<分区>
在s6和s7的定义中，s6中的每一个+怎么都有一个字符串，为什么还不是这样s7?
#include <string>
using std::string;
int main()
{
string s1 = "hello", s2 = "world";
string s3 = s1 + ", " + s2 + '\n';
string s4 = s1 + ", "; // ok: adding a string and a literal
string s5 = "hello" + ", "; // error: no string operand
string s6 = s1 + ", " + "world"; // ok: each + has a string operand
string s7 = "hello" + ", " + s2; // error: can't add string literal
}