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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
cpp
ssssssssssssss
aaaaaaaaaaaaaa
dddddddddddddd
测试程序:
cpp
#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:
- 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)
- tellg(),没啥好说的,就是返回文件流指针所在位置
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