C++文件操作典型案例

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案例一：

1.打开文件，显示其中内容

2.向其中追加记录

3.显示追加之后的文件内容

#include<iostream>

#include<fstream>

#include<iomanip>

#include<stdlib.h>

using namespace std;

inline void eatline() { while (std::cin.get() != '\n')continue; }

struct planet

{

char name[20];

double population;

double g;

};

const char* file = "planets.dat";

int main()

{

planet pl;

cout << fixed << right;

ifstream fin;

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

if (fin.is_open())

{

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

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();

}

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

if (!fout.is_open())

{

cerr << "Can't Open " << file << "fiel for output:\n";

exit(EXIT_FAILURE);

}

cout << "Enter plant name(Enter a blank line to quit):\n";

cin.get(pl.name, 20);

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

{

eatline();

cout << "Enter planetary 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):\n";

cin.get(pl.name, 20);

}

fout.close();

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:\n";

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!\n";

}

案例二：

1.打开文件显示其中内容

2.选择要修改的记录

3.显示选中的要修改的记录

4.修改记录

5.显示修改之后的内容

#include<iostream>

#include<fstream>

#include<iomanip>

#include<stdlib.h>

using namespace std;

const int LIM = 20;

struct planet

{

char name[LIM];

double population;

double g;

};

const char* file = "Planet.dat";

inline void eatline() { while (cin.get() != '\n')continue; }

int main()

{

/*ofstream fout("Planet.dat");

fout.close();*/

planet pl;

cout << fixed;

fstream finout;

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

int ct = 0;

if (finout.is_open())

{

finout.seekg(0);

cout << "Here are the current 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;

}

if (finout.eof())

{

finout.clear();

}

else

{

cerr << "Error in reading" << file << ".\n";

exit(EXIT_FAILURE);

}

else

{

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

exit(EXIT_FAILURE);

}

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

long rec;

cin >> rec;

eatline();

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

{

cerr << "Invalid record bumber --bye \n";

exit(EXIT_FAILURE);

}

streampos place = rec * sizeof pl;

finout.seekg(place);

if (finout.fail())

{

cerr << "Error on attempted seek\n";

exit(EXIT_FAILURE);

}

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

cout << "Your selection:\n";

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

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

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

if (finout.eof())

{

finout.clear();

}

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);

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

if (finout.fail())

{

cerr << "Error on attempted write\n";

exit(EXIT_FAILURE);

}

ct = 0;

finout.seekg(0);

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;

}

cout << "Done!\n";

return 0;

}

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