文件流对象有两个可用于随机文件访问的成员函数:tellp 和 tellg。它们的目的是将文件读写位置的当前字节编号作为一个 long 类型整数返回。
如果你了解 seekp 和 seekg 不难猜到,tellp 用于返回写入位置,tellg 则用于返回读取位置。假设 pos 是一个 long 类型的整数,那么以下就是该函数的用法示例:
下面的程序演示了 tellg 函数的用法。它打开了一个名为 letters.txt 文件。文件包含以下字符:
//This program demonstrates the tellg function.
#include <iostream>
#include <fstream>
#include <cctype> // For toupper
using namespace std;
int main()
{
// Variables used to read the file
long offset;
char ch;
char response; //User response
// Create the file object and open the file
fstream file("letters.txt", ios::in);
if (!file)
{
cout << "Error opening file.";
return 0;
}
// Work with the file
do {
// Where in the file am I?
cout << "Currently at position " << file.tellg() << endl;
// Get a file offset from the user.
cout << "Enter an offset from the " << "beginning of the file: ";
cin >> offset;
// Read the character at the given offset
file.seekg(offset, ios::beg);
file.get(ch);
cout << "Character read: " << ch << endl;
cout << "Do it again? ";
cin >> response;
} while (toupper(response) == 'Y');
file.close ();
return 0;
}
程序输出结果: