C++文件操作

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ofstream fout; 创建一个对象

fout.open("jar.txt"); 打开一个文件

ofstream fout("jar,txt"); 等价于上面两步操作

然后，以使用cout的方式使用fout（或其他名称）。例如 fout<<" Dull Data";

ostream是ofstream的基类，既可以使用任何格式控制。

ifstream fin;

fin.open("Test.txt");

ifstream fin("Test.txt"); 与上面两步操作等价

char ch;

fin>>ch; 读取一个字母

char buff[80];

cin>>buff; 读取一个word

fin.getline(buff,80); 从文件中读取一行

string line;

getline(fin,line); 从文件中读取一行

fout.close(); 显式的关闭文件

fin.close() 显式的关闭文件

控制台中可以用ctrl+z代替EOF

逐词读取空格隔开

ifstream fin("data.txt");

string s;

while( fin >> s )

{

cout << "Read from file: " << s << endl;

}

逐行读取回车换行隔开

方式一： //读取到string对象不用指定长度

ifstream fin("data.txt");

string s;

while( getline(fin,s) )

{

cout << "Read from file: " << s << endl;

}

方式二： //读取到字符数组指定长度

ifstream fin("data.txt");

const int LINE_LENGTH = 100;

char str[LINE_LENGTH];

while( fin.getline(str,LINE_LENGTH) )

{

cout << "Read from file: " << str << endl;

}

getline()函数详解

getline()的原型是istream& getline ( istream &is , string &str , char delim );

从一个流中读取一串字符放到指定位置，读取时遇到delim停止（默认为\n）;

例程：

#include<fstream>

#include<iostream>

#include<string>

using namespace std;

int main()

{

string filename;

cout << "Enter name for new file:";

cin >> filename;

ofstream fout(filename.c_str());

fout << "For your eyes only!\n";

cout << "Enter your secret number:";

float secret;

cin >> secret;

fout << "Your secret number is " << secret << endl;

fout.close();

ifstream fin(filename.c_str());

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

char ch;

while (fin.get(ch))

{

cout << ch;

}

cout << "Done!\n";

fin.close();

return 0;

}

检测文件是否成功打开。

fin.open(argv[file]);

if(fin.fail())

{

......

}

或者

if(!fin)

{

......

}

或者

if(!fin.is_open())

{

......

}

或者

if(!fin.good())

{

......

}

依次打开多个文件

ifstream fin;

fin.open("test.txt");

...

fin.close();

fin.clear();

fin.open("test.txt");

...

fin.close();

文件打开模式选择

#include<fstream>

#include<iostream>

#include<string>

#include<stdlib.h>

using namespace std;

const char* file = "guests.txt";

int main()

{

char ch;

ifstream fin;

fin.open(file);

if (fin.is_open())

{

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

while (fin.get(ch))

{

cout << ch;

}

fin.close();

}

ofstream fout(file, ios::out | ios::app);

if (!fout.is_open())

{

cerr << "Can't open" << file << "file for out put.\n";

exit(EXIT_FAILURE);

}

cout << "Enter guest names (enter a blank line to quit):\n";

string name;

while (getline(cin,name)&&name.size()>0)

{

fout << name << endl;

}

fout.close();

fin.clear();

fin.open(file);

if (fin.is_open())

{

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

while (fin.get(ch))

{

cout << ch;

}

fin.close();

}

cout << "Done!\n";

return 0;

}

文本形式和二进制形式保存

const int LIM = 20;

struct planet

{

char name[LIM];

double population;

double g;

}

planet pl;

保存方式一：

ofstream fout("planet.dat",ios_base::out|ios_base::app);

fout<<pl.name<<" "<<pl.population<<" "<<pl.g<<"\n";

保存方式二：

ofstream fout("planet.dat",ios_base::out|ios_base::app|ios_base::binary);

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

采用二进制文件模式时：速度快，不丢失数据，不发生隐含转换。

采用文本模式时：会发生隐式转换，可能丢失数据。在不同的系统上转换结果可能不一样。

采用write()函数的时候，第一个参数是地址，第二个参数是字节。地址必须转换为char*类型

read()和write()的功能是相反的，可以用read()来恢复write()写入的数据。

案例：

#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";

}

打开文件用于读写:

fstream finout;

finout.Open(file,ios::in|ios::out|ios::binary);

打开文件，移动到文件开头并显示文件内容。

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.population

<<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 ont be opened---bye.\n";

exit(EXIT_FAILURE);

}

选择修改记录：

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);

ct 表示记录号。如果试图超过文件尾，程序将退出。

接下来显示当前记录：

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();

}

显示记录之后，现在可以执行修改记录。

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);

}

总结案例：

#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()

{

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;

}

临时文件：

#include<iostream>

#include<stdlib.h>

using namespace std;

int main()

{

cout << "This system can generate up to " << TMP_MAX

<< " temporary names of up to " << L_tmpnam

<< " characters.\n";

char pasName[ L_tmpnam ] = { '\0' };

cout << "Here are ten names:\n";

for (int i = 0;i < 10;i++)

{

tmpnam(pasName);

cout << pasName <<endl;

}

return 0;

}

上面代码可以生成10个零时文件名。

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