C++ fstream 类

❮ fstream 类

#include <iostream>
#include <fstream>
using namespace std;

int main() {
  // Create and open a text file
  fstream MyFile("filename.txt");

  // Write to the file
  MyFile << "Files can be tricky, but it is fun enough!";
  
  // Read from the file
  string myText;
  getline(MyFile, myText);
  cout << myText;

  // Close the file
  MyFile.close();
} 

定义和用法

The fstream 类（简称“文件流”）用于读写文件。

The fstream 类在 <fstream> 头文件中定义。

要打开文件，将文件路径传递到构造函数中

fstream MyFile("filename.txt");

The fstream 类具有多种用于读写文件的函数，这些函数将在下面列出。

文件指针函数

文件指针是内部变量，指示在文件中读取或写入的位置。

文件指针函数用于操作文件指针。有用于读取文件指针和写入文件指针的函数，但是 fstream 类对这两个操作使用相同的指针，因此更改其中一个也会更改另一个。

seekg()

The seekg(position) 方法将读取文件指针移动到相对于文件开头的指定位置。

MyFile.seekg(6)

The seekg(position, origin) 方法将读取文件指针移动到相对于origin的文件中的指定position。origin 有三个可能的值

• fstream::beg - 位置相对于文件的开头。
• fstream::cur - 位置相对于当前文件位置。
• fstream::end - 位置相对于文件的结尾。

MyFile.seekg(6, fstream::beg);
cout << MyFile.tellg() << "\n";
MyFile.seekg(-3, fstream::cur);
cout << MyFile.tellg() << "\n";
MyFile.seekg(-4, fstream::end);
cout << MyFile.tellg() << "\n";

tellg()

The tellg() 方法返回文件指针在文件中的当前位置。

cout << MyFile.tellg();

seekp()

The seekp(position) 方法将写入文件指针移动到相对于文件开头的指定位置。

MyFile.seekp(6)

The seekp(position, origin) 方法将写入文件指针移动到相对于origin的文件中的指定position。origin 有三个可能的值

• fstream::beg - 位置相对于文件的开头。
• fstream::cur - 位置相对于当前文件位置。
• fstream::end - 位置相对于文件的结尾。

MyFile.seekp(6, fstream::beg);
cout << MyFile.tellp() << "\n";
MyFile.seekp(-3, fstream::cur);
cout << MyFile.tellp() << "\n";
MyFile.seekp(-4, fstream::end);
cout << MyFile.tellp() << "\n";

tellp()

The tellp() 方法返回写入文件指针在文件中的当前位置。

cout << MyFile.tellp();

文件读取函数

文件读取函数从文件提取字符并移动文件指针。

get()

The get() 方法从文件中读取一个字符，并将其 ASCII 值作为 int 值返回。将其转换为 char 类型以查看字符。文件指针移动到文件中的下一个字符。

char myChar = MyFile.get();
cout << myChar;

The get(destination, size, delimiter) 方法将从文件中读取的数据写入最多 size 个字符到目标。它一遇到换行符、文件结尾或由 delimiter 参数给出的可选字符就会停止读取。写入 destination 的值始终以 \0 空终止符结尾。此方法将文件指针移动到它停止读取的换行符或分隔符。

char destination[20];
MyFile.get(destination, 20);
cout << destination << "\n";

// Stop reading when a '.' is found
MyFile.get(destination, 20, '.');
cout << destination << "\n";

getline()

The getline(destination, size, delimiter) 方法与 get(destination, size, delimiter) 方法相同，只是换行符或分隔符被丢弃，文件指针移动到它后面的字符。

char destination[20];
MyFile.getline(destination, 20);
cout << destination << "\n";

// Stop reading when a '.' is found
MyFile.getline(destination, 20, '.');
cout << destination << "\n";

有一个类似的 getline(stream, destination, delimiter) 函数，它从由 stream 参数中的 fstream 对象指定的文件中读取所有字符，直到下一个换行符（或可选的 delimiter），并将它们写入由 destination 指定的字符串中。

string destination;
getline(MyFile, destination);
cout << destination << "\n";

// Stop reading when a '.' is found
getline(MyFile, destination, '.');
cout << destination << "\n";

read()

The read(destination, n) 方法从文件中读取 n 个字符，并将它们写入由 destination 参数指定的 char 数组中。与其他函数不同，它不会在换行符处停止读取，也不会在数据中添加空终止符。

char destination[20];
MyFile.read(destination, 19);
destination[20] = '\0'; // Make sure it ends with a null terminating character
cout << destination << "\n";

peek()

The peek() 方法从文件中读取一个字符，并将其 ASCII 值作为 int 值返回。将其转换为 char 类型以查看字符。与 get() 方法不同，此方法不会移动文件指针。

char myChar = MyFile.peek();
cout << myChar;

gcount()

The gcount() 方法返回最近调用的文件读取方法从文件中提取的字符数。

char destination[20];
MyFile.getline(destination, 20);
cout << MyFile.gcount() << "\n";

文件写入函数

文件写入函数将数据写入文件并将文件指针移动到写入内容后的第一个位置。

write()

The write(str, n) method writes n characters from the char array str into the file and moves the file pointer forward by n characters.

char myStr[] = "Hello World!";
MyFile.write(myStr, 5);

put()

The put(c) method writes the specified character c into the file and moves the file pointer forward by one character.

char grade = 'B';
MyFile.put(grade);

文件处理函数

文件处理函数打开和关闭文件。

open()

The open(filepath) method opens the file at the path specified by filepath. If a file is already open then this method has no effect.

ofstream MyFile;
MyFile.open("filename.txt");

is_open()

The is_open() method returns true if a file is open and false if there is no file open.

fstream MyFile;
cout << MyFile.is_open(); << "\n"; // Displays 0 because the file is not open
MyFile.open("filename.txt");
cout << MyFile.is_open(); << "\n"; // Displays 1 because the file is open

close()

The close() method closes a file. It is good to close a file when you are finished working with it to free up resources.

MyFile.close();

rdbuf()

The rdbuf() method returns a pointer to the internal filebuf object which directly handles the file.

filebuf * buf = MyFile.rdbuf();

提取运算符

The >> extraction operator reads a number of characters from the current position in the file, interprets them and writes the interpreted value into a variable. Then the file pointer is moved to the next character which has not yet been read. The way that the characters are interpreted depends on the data type of the variable.

MyFile >> variable

It can also be used multiple times to read parts of a file one after another.

MyFile >> variable1 >> variable2 >> variable3

The >> extraction operator starts by skipping over whitespace characters (spaces, tabs and line breaks) until it reaches the first character that is not whitespace. After that, it follows the rules shown in the following table based on the data type of the variable.

操作符

A manipulator can be used in place of a variable. When manipulators are used they change how data is interpreted by the fstream object. The effect of a manipulator remains until another manipulator changes it.

The following table has a list of manipulators that can be used with the >> extraction operator.

bool myBool;
int myInt;

// Interpret character sequences "true" and "false" as boolean values
MyFile >> boolalpha >> myBool;

// Revert to reading booleans normally
MyFile >> noboolalpha;

// Read hexadecimal numbers from the file and interpret them as integers
MyFile >> hex >> myInt;

// Revert to reading integers normally
MyFile >> dec;

插入运算符

The << insertion operator writes a literal value or the contents of a variable into the file.

int year = 2024;
MyFile << year << "\n";
MyFile << "Files can be tricky, but it is fun enough!";

操作符

Manipulators change the formatting of the data that is written to the file. They are used with the << insertion operator in the same way as literal values and variables.

Except for setw(), the effect of a manipulator remains until another another manipulator changes it.

The table below shows a list of useful manipulators

❮ fstream 类