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0 File I/O operations
The data generated when the program is running are all temporary data , and the temporary data will be released when the program is executed. accessiblefilePersist data .
Note: in C++Operating fileAt the time, header files related to the file stream must be included
<fstream>.
Document classification :
(1) 文本文件: Documents in textASCII codeStored in the form;
(2) 二进制文件: the file is in textBinaryStored in form, users generally cannot read it directly.
Types of file operations :
(1) ofstream: Write operation (Output File Stream)
(2) ifstream: Read operation (Input File Stream)
(3) fstream:Read/write operation（File Stream）
1 text file
1.1 Writing files
Steps to write a file :
(1) Include the header file : #include <fstream>
(2) Create an output stream object : ofstream ofs;(Create a file output stream object)
(3) UseSpecify path and methodOpen the file : ofs.open("文件路径", 打开方式);
(4) Output stream objects and 左移运算符<<write data through the file : ofs << "写入数据";
(5) Close the file :ofs.close();
Note: Through
ofstreamthe constructor of the file output stream class , specify the path and opening method of the file when creating the file output stream object , thereby omitting theofs.open("文件路径", 打开方式);function:
ofstream ofs("文件路径", 打开方式);
File opening method :
| Open method | Explanation |
|---|---|
| ios::in | Read file |
| ios::out | Write file |
| ios::ate | Initial position of opening: end of file |
| ios::app | Write files in append mode ( append write ) |
| ios::trunc | If the file already exists, delete it and then create it |
| ios::binary | Binary way |
Note: The file opening method can be used in
|combination with bit or operator .
Example: write files in binary mode:ios::binary | ios:: out.
Text file-write operation summary :
(1) File operations must include header files <fstream>;
(2) Write files can use ofstreamor fstreamclasses;
(3) When opening files, you need to specify the file path and opening method ;
(4) File output streams can be used Object and write data左移运算符<< to the file ; (5) After the operation is completed, the file needs to be closed .
Example : text file-write operation
#include <iostream>
using namespace std;
#include <fstream> //1.包含文件流相关的头文件
void writeTextFile() {
//2.创建文件输出流对象
ofstream ofs;
//3.以指定方式打开
ofs.open("test.txt",ios::out);
//4.写内容
ofs << "姓名：Tom" << endl;
ofs << "时间：2021年2月14日" << endl;
//5.关闭文件流对象
ofs.close();
}
int main() {
writeTextFile();
return 0;
}
1.2 Read files
Steps to read the file :
(1) Include the header file : #include <fstream>
(2) Create an input stream object : ifstream ifs;(Create a file input stream object)
(3) UseSpecify path and methodOpen the file and determine whether the file is opened successfully :
Open the file: ifs.open("文件路径", 打开方式);
Determine whether the file is opened successfully: ifs.is_open()
(4) Input the stream object through the file , and read the data in 4 ways :
①Character array and右移运算符>>
char buffer[1024] = {
0}; //初始化字符数组
while(ifs >> buffer){
cout << buffer << endl;
}
② array of characters and member functions ifstream classifstream& getline(char *_Str, std::streamsize _Count)
ifstream :: getline (char array buffer, the buffer size)
char buffer[1024] = {
0 }; //初始化字符数组
while (ifs.getline(buffer, sizeof(buffer))) {
cout << buffer << endl;
}
③ string and global functiongetline(istream &_istr, string &_str)
getline (input stream objects, string)
string str;
while(getline(ifs, str)){
cout << str << endl;
}
④Read character by character , member functions of the ifstream classget() and 文件结束符EOF(end of file) [not recommended]
Note: The efficiency of character-by-character reading is low, so it is not recommended.
char ch;
while ((ch = ifs.get()) != EOF) {
cout << ch;
}
(5) Close the file :ifs.close();
Determine whether the content of the text file is empty (the file exists):
Read 1 character of the text file and judge whether the character is EOF mark.
(1) Create a file input stream object : ifstream ifs;
(2) Read a text file1 character: char ch; ifs >> ch;Or ch = ifs.get();
(3) Determine whether the currently read character has reached the end of the file (ie 文件结束符EOF):
①Use the member function of the file input stream class ; ② Determine whether the current character is .bool ifstream::eof();文件结束符EOF
Example : Determine whether the content of a text file is empty
//方式1：文件输入流对象调用成员函数ifs.eof()
ifstream ifs(FILENAME, ios::in);
char ch;
ifs >> ch;
if(ifs.eof()){
cout << "文本文件内容为空..." << endl;
}
//方式2：判断当前字符是否为EOF文件结束符
ifstream ifs(FILENAME, ios::in);
char ch;
if((ch = ifs.get()) == EOF){
cout << "文本文件内容为空..." << endl;
}
Text file-read operation summary :
(1) The file operation must include the header file <fstream>;
(2) The ifstreamor fstreamclass can be used to write the file ;
(3) After the is_open()file is opened , the ifstream class function should be used to determine whether the file is opened successfully;
(4) After the operation is completed, the file needs to be closed .
Example : text file-read operation
#include <iostream>
using namespace std;
#include <fstream> //1.包含文件流相关的头文件
#include <string>
void readTextFile() {
//2.创建文件输入流对象
ifstream ifs;
//3.打开文件，并判断是否打开成功
ifs.open("test.txt", ios::in);
if (!ifs.is_open()) {
cout << "文件打开失败" << endl;
return;
}
//4.读数据
//第1种：字符数组和右移运算符>>
//char buffer[1024] = { 0 }; //初始化字符数组
//while (ifs >> buffer) {
// cout << buffer << endl;
//}
//第2种：字符数组和istream类成员函数getline()
//char buffer[1024] = { 0 }; //初始化字符数组
//while (ifs.getline(buffer, sizeof(buffer))) {
// cout << buffer << endl;
//}
//第3种：字符串和全局函数getline()
string str;
while(getline(ifs, str)){
cout << str << endl;
}
//第4种：按字符读取和EOF文件尾标志（end of file）【不建议】
//char ch;
//while ((ch = ifs.get()) != EOF) {
// cout << ch;
//}
//5.关闭文件
ifs.close();
}
int main() {
readTextFile();
return 0;
}
2 Binary files
The file is read and written in binary mode, open()函数and the opening method in the file needs to be used additionally ios::binary.
2.1 Writing files
Write files in binary mode, passFile output stream objectofstream ofs;Call member functionwrite() .
Function prototype : ostream& write(const char* buffer, int len);
Parameter explanation : bufferthe memory address pointed to by the character pointer ; it lenis the number of bytes read and written.
Note 1: When writing files in binary mode, writing is supportedCustom data type, The address of the custom data type object (for example
Object *)Forced transferIs theconst char *type.
Example:ofs.write((const char*) &obj, sizeof(obj));
Note 2: For files with binary write operations, the content may be displayed as garbled characters , and binary read operations need to be used for analysis.
Note 3: Throughofstreamthe constructor of the file output stream class , specify the path and opening method of the file when creating the file output stream object , thereby omitting theofs.open("文件路径", 打开方式);function:
ofstream ofs("文件路径", 打开方式);
Example : Write custom data type in binary mode
#include <iostream>
using namespace std;
#include <fstream> //1.包含文件流相关的头文件
//自定义数据类型
class Student{
public:
char name[32];
int score;
};
int main() {
/*
//2.创建文件输出流对象
ofstream ofs;
//3.以二进制方式打开文件，并执行写操作
ofs.open("student.txt", ios::binary | ios::out);
*/
//简化写法
/* 通过输出流类ofstream的构造函数，创建对象时指定文件路径及打开方式 */
ofstream ofs("student.txt", ios::binary | ios::out);
//4.写文件
Student stu = {
"Tom", 99 };
//将自定义的数据类型指针，强转为const char*
ofs.write((const char*)&stu, sizeof(stu));
//5.关闭文件
ofs.close();
return 0;
}
2.2 Read files
Read the file in binary mode, passFile input stream objectifstream ifs;Call member functionread() .
Function prototype : istream& read(char* buffer, int len);
Parameter explanation : bufferthe memory address pointed to by the character pointer ; it lenis the number of bytes read and written.
Note 1: When reading a file in binary mode , it supports readingCustom data type, The address of the custom data type object (for example
Object *)Forced transferIs thechar *type.
Example:ifs.read((char*) &obj, sizeof(obj));
Note 2: After opening the file, you should use theis_open()function of the ifstream class to determine whether the file is opened successfully.
Example : Read custom data type in binary mode
#include <iostream>
using namespace std;
#include <fstream> //1.包含文件流相关的头文件
//自定义数据类型
class Student {
public:
char name[32];
int score;
};
int main() {
/*
//2.创建输入流对象
ifstream ifs;
//3.打开文件并判断是否打开成功
ifs.open("student.txt", ios::binary | ios::in);
*/
//简化写法
/* 通过输出流类ofstream的构造函数，创建对象时指定文件路径及打开方式 */
ifstream ifs("student.txt", ios::binary | ios::in);
if (!ifs.is_open()) {
cout << "文件打开失败..." << endl;
return 0;
}
//4.读文件
Student stu;
ifs.read((char*)&stu, sizeof(stu));
cout << "姓名：" << stu.name << endl;
cout << "成绩：" << stu.score << endl;
//5.关闭文件
ifs.close();
return 0;
}