输入流迭代器和异常
几天前，我在玩 istream 迭代器和异常处理，我遇到了这个好奇：
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
using namespace std;
int main(int argc, char* argv[])
{
if (argc < 2) {
cout << argv[0] << " <file>" << endl;
return -1;
}
try {
ifstream ifs(argv[1]);
ifs.exceptions(ios::failbit | ios::badbit);
istream_iterator<string> iss(ifs), iss_end;
copy(iss, iss_end, ostream_iterator<string>(cout, "\n"));
}
catch (const ios_base::failure& e) {
cerr << e.what() << endl;
return -2;
}
return 0;
}
为什么在读取输入文件的最后一个字后总是引发失败位异常？
I was playing around with istream iterators and exception handling a few days ago and I came across with this curiousity:
#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
using namespace std;
int main(int argc, char* argv[])
{
if (argc < 2) {
cout << argv[0] << " <file>" << endl;
return -1;
}
try {
ifstream ifs(argv[1]);
ifs.exceptions(ios::failbit | ios::badbit);
istream_iterator<string> iss(ifs), iss_end;
copy(iss, iss_end, ostream_iterator<string>(cout, "\n"));
}
catch (const ios_base::failure& e) {
cerr << e.what() << endl;
return -2;
}
return 0;
}
Why a failbit exception is always raised after reading the last word of the input file?
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每当读取操作无法提取任何字符时，就会设置失败位，无论这是因为它是否达到了 EOF。
failbitis set whenever a read operation fails to extract any characters, whether this is because it hit EOF or not.好问题。如果能够捕获该调用中的其他失败，但在遇到 eof 时让它正常继续，那就太好了。
也就是说，我以前没有在流中使用过异常。我认为您可以进行复制并随后检查流的状态以检测其他错误，例如：
Good question. It would be nice to be able to catch other failures in that call, but have it continue normally when it hits eof.
That said, I haven't used exceptions with streams before. I think you could do the copy and check the state of the stream afterwards to detect other errors, for example:
检测 EOF 条件的方法是读取直到发生故障（这会触发异常），然后检查故障原因。
扩展一下：当使用 >> 读取值后，流运算符 void* 返回 NULL 时，istream_iterator 变得无效。但为此，运算符>>必须设置失败位，因此引发异常。
One detect the EOF condition by reading until a failure -- which triggers the exception -- and then checking the cause of the failure.
To expand: the istream_iterator becomes invalid when after having read a value with >>, the stream operator void* returns NULL. But for that, the operator >> has to set the fail bit, so raise the exception.