C++ Primer 学习４：string类使用重载操作符＋的注意

#include <iostream>
#include <string>
using namespace std;

int main()
{
string s1 = "12";
string s2 = s1 + "34";
string s3 = s1 + s2;
//  string s4 = "12" + "34";
//error:invalid operands of types ‘const char [3]’ and ‘const char [3]’ to binary ‘operator+’

string s5 = s1 + "34" + "56";
//true:因为s1是string类与“34”返回一个string类，再继续与“56”进行操作

//  string s6 = "34" + "56" + s1;  //error 运算从前２个进行运算　由于钱２个均不含string类
string s7 = "11" + s2;
cout << s1 <<endl;
cout << s2 <<endl;
cout << s3 <<endl;
cout << s5 <<endl;
return 0;
}