ifstream之seekg/tellg

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在文件读写的时候，一般需要借助fstream来进行文件操作，常见的操作有seekg()和tellg()，但是这两个函数有一些需要注意的地方，如下：

主要参考：

https://stackoverflow.com/questions/20506771/get-file-size-with-ifstreamseekg-and-tellg

https://stackoverflow.com/questions/28823258/which-of-these-if-the-correct-way-to-use-seekg

https://stackoverflow.com/questions/11714973/istream-seekg-offsets-and-iosend

定义，参考cppreference
seekg: Sets input position indicator of the current associated streambuf object.中文的意思是：设置当前关联streambuf对象的输入位置指示器
tellg: Returns input position indicator of the current associated streambuf object.中文的意思是：返回当前关联streambuf对象的输入位置指示器

首先准备一个test.txt，每行15个字符，共45个字符

ssssssssssssss
aaaaaaaaaaaaaa
dddddddddddddd

测试程序：

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

int main() {
    int size = 0;
    std::string fileName = "../test.txt";
    ifstream in(fileName.c_str(), ifstream::in | ifstream::binary);
    if(in)
    {
        in.seekg(0,ifstream::end);
        size = in.tellg();
        cout <<"********** size stream1*** =" << size << endl;  // ********** size stream1*** =44
        in.seekg(0,ios::end);
        size = in.tellg();
        cout <<"********** size stream2*** =" << size << endl; // ********** size stream2*** =44
        in.seekg(ios::end);
        size = in.tellg();
        cout <<"********** size stream3*** =" << size << endl; // ********** size stream3*** =2
        in.seekg(10,ios::end);
        size = in.tellg();
        cout <<"********** size stream4*** =" << size << endl; //  ********** size stream4*** =54
        in.seekg(-10,ios::end);
        size = in.tellg();
        cout <<"********** size stream5*** =" << size << endl; // ********** size stream5*** =34
        in.seekg(0,ios::beg);
        size = in.tellg();
        cout <<"********** size stream6*** =" << size << endl; // ********** size stream6*** =0
        in.seekg(ios::beg);
        size = in.tellg();
        cout <<"********** size stream7*** =" << size << endl; // ********** size stream7*** =0
        in.seekg(14);
        in.seekg(0, ios::end);
        size = in.tellg();
        cout <<"********** size stream8*** =" << size << endl; // ********** size stream8*** =44
        in.seekg(10);
        in.seekg(0, ios::cur);
        size = in.tellg();
        cout <<"********** size stream9*** =" << size << endl; // ********** size stream9*** =10
        in.seekg(ios::beg,ios::end);
        size = in.tellg();
        cout <<"********** size stream10*** =" << size << endl; // ********** size stream10*** =44
        in.seekg(ios::beg);
        in.seekg(ios::end);
        size = in.tellg();
        cout <<"********** size stream11*** =" << size << endl; // ********** size stream11*** =2
        in.seekg(ios::cur);
        size = in.tellg();
        cout <<"********** size stream12*** =" << size << endl; // ********** size stream12*** =1
    }
}

代码分析：

1. seekg()，用来设置stream的文件指针位置，如in.seekg(14)就是将文件指针设置到14位置处（相对begin位置），这里相当于in.seekg(14, ios::beg)，但是对于in.seekg(ios::end)却截然不同，如stream3输出为2，这是因为enum seekdir {beg, cur, end}，所以当使用ios::cur和ios::end且只有一个参数的时候，会将ios::cur隐式转换为1，ios::end隐式转换为2，所以stream3输出2，stream12输出1，这告诉我们当使用beg, cur, end，为了避免错误，请使用两个参数，如in.seekg(0,ios::end)代替in.seekg(ios::end)
2. tellg()，没啥好说的，就是返回文件流指针所在位置

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