public : 
    int a; 
    string b; 
    A(int num , string text) 
    { 
     a = num; 
     b = text; 
    } 
}; 

int main() 
{ 
    A myA(1,"Hello"); 
    A myA2(2,"test"); 

    cout << "Num: " << myA.a<<endl<<"Text: "<<myA.b<<endl; 

    wofstream output; //I used wfstream , becuase I need to wite a unicode file 
    output.open("12542.dat" , ios::binary); 
    if(! output.fail()) 
    { 
     output.write((wchar_t *) &myA , sizeof(myA)); 
     cout << "writing done\n"; 
      output.close(); 

    } 
    else 
    { 
     cout << "writing failed\n"; 
    } 


    wifstream input; 
    input.open("12542.dat" , ios::binary); 
    if(! input.fail()) 
    { 
    input.read((wchar_t *) &myA2 , sizeof(myA2)); 
    cout << "Num2: " << myA2.a<<endl<<"Text2: "<<myA2.b<<endl; 
    cout << "reading done\n"; 
    } 

    else 
    { 
     cout << "reading failed\n"; 
    } 

    cin.get(); 
} 

和输出是：

Num: 1 
Text: Hello 
writing done 
Num2: 1 
Text2: test 
reading done 

但我希望Text2: Hello。 有什么问题？

顺便说一句，我如何在我的课程内（在一个函数中）output.write？

感谢

Answer 1

A不POD，你不能粗暴地投非POD对象char*然后写入流。 需要序列A，例如：

class A 
{ 
public : 
    int a; 
    wstring b; 
    A(int num , wstring text) 
    { 
     a = num; 
     b = text; 
    } 
}; 

std::wofstream& operator<<(std::wofstream& os, const A& a) 
{ 
    os << a.a << " " << a.b; 
    return os; 
} 

int main() 
{ 
    A myA(1, L"Hello"); 
    A myA2(2, L"test"); 

    std::wcout << L"Num: " << myA.a<<endl<<L"Text: "<<myA.b<<endl; 

    wofstream output; //I used wfstream , becuase I need to wite a unicode file 
    output.open(L"c:\\temp\\12542.dat" , ios::binary); 
    if(! output.fail()) 
    { 
     output << myA; 
     wcout << L"writing done\n"; 
     output.close(); 
    } 
    else 
    { 
     wcout << "writing failed\n"; 
    } 

    cin.get(); 
} 

此示例对象序列化到妙文件，你可以想想如何读出来。