修改与不修改putback()的区别

问题来自 https://en.cppreference.com/w/cpp/io/basic_istream/putback ,示例代码。

#include <sstream>
#include <iostream>

int main()
{
    std::istringstream s2("Hello, world"); // input-only stream
    s2.get();
    if (s2.putback('Y')) // cannot modify input-only buffer
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";

    s2.clear();
    if (s2.putback('H')) // non-modifying putback
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";
}

s2.putback('Y') 失败但是 s2.putback('H') 成功了吗?后者不也是修改仅输入流缓冲区的操作吗?

另外,我在做实验的时候发现了一些令人困惑的事情。 与上面的示例相比,我添加了一行代码,第二个结果失败了。。为什么会这样?

#include <sstream>
#include <iostream>

int main()
{
    std::istringstream s2("Hello, world"); // input-only stream
    s2.get();
    if (s2.putback('Y')) // cannot modify input-only buffer
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";

    std::cout << s2.rdbuf() << '\n';   //1 line code added

    s2.clear();
    if (s2.putback('H')) // non-modifying putback
        std::cout << s2.rdbuf() << '\n';
    else
        std::cout << "putback failed\n";

}