std::string s = "hello"; s.substr(1, 3)的返回值为字符串"ell"

std::string s = "hello world"; s.find("a")的返回值为0

std::string s = "abc"; s.append(2, 'd', 'e')执行后s的值为"abcdde"

std::string s = "test"; s.erase(2)执行后s的值为"tet"