这个程序编译未报错，但运行时显示如下内容

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct null not valid

异常返回值为3

代码如下

#include <bits/stdc++.h>
using namespace std;
int main(int argc, char* argv[])
{
	string s = argv[1];char c;
	if(s.substr(s.size() - 3, s.size()) == ".ed")
	{
		s = s.substr(0, s.size() - 3);
		ifstream i(argv[1], ios::binary);
		ofstream o(s.c_str(), ios::out);
		while(i>>c) o<<char(c - 1);
	}
	else
	{
		s = s + ".ed";
		ifstream i(argv[1], ios::in);
		ofstream o(s.c_str(), ios::binary);
		while(i>>c) o<<char(c + 1);
	}
	return 0;
}