why the reference of temporary object is valid here?

#include <iostream>
 #include <string>
 using namespace std;
 class test
 {
 public:
     string a;
 public:
     test(string b){a=b;}
     friend string operator+(test);
 };
 string operator+(string &c,test a)
 {
     c=c+a.a;
     return c;
 }
 void main()
 {
     test d("the ");
     test e("world!");
     string s="Hello ";
      s=s+d+e;
     cout<<s<<endl;
 }
 

the second last line s=s+d+e; after the fist overloaded operator + it returned a temporary object,and the second overloaded operator + unexpectedly worked!But the first parameter of operator+ function is a reference. why the reference of temporary object is valid here,or there is something i have missed?

P.S: It's compiled by VC++6.0 and here is the running result.

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