C ++中隨機訪問歸檔的奇怪問題

Question

這是一個簡單的隨機存取文件程序。 問題出現在我想隨機寫數據的地方。 如果我在文件中的任何位置寫入任何內容，則先前的記錄將設置為0。正在輸入的最后1個當前具有正確的值，其他所有值都為= 0。

這是代碼

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

using namespace std;

class name
{
int id;
int pass;
public:
    void writeBlank();
    void writedata();
    void readdata();
    void readall();
    int getid()
{
    return id;
}

int getpass()
{
    return pass;
}

void setid(int i)
{
    id=i;
}

void setpass(int p)
{
    pass=p;
}
};
void name::writeBlank()
{
name person;
person.setid(0);
person.setpass(0);

int i;

ofstream out("pass.txt",ios::binary);

if ( !out ) 
{
    cout << "File could not be opened." << endl;
}

for(i=0;i<10;i++)  //make 10 records
{
    cout<<"Put pointer is at: "<<out.tellp()<<endl;
    cout<<"Blank record "<<i<<" is: "<<person.getid()<<" "<<person.getpass()<<" and size: "<<sizeof(person)<<endl;
    cout<<"Put pointer is at: "<<out.tellp()<<endl;
    out.write(reinterpret_cast< const char * >(&person),sizeof(name));

}
}

void name::writedata()
{
ofstream out("pass.txt",ios::binary|ios::out);
name n1;
int iD,p;
    cout<<"ID?";
    cin>>iD;
    n1.setid(iD);
    cout<<"Enter password";
    cin>>p;
    n1.setpass(p);

if (!out ) 
{
    cout << "File could not be opened." << endl;
}

out.seekp((n1.getid()-1)*sizeof(name),ios::beg);   //pointer moves to desired location where we have to store password according to its ID(index)
cout<<"File pointer is at: "<<out.tellp()<<endl;

out.write(reinterpret_cast<const char*> (&n1), sizeof(name));  //write on that pointed location
}

void name::readall()
{
name n1;

ifstream in("pass.txt",ios::binary);
if ( !in ) 
{
    cout << "File could not be opened." << endl;
}

in.read( reinterpret_cast<char *>(&n1), sizeof(name) );    
  while ( !in.eof() ) 
  {    
      // display record
      cout<<endl<<"password at this index is:"<<n1.getpass()<<endl;
      cout<<"File pointer is at: "<<in.tellg()<<endl;

      // read next from file                               
      in.read( reinterpret_cast< char * >(&n1), sizeof(name));                               
   } // end while
}

void name::readdata()
{
ifstream in("pass.txt",ios::binary);
if ( !in ) 
{
    cout << "File could not be opened." << endl;
}
in.seekg((getid()-1)*sizeof(name));   //pointer moves to desired location where we have to read password according to its ID(index)
cout<<"File pointer is at: "<<in.tellg()<<endl;
in.read((char* )this,sizeof(name));  //reads from that pointed location
cout<<endl<<"password at this index is:"<<getpass()<<endl;
}


int main()
{

name n1;
cout<<"Enter 0 to write blank records"<<endl;
cout<<"Enter 1 for new account"<<endl;
cout<<"Enter 2 to login"<<endl;
cout<<"Enter 3 to read all"<<endl;
cout<<"Enter 9 to exit"<<endl;
int option;
cin>>option;
while(option==0 || option==1 || option==2 || option==3)
{
    if (option == 0)
        n1.writeBlank();

    if(option==1)
    {
        /*int iD,p;
        cout<<"ID?";
        cin>>iD;
        n1.setid(iD);
        cout<<"Enter password";
        cin>>p;
        n1.setpass(p);*/
        n1.writedata();
    }
    int ind;

    if(option==2)
    {
        cout<<"Index?";
        cin>>ind;
        n1.setid(ind);
        n1.readdata();

    }
    if(option == 3)
        n1.readall();

    cout<<"Enter 0 to write blank records"<<endl;
    cout<<"Enter 1 for new account"<<endl;
    cout<<"Enter 2 to login"<<endl;
    cout<<"Enter 3 to read all"<<endl;
    cout<<"Enter 9 to exit"<<endl;
    cin>>option;

}
}

我不能理解Y以前的記錄是0。

Answer 1

編輯：在MSDN上找到文檔。

ios :: trunc - 如果文件已存在，則丟棄其內容。 如果指定了ios :: out並且未指定ios :: ate，ios :: app和ios：in，則隱含此模式 。

來自http://msdn.microsoft.com/en-us/library/aa277521%28v=vs.60%29.aspx ，但由於官方標准沒有改變，這應該是有效的。

Answer 2

包括

包括

包括

using namespace std;

class name {int id; int pass; 公共：void writeBlank（）; void writedata（）; void readdata（）; void readall（）; int getid（）{返回ID； }

int getpass（）{返回密碼； }

void setid（int i）{id = i; }

void setpass（int p）{pass = p; }}; 無效name :: writeBlank（）{name person; person.setid（0）; person.setpass（0）;

int i;

ofstream out（“ pass.txt”，ios :: binary）;

if（！out）{cout <<“無法打開文件。” << endl; }

for（i = 0; i <10; i ++）//制作10條記錄{cout <<“把指針放在：”<（＆person），sizeof（name））;

}}

void name :: writedata（）{ofstream out（“pass.txt”，ios :: binary | ios :: ate）; 名字n1; int iD，p; COUT << “ID？”; CIN >>標識; n1.setid（ID）; cout <<“輸入密碼”; CIN >> P; n1.setpass（P）;

如果（！out）{cout <<“無法打開文件。” << endl; }

out.seekp（（n1.getid（） - 1）*的sizeof（名稱），IOS :: BEG）; //指針移動到我們必須根據其ID（索引）存儲密碼的所需位置cout <<“文件指針位於：”<

out.write（reinterpret_cast（＆n1），sizeof（name））; //寫在那個尖頭的位置}

void name :: readall（）{name n1;

ifstream in（“ pass.txt”，ios :: binary）; if（！in）{cout <<“無法打開文件。” << endl; }

in.read（reinterpret_cast（＆n1），sizeof（name））;
while（！in.eof（））{
//顯示記錄cout <

  // read next from file                               
  in.read( reinterpret_cast< char * >(&n1), sizeof(name));                               

} //結束於}

void name :: readdata（）{ifstream in（“ pass.txt”，ios :: binary）; if（！in）{cout <<“無法打開文件。” << endl; } in.seekg（（getid（） - 1）sizeof（name））; //指針移動到我們必須根據其ID（索引）讀取密碼所需的位置cout <<“文件指針位於：”<）this，sizeof（name））; //從該指向的位置讀取cout <

int main（）{

名字n1; cout <<“輸入0以寫入空白記錄” <>選項； while（選項== 0 ||選項== 1 ||選項== 2 ||選項== 3）{if（選項== 0）n1.writeBlank（）;

if(option==1)
{
    /*int iD,p;
    cout<<"ID?";
    cin>>iD;
    n1.setid(iD);
    cout<<"Enter password";
    cin>>p;
    n1.setpass(p);*/
    n1.writedata();
}
int ind;

if(option==2)
{
    cout<<"Index?";
    cin>>ind;
    n1.setid(ind);
    n1.readdata();

}
if(option == 3)
    n1.readall();

cout<<"Enter 0 to write blank records"<<endl;
cout<<"Enter 1 for new account"<<endl;
cout<<"Enter 2 to login"<<endl;
cout<<"Enter 3 to read all"<<endl;
cout<<"Enter 9 to exit"<<endl;
cin>>option;

}}