Дело в том что я сейчас только изучаю файловый ввод/вывод, у меня один пример и тщательно изучаю его. Пример такой…. | Код |
// empl_io.cpp
// performs file I/O on employee objects
// handles different sized objects
#include <fstream> //for file-stream functions
#include <iostream>
#include <typeinfo> //for typeid()
using namespace std;
#include <process.h> //for exit()
const int LEN = 32; //maximum length of last names
const int MAXEM = 100; //maximum number of employees
enum employee_type {tmanager, tscientist, tlaborer};
////////////////////////////////////////////////////////////////
class employee //employee class
{
private:
char name[LEN]; //employee name
unsigned long number; //employee number
static int n; //current number of employees
static employee* arrap[]; //array of ptrs to emps
public:
virtual void getdata()
{
cin.ignore(10, '\n');
cout << " Enter last name: "; cin >> name;
cout << " Enter number: "; cin >> number;
}
virtual void putdata()
{
cout << "\n Name: " << name;
cout << "\n Number: " << number;
}
virtual employee_type get_type(); //get type
static void add(); //add an employee
static void display(); //display all employees
static void read(); //read from disk file
static void write(); //write to disk file
};
//--------------------------------------------------------------
//static variables
int employee::n; //current number of employees
employee* employee::arrap[MAXEM]; //array of ptrs to emps
////////////////////////////////////////////////////////////////
//manager class
class manager : public employee
{
private:
char title[LEN]; //"vice-president" etc.
double dues; //golf club dues
public:
void getdata()
{
employee::getdata();
cout << " Enter title: "; cin >> title;
cout << " Enter golf club dues: "; cin >> dues;
}
void putdata()
{
employee::putdata();
cout << "\n Title: " << title;
cout << "\n Golf club dues: " << dues;
}
};
////////////////////////////////////////////////////////////////
//scientist class
class scientist : public employee
{
private:
int pubs; //number of publications
public:
void getdata()
{
employee::getdata();
cout << " Enter number of pubs: "; cin >> pubs;
}
void putdata()
{
employee::putdata();
cout << "\n Number of publications: " << pubs;
}
};
////////////////////////////////////////////////////////////////
//laborer class
class laborer : public employee
{
};
////////////////////////////////////////////////////////////////
//add employee to list in memory
void employee::add()
{
char ch;
cout << "'m' to add a manager"
"\n's' to add a scientist"
"\n'l' to add a laborer"
"\nEnter selection: ";
cin >> ch;
switch(ch)
{ //create specified employee type
case 'm': arrap[n] = new manager; break;
case 's': arrap[n] = new scientist; break;
case 'l': arrap[n] = new laborer; break;
default: cout << "\nUnknown employee type\n"; return;
}
arrap[n++]->getdata(); //get employee data from user
}
//--------------------------------------------------------------
//display all employees
void employee::display()
{
for(int j=0; j<n; j++)
{
cout << (j+1); //display number
switch( arrap[j]->get_type() ) //display type
{
case tmanager: cout << ". Type: Manager"; break;
case tscientist: cout << ". Type: Scientist"; break;
case tlaborer: cout << ". Type: Laborer"; break;
default: cout << ". Unknown type";
}
arrap[j]->putdata(); //display employee data
cout << endl;
}
}
//--------------------------------------------------------------
//return the type of this object
employee_type employee::get_type()
{
if( typeid(*this) == typeid(manager) )
return tmanager;
else if( typeid(*this)==typeid(scientist) )
return tscientist;
else if( typeid(*this)==typeid(laborer) )
return tlaborer;
else
{ cerr << "\nBad employee type"; exit(1); }
return tmanager;
}
//--------------------------------------------------------------
//write all current memory objects to file
void employee::write()
{
int size;
cout << "Writing " << n << " employees.\n";
ofstream ouf; //open ofstream in binary
employee_type etype; //type of each employee object
ouf.open("EMPLOY.DAT", ios::trunc | ios::binary);
if(!ouf)
{ cout << "\nCan't open file\n"; return; }
for(int j=0; j<n; j++) //for every employee object
{ //get its type
etype = arrap[j]->get_type();
//write type to file
ouf.write( (char*)&etype, sizeof(etype) );
switch(etype) //find its size
{
case tmanager: size=sizeof(manager); break;
case tscientist: size=sizeof(scientist); break;
case tlaborer: size=sizeof(laborer); break;
} //write employee object to file
ouf.write( (char*)(arrap[j]), size );
if(!ouf)
{ cout << "\nCan't write to file\n"; return; }
}
}
//--------------------------------------------------------------
//read data for all employees from file into memory
void employee::read()
{
int size; //size of employee object
employee_type etype; //type of employee
ifstream inf; //open ifstream in binary
inf.open("EMPLOY.DAT", ios::binary);
if(!inf)
{ cout << "\nCan't open file\n"; return; }
n = 0; //no employees in memory yet
while(true)
{ //read type of next employee
inf.read( (char*)&etype, sizeof(etype) );
if( inf.eof() ) //quit loop on eof
break;
if(!inf) //error reading type
{ cout << "\nCan't read type from file\n"; return; }
switch(etype)
{ //make new employee
case tmanager: //of correct type
arrap[n] = new manager;
size=sizeof(manager);
break;
case tscientist:
arrap[n] = new scientist;
size=sizeof(scientist);
break;
case tlaborer:
arrap[n] = new laborer;
size=sizeof(laborer);
break;
default: cout << "\nUnknown type in file\n"; return;
} //read data from file into it
inf.read( (char*)arrap[n], size );
if(!inf) //error but not eof
{ cout << "\nCan't read data from file\n"; return; }
n++; //count employee
} //end while
cout << "Reading " << n << " employees\n";
}
////////////////////////////////////////////////////////////////
int main()
{
char ch;
while(true)
{
cout << "'a' -- add data for an employee"
"\n'd' -- display data for all employees"
"\n'w' -- write all employee data to file"
"\n'r' -- read all employee data from file"
"\n'x' -- exit"
"\nEnter selection: ";
cin >> ch;
switch(ch)
{
case 'a': //add an employee to list
employee::add(); break;
case 'd': //display all employees
employee::display(); break;
case 'w': //write employees to file
employee::write(); break;
case 'r': //read all employees from file
employee::read(); break;
case 'x': exit(0); //exit program
default: cout << "\nUnknown command";
} //end switch
} //end while
return 0;
} //end main()
|
потом там написано :” Мы умеем определять класс объекта, находящегося в памяти, но как узнать класс объекта, если мы собираемся читать его из файла? Поэтому при записи данных на диск необходимо записывать код (перечисляемую переменную типа employee_type) прямо перед данными объекта. До начала чтения из файла надо прочитать его значение и создать объект соответствующего типа. И только после этого можно копировать данные из файла в этот новый объект. ” мне интересно узнать если определит sizeof(employee) все байты равны, если равны значит все байты совпадают. Тогда почему нам определит класс объекта…….. |
class employee, fstream 
Загрузка ...
