C ++如何添加/减去tellp（），tellg（）返回 [英] C++ how to add/subtract tellp(), tellg() return

问题描述

说我想要得到两个tellp（）输出之间的差异（int）。

Say I wanted to get the difference (in int) between two tellp() outputs.

如果写一个大文件，tellp所以说它不能安全地存储在一个很长的长。是否有安全的方式来执行这样的操作：

The tellp() output could be huge if a big file is written so it is not safe to say store it inside a long long. Is there a safe way to perform a operation like this:

ofstream fout;
fout.open("test.txt",ios::out | ios::app);
int start = fout.tellp();
fout<<"blah blah "<<100<<","<<3.14;
int end = fout.tellp();
int difference = end-start;

在这里，我知道end和start之间的区别绝对可以适合int。

Here, I know that the difference between end and start can definitely fit in an int. But end and start itself could be very massive.

推荐答案

的返回类型ofstream :: tellp （和 ifstream :: tellg ）是 char_traits< char> :: pos_type 。除非你真的需要你的最终结果是 int ，你可能想使用 pos_type 。如果你确实需要你的最终结果作为 int ，你仍然可能想存储中间值在 pos_type s，然后减去结果并将结果转换为 int 。

The return type from ofstream::tellp (and ifstream::tellg) is a char_traits<char>::pos_type. Unless you really need your final result to be an int, you probably want to use pos_type throughout. If you do need your final result as an int you still probably want to store the intermediate values in pos_types, then do the subtraction and cast the result to int.

typedef std::char_traits<char>::pos_type pos_type;

ofstream fout;
fout.open("test.txt",ios::out | ios::app);
pos_type start = fout.tellp();
fout<<"blah blah "<<100<<","<<3.14;
pos_type end = fout.tellp();
int difference = int(end-start);
// or: pos_type difference = end-start;

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