以下代码的输出让我感到困惑：
const std::string str = "Modern C++";
std::string s1 {"Modern C++", 3};
std::string s2 {str, 3};
std::cout << "S1: " << s1 << "\n";
std::cout << "S2: " << s2 << "\n";
输出：
> S1: Mod
> S2: ern C++
谁能解释这个结果？