Seekg, tellg related file operations

seekg（） And tellg（） Related file operations
Operate on the input stream ：seekg（） And tellg（）
Operate on the output stream ：seekp（） And tellp（）
The following uses the input stream function as an example ：
seekg（） Is to locate the input file , It has two parameters ： The first parameter is the offset , The second parameter is the base address .
For the first parameter , It can be a positive or negative value , Positive indicates a backward offset , Negative indicates forward offset . The second parameter can be ：
ios：：beg： Indicates the starting position of the input stream
ios：：cur： Indicates the current position of the input stream
ios：：end： Indicates the end position of the input stream
tellg（） The function does not need to take arguments , It returns the position of the current positioning pointer , It also represents the size of the input stream .

The procedure is ：

#include <iostream>
#include <fstream>
#include <assert.h>

using namespace std;
int main()
{
    
    ifstream in("test.txt");
    assert(in);
  
    in.seekg(0,ios::end);       // The base address is at the end of the file , The offset address is 0, So the pointer is at the end of the file 
    streampos sp=in.tellg(); //sp Position pointer for , Because it's at the end of the file , So that's the size of the file 
    cout<<"file size:"<<endl<<sp<<endl;

    in.seekg(-sp/3,ios::end); // The base address is the end of the file , The offset address is negative , So move forward sp/3 Bytes 
    streampos sp2=in.tellg();
    cout<<"from file to point:"<<endl<<sp2<<endl;

    in.seekg(0,ios::beg);        // Base address is file header , The offset for the 0, So it is located in the file header 
    cout<<in.rdbuf();             // Read the contents of the file from the beginning 
    in.seekg(sp2);

    cout<<in.rdbuf()<<endl; // from sp2 Start reading the contents of the file 

    return 0;
}

int main()
{
    
 // Get the file size ：C++ The way 
 ifstream ifs;
 ifs.open("log.txt");
 assert(ifs.is_open());
 ifs.seekg( 0 , std::ios::end );
 cout<<ifs.tellg()<<endl;
 ifs.close();

 //  Get the file size ：C The way 
 FILE* fp = fopen("log.txt", "rb");
 assert ( 0 == fseek(fp, 0, SEEK_END));
 unsigned int usize = ftell(fp);
 cout<<usize<<endl;
 fclose(fp);

 return 0;
}