C++分析——文件操作

- 日理万妓 2022-06-06 07:45 448阅读 0赞

*  文件操作方法
    
     *  打开文件属性open
     *  设置文件当前指针
 *  写文件操作类stdofstream
    
     *  写入数据
     *  写入数据块
 *  读文件操作类stdifstream
    
     *  读出数据
        
         *  读出一个字符
         *  读到空格结束
         *  读到回车字符结束
         *  读到文件末尾结束
     *  读出数据块

C++文件操作主要依靠两个类来进行

1、 写文件操作类：std::ofstream  
2、 读文件操作类：std::ifstream

std::fstream类可以进行读写操作，但一般用上面两个类进行，分工明确。

需要包括的头文件：

#include <iostream>
    #include <fstream>

C++操作函数像C需要将相对地址转换成绝对地址。

文件操作结构体

typedef struct Cpp_sys_info
    {
        char name[10];
        char group[10];
        int  permissions;
    }Cpp_User_info;

# 文件操作方法 #

## 打开文件属性open ##

ios::in     ：以输入方式打开文件
    ios::out    ：以输出方式打开文件
    ios::app    ：以追加的方式打开文件，所有输出附加在文件末尾
    ios::ate    ：文件打开后，文件流指针定位到文件尾，和ios::app功能相同
    ios::trunc  ：如果文件已存在则先删除该文件 
    ios::binary ：以二进制的方式打开文件

具体用法在后面的示例中展现。

## 设置文件当前指针 ##

seekp：设置写文件指针  
seekg：设置读文件指针

ios::beg：　　文件开头 
    ios::cur：　　文件当前位置 
    ios::end：　　文件结尾

m_steam_read.seekg(-8L,ios::end);   //把读文件指针移动到从结尾位置起始，往负方向的6个字节处
    m_steam_write.seekp(5L,ios::beg);   //把写文件指针移动到从文件开头起始，往正方向的5个字节处

# 写文件操作类std::ofstream #

## 写入数据 ##

源码

//std::ofstream steam_write(p_argv[1],ios::in | ios::trunc); //比较简单的初始化函数，直接读取文件
    std::ofstream steam_write;
    m_steam_write.open(p_argv[1],ios::in | ios::trunc);         //比较正式的写法，更加直观
    
     if (!steam_write.is_open())                            //判断文件是否打开成功
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
     steam_write << __func__ << " func is wirte test " << 45 << 0x61 <<endl;        //写入数据
     steam_write << "Test C++ wirte number " << 54.79;                  //写入数据
    
     steam_write.close();       //关闭文件

执行结果

$ ./project.o write.txt
    $ cat write.txt 
    write_file func is wirte test 4597
    Test C++ wirte number 54.79
    $

## 写入数据块 ##

源码

Cpp_User_info Test_User_info[2] = {     //需要写入的数据块
            {
       "admin_cpp","root_cpp",3},
            {
       "guest_cpp","user_cpp",4}
        };
    
        std::ofstream steam_write;
        steam_write.open(p_argv[1],ios::in | ios::binary | ios::app);
    
     if (!steam_write.is_open())
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
        steam_write.write(reinterpret_cast<char *>(&Test_User_info),static_cast<int>(sizeof(Test_User_info)));
        steam_write.close();

执行结果

$ ./project.o writetable.bin 
    $ ./project.o writetable.bin 
    $ ./project.o writetable.bin 
    $ hexdump writetable.bin 
    0000000 6461 696d 5f6e 7063 0070 6f72 746f 635f
    0000010 7070 0000 0003 0000 7567 7365 5f74 7063
    0000020 0070 7375 7265 635f 7070 0000 0004 0000
    0000030 6461 696d 5f6e 7063 0070 6f72 746f 635f
    0000040 7070 0000 0003 0000 7567 7365 5f74 7063
    0000050 0070 7375 7265 635f 7070 0000 0004 0000
    0000060 6461 696d 5f6e 7063 0070 6f72 746f 635f
    0000070 7070 0000 0003 0000 7567 7365 5f74 7063
    0000080 0070 7375 7265 635f 7070 0000 0004 0000
    0000090 6461 696d 5f6e 7063 0070 6f72 746f 635f
    00000a0 7070 0000 0003 0000 7567 7365 5f74 7063
    00000b0 0070 7375 7265 635f 7070 0000 0004 0000
    00000c0
    $

hex格式查询即为  
![这里写图片描述][SouthEast]

# 读文件操作类std::ifstream #

读文件就分几种读法，读到空格结束，读到回车字符结束，读数据块

## 读出数据 ##

读数据的文件

$ cat write.txt 
    write_file func is wirte test 4597
    Test C++ wirte number 54.79
    $

### 读出一个字符 ###

源码

std::ifstream steam_read;
        steam_read.open(p_argv[1],ios::out);
    
     if (!steam_read.is_open())
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
     char ch = 0;
     while(!steam_read.eof()) 
     {
         steam_read.get(ch);    //读取一个字符
         cout << ch;
     }
        cout << endl;
    
        steam_read.close();

执行结果

$ ./project.o write.txt 
    write_file func is wirte test 4597
    Test C++ wirte number 54.799
    $

### 读到空格结束 ###

源码

std::ifstream steam_read;
        steam_read.open(p_argv[1],ios::out);
    
     if (!steam_read.is_open())
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
        string read_data;
    
        steam_read >> read_data;        //读到空格结束
        cout << read_data << endl;
    
        steam_read.close();

执行结果

$ ./project.o write.txt 
    write_file
    $

### 读到回车字符结束 ###

源码

std::ifstream steam_read;
        steam_read.open(p_argv[1],ios::out);
    
        if (!steam_read.is_open())
        {
            cout << "wirte is NO" << endl;
            return;
        }
    
        string read_data;
    
        std::getline(steam_read,read_data); //读到回车字符结束
        cout << read_data << endl;
    
        steam_read.close();

执行结果

$ ./project.o write.txt 
    write_file func is wirte test 4597
    $

### 读到文件末尾结束 ###

源码

std::ifstream steam_read;
        steam_read.open(p_argv[1],ios::out);
    
     if (!steam_read.is_open())
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
        string read_data;
    
        while(!steam_read.eof())    //判断是否到达文件末尾
        {
            std::getline(steam_read,read_data);
            cout << read_data << endl;
        }
    
        steam_read.close();

执行结果

$ ./project.o write.txt 
    write_file func is wirte test 4597
    Test C++ wirte number 54.79
    $

## 读出数据块 ##

源码

std::ifstream steam_read;
        steam_read.open(p_argv[1],ios::out | ios::binary);
    
     if (!steam_read.is_open())
     {
            cout << "wirte is NO" << endl;
            return;
     }
    
        string read_data;
    
        while(!steam_read.eof())    //判断是否到达文件末尾
        {
            std::getline(steam_read,read_data);
            cout << read_data << endl;
        }
    
        steam_read.close();

执行结果

$ ./project.o writetable.bin 
    name is         admin_cpp
    group is        root_cpp
    permissions is  3
    name is         guest_cpp
    group is        user_cpp
    permissions is  4           //文件函数判断多一次，读取不成功，打印上一次的值
    name is         guest_cpp
    group is        user_cpp
    permissions is  4
    $

hex格式查询即为  
![这里写图片描述][SouthEast 1]

[SouthEast]: /images/20220606/dd8b568a486147cb9051713948b48be5.png
[SouthEast 1]: /images/20220606/4f057cb7c677447a922df756859eb5fa.png