使用read(),write(),seekg(),seekp()实现二进制方式文件随机存取

// binary.cpp -- binary file I/O

#include <iostream>

#include <fstream>

#include <iomanip>

#include <cstdlib>  //for exit()

using namespace std;



const char * file = "planets.dat";



struct planet

{

    char name[20];      //name of planet

    double population;  //its population

    double g;           //its acceleration of gravity

};

inline void eatline()

{

    while(cin.get() != '\n')

    {

        continue;

    }

}



int main()

{

    planet pl;

    cout << fixed << right;



    //show initial contents

    ifstream fin;



    fin.open(file, ios_base::in | ios_base::binary);    //binary file

    if(fin.is_open())

    {

        cout << "Here are the current contents of the " << file << " file:" << endl;

        while(fin.read((char* )&pl, sizeof pl))

        {

            cout << setw(20) << pl.name << ": "

                 << setprecision(0) << setw(12) << pl.population

                 << setprecision(2) << setw(6) << pl.g << endl;

        }

        fin.close();

    }



    //add new data

    ofstream fout;

    fout.open(file, ios_base::out | ios_base::app | ios_base::binary);

    if(!fout.is_open())

    {

        cerr << "Can't open " << file << " file for output:" << endl;

        exit(EXIT_FAILURE);

    }

    cout << "Enter planet name (enter a blank line to quit):" << endl;

    cin.get(pl.name, 20);

    while(pl.name[0] != '\0')

    {

        eatline();

        cout << "Enter planet's population: ";

        cin >> pl.population;

        cout << "Enter planet's acceleration of gravity: ";

        cin >> pl.g;

        eatline();

        fout.write((char *)&pl, sizeof pl);

        cout << "Enter planet name (enter a blank line to quit):" << endl;

        cin.get(pl.name, 20);

    }

    fout.close();



    //show revised file

    fin.clear();

    fin.open(file, ios_base::in | ios_base::binary);

    if(fin.is_open())

    {

        cout << "Here are the new contents of the " << file << " file:" << endl;

        while(fin.read((char *)&pl, sizeof pl))

        {

            cout << setw(20) << pl.name << ": "

                 << setprecision(0) << setw(12) << pl.population

                 << setprecision(2) << setw(6) << pl.g << endl;

        }

        fin.close();

    }

    cout << "Done." << endl;



    return 0;

}

//random.cpp -- random access to a binary file

#include <iostream>

#include <fstream>

#include <iomanip>

#include <cstdlib>

using namespace std;



const int LIM = 20;

const char * file = "planets.dat";



struct planet

{

    char name[LIM];     //name of planet

    double population;  //its population

    double g;           //its acceleration

};

inline void eatline()

{

    while(cin.get() != '\n')

    {

        continue;

    }

}



int main()

{

    planet pl;

    cout << fixed;



    //show initial contents

    fstream finout;     //read and write streams

    finout.open(file, ios_base::in | ios_base::out | ios_base::binary);



    int ct = 0;

    if(finout.is_open())

    {

        finout.seekg(0);    //go to beginning

        cout << "Here are the current contents of the " << file << " file:" << endl;

        while(finout.read((char *)&pl, sizeof pl))

        {

            cout << ct++ << ": " << setw(LIM) << pl.name << ": "

                 << setprecision(0) << setw(12) << pl.population

                 << setprecision(2) << setw(6) << pl.g << endl;

        }

        if(finout.eof())

        {

            finout.clear();     //clean eof flag

        }

        else

        {

            cerr << "Error in reading " << file << " file." << endl;

            exit(EXIT_FAILURE);

        }

    }

    else

    {

        cerr << file << " could not be opened -- bye." << endl;

        exit(EXIT_FAILURE);

    }



    //change a record

    cout << "Enter the record number you wish to change: ";

    long rec;

    cin >> rec;

    eatline();  //get rid of newline

    if(rec < 0 || rec >= ct)

    {

        cerr << "Invalid record number -- bye" << endl;

        exit(EXIT_FAILURE);

    }

    streampos place = rec * sizeof pl;  //convert to streampos type

    finout.seekg(place);    //random access

    if(finout.fail())

    {

        cerr << "Error on attempted seek" << endl;

        exit(EXIT_FAILURE);

    }



    finout.read((char *)&pl, sizeof pl);

    cout << "Your selection: " << endl;

    cout << rec << ": " << setw(LIM) << pl.name << ": "

         << setprecision(0) << setw(12) << pl.population

         << setprecision(2) << setw(6) << pl.g << endl;

    if(finout.eof())

    {

        finout.clear(); //clear eof flag

    }



    cout << "Enter planet name: ";

    cin.get(pl.name, LIM);

    eatline();

    cout << "Enter planetary population: ";

    cin >> pl.population;

    cout << "Enter planet's acceleration of gravity: ";

    cin >> pl.g;

    finout.seekp(place);    //go back

    finout.write((char *)&pl, sizeof pl) << flush;

    if(finout.fail())

    {

        cerr << "Error on attempted write" << endl;

        exit(EXIT_FAILURE);

    }



    //show revised file

    ct = 0;

    finout.seekg(0);    //go to beginning of file

    cout << "Here are the new contents of the " << file << " file:\n";

    while(finout.read((char *)&pl, sizeof pl))

    {

        cout << ct++ << ": " << setw(LIM) << pl.name << ": "

             << setprecision(0) << setw(12) << pl.population

             << setprecision(2) << setw(6) << pl.g << endl;

    }

    finout.close();

    cout << "Done." << endl;



    return 0;

}