c++ - iostream sentry 在到达末尾时不设置 failbit

Question

对于以下代码:

#include <iostream>
#include <sstream>

using namespace std;

int main() {
    istringstream iss("a");
    iss.get();
    cout << iss.tellg() << endl;  // 1
    cout << iss.fail() << endl;   // 0
}

我希望结果是 -1,1 而不是 1, 0。

tellg会先构造一个sentry，然后检查fail()。

根据 http://eel.is/c++draft/istream::sentry#2

If is.rdbuf()->sbumpc() or is.rdbuf()->sgetc() returns traits​::​eof(), the function calls setstate(failbit | eofbit) (which may throw ios_­base​::​failure).

所以 fail() 应该为真，tellg 应该返回 -1。

Answer 1

来自 std::istream::tellg 的引用:

if member fail returns true, the function returns -1.

来自 std::ios::fail 的引用:

Check whether either failbit or badbit is set

Returns true if either (or both) the failbit or the badbit error state flags is set for the stream.

At least one of these flags is set when an error occurs during an input operation.

检查此代码:

#include <iostream>
#include <sstream>

using namespace std;

int main() {
    istringstream iss("a");
    iss.get();
    cout << iss.fail() << endl;   // 0
    cout << iss.tellg() << endl;  // 1
    cout << iss.eof() << endl;    // 0
}