File operations

1.File Operations

Data generated during program execution is temporary data, and it will be released once the program finishes running.

Throughfiles, data can be persisted

C++ File operations require including the header file ==< fstream >==

File types are divided into two categories:

1. 文本文件 - 文件按照某种字符编码保存文本，例如本教程使用的 UTF-8
2. 二进制文件 - 文件以文本的二进制形式存储在计算机中，用户一般不能直接读懂它们

Three major categories of file operations:

1. ofstream: write operations
2. ifstream: read operations
3. fstream: read and write operations

1.1.Linux 和 Windows 中的文件路径

本章的 test.txt 这样的相对路径，两种系统都可以使用。需要注意，路径相对于程序运行时的当前工作目录，不一定相对于 .cpp 或 EXE 所在目录。用 VSCode 运行和自己打开终端运行时，工作目录也可能不同。

如果不确定文件生成在哪里，可以先打印：

#include <filesystem>
#include <iostream>

int main()
{
    std::cout << std::filesystem::current_path() << '\n';
    return 0;
}

运行结果： 输出当前工作目录的绝对路径，具体内容以你的电脑为准。

Windows 的反斜杠在普通 C++ 字符串里需要转义，例如 "C:\\work\\test.txt"；也可以写原始字符串 R"(C:\work\test.txt)"。入门练习优先使用项目中的相对路径。

本教程源文件和示例文本统一保存为 UTF-8。文本模式下，Windows 与 Linux 的换行处理可能不同；逐字节读写数据时要使用二进制模式。后面的结构体直接读写示例用于理解 read / write，跨平台通信或长期保存数据时，还需要明确字段大小、字节序和数据格式。

1.2.Text file

1.2.1.Write file

The steps to write a file are as follows:

1. Include header file

   \#include <fstream\>

2. Create stream object

   ofstream ofs;

3. Open file

   ofs.open("file path", open mode);

4. Write data

   ofs << "data to write";

5. Close file

   ofs.close();

   ​

File open method:

Note: File opening modes can be used in combination with the | operator.

**For example:** Write a file in binary mode ios::binary | ios:: out

Example:

#include <fstream>

void test01()
{
	ofstream ofs;
	ofs.open("test.txt", ios::out);

	ofs << "姓名：张三" << endl;
	ofs << "性别：男" << endl;
	ofs << "年龄：18" << endl;

	ofs.close();
}

int main() {
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。

	test01();


	// 返回 0 表示程序正常结束。
	return 0;
}

Run result: No output in the terminal; the program will generate in the current directory test.txt, and the file content is:

姓名：张三
性别：男
年龄：18

Summary:

• File operations must include the header file fstream
• To read a file, you can use ofstream, or the fstream class
• When opening a file, you need to specify the path to the file to be operated on, as well as the open mode
• You can use << to write data to the file
• After the operation is complete, close the file

1.2.2.Read file

The steps for reading a file are similar to those for writing a file, but there are relatively more reading methods.

The steps to read a file are as follows:

1. Include header file

   \#include <fstream\>

2. Create stream object

   ifstream ifs;

3. Open the file and check whether it was opened successfully

   ifs.open("file path", open mode);

4. Read data

   Four ways to read

5. Close the file

   ifs.close();

Example:

#include <fstream>
#include <string>
void test01()
{
	ifstream ifs;
	ifs.open("test.txt", ios::in);

	if (!ifs.is_open())
	{
		cout << "文件打开失败" << endl;
		return;
	}

	//第一种方式
	//char buf[1024] = { 0 };
	//while (ifs >> buf)
	//{
	//	cout << buf << endl;
	//}

	//第二种
	//char buf[1024] = { 0 };
	//while (ifs.getline(buf,sizeof(buf)))
	//{
	//	cout << buf << endl;
	//}

	//第三种
	//string buf;
	//while (getline(ifs, buf))
	//{
	//	cout << buf << endl;
	//}

	char c;
	while ((c = ifs.get()) != EOF)
	{
		cout << c;
	}

	ifs.close();

}

int main() {

	test01();


	return 0;
}

Running results (the following is an example when entering 10 , 20 , 30 ... in order as prompted; the type size or type name may vary depending on the compiler and platform):

姓名：张三
性别：男
年龄：18

Summary:

• To read a file, you can use ifstream, or the fstream class
• The is_open function can be used to determine whether the file was opened successfully
• close closes the file

1.3.Binary file

Read and write files in binary mode

The opening mode must be specified as ==ios::binary==

1.3.1.Write file

Writing to a file in binary mode mainly uses a stream object to call the member function write.

Function prototype:ostream& write(const char * buffer,int len);

Parameter explanation: The character pointer buffer points to a segment of storage space in memory. len is the number of bytes to read or write.

Example:

#include <fstream>
#include <string>

class Person
{
public:
	char m_Name[64];
	int m_Age;
};

//二进制文件  写文件
void test01()
{
	//1、包含头文件

	//2、创建输出流对象
	ofstream ofs("person.txt", ios::out | ios::binary);
	
	//3、打开文件
	//ofs.open("person.txt", ios::out | ios::binary);

	Person p = {"张三"  , 18};

	//4、写文件
	ofs.write((const char *)&p, sizeof(p));

	//5、关闭文件
	ofs.close();
}

int main() {

	test01();


	return 0;
}

Execution result: The terminal has no output; the program will generate a binary file in the current directory person.txt, into which a 18 object with name "Zhang San" and age Person is written.

Summary:

• The file output stream object can write data in binary mode through the write function.

1.3.2.Read file

Reading files in binary mode mainly uses a stream object to call the member function read.

Function prototype:istream& read(char *buffer,int len);

Parameter explanation: The character pointer buffer points to a segment of storage space in memory. len is the number of bytes to read or write.

Example:

#include <fstream>
#include <string>

class Person
{
public:
	char m_Name[64];
	int m_Age;
};

void test01()
{
	ifstream ifs("person.txt", ios::in | ios::binary);
	if (!ifs.is_open())
	{
		cout << "文件打开失败" << endl;
	}

	Person p;
	ifs.read((char *)&p, sizeof(p));

	cout << "姓名： " << p.m_Name << " 年龄： " << p.m_Age << endl;
}

int main() {
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。

	test01();


	// 返回 0 表示程序正常结束。
	return 0;
}

Run result (type size or type name may vary depending on compiler and platform):

姓名： 张三 年龄： 18

• The file input stream object can read data in binary mode through the read function.