第15周 程序阅读-二进制及二进制文件的读取3

3、阅读下面的程序，指出其功能，体会seekg()、tellg()等函数的功能及其用法

（1）

#include<iostream>
#include <fstream>
using namespace std;
const char * filename = "a.txt";
int main ()
{
    long l,m;
    ifstream file (filename, ios::in|ios::binary);
    l = file.tellg();
    file.seekg (0, ios::end);
    m = file.tellg();
    file.close();
    cout << "size of " << filename;
    cout << " is " << (m-l) << " bytes.\n";
    return 0;
}

运行结果：

（2）

#include <fstream>
using namespace std;
int main (){
    long pos;
    ofstream outfile;
    outfile.open ("test.txt");
    outfile.write ("This is an apple",16);
    pos=outfile.tellp();
    outfile.seekp (pos-7);
    outfile.write (" sam",4);
    outfile.close();
    return 0;
}

运行结果：

（3）

#include <iostream>
#include <fstream>
using namespace std;
int main()
{
    fstream outfile,infile;
    outfile.open("data.txt",ios::out);
    for (int i=0;i<26;i++)
       outfile<<(char)('A'+i);
    outfile.close();
    infile.open("data.txt",ios::in);
    char ch;
    infile.seekg(6,ios::beg);
    if(infile.get(ch))
        cout<<ch;
    infile.seekg(8,ios::beg);
    if(infile.get(ch))
        cout<<ch;
    infile.seekg(-8,ios::end);
    if(infile.get(ch))
        cout<<ch;
    cout<<endl;
    infile.close();
    return 0;
}

运行结果：