Problem of returning an object at friend operator and general function

Magi Su <>
Thu, 23 Aug 2007 13:02:30 CST
Hello, everyone. I have a problem while programming. Please refer to the
code below:

#include <iostream>
using namespace std;

class T
      T() { id=++count;cout<<"T constructor! ID="<<id<<endl;}
      ~T() { cout<<"T destructor! ID="<<id<<endl;}
      T(const T& cp) { cout<<"T copy-constructor! source ID="<<<<"
this ID="<<id<<endl;; }
      T operator=(const T& data) { cout<<"T operator=! source
ID="<<<<" this ID="<<id<<endl;
          return *this;}
      double ShowID(void) { return id; }
      operator double() { return id; }
      friend T operator+(const T& a,const T& b);
      static int count;
      double id;

int T::count=0;
T operator+(const T& a,const T& b)
    T c;;
    return c;
T x(T t)
   cout<<"inside x(): t ID="<<t.ShowID()<<endl;
   return t;

int main(void)
    T a,b;

The result of the program is (compiled by g++ 4.1.2):

T constructor! ID=1
T constructor! ID=2
T constructor! ID=3
T destructor! ID=1.2
T copy-constructor! source ID=1 this ID=-1.44066e-39
inside x(): t ID=11
T copy-constructor! source ID=11 this ID=-3.09432e-39
T destructor! ID=21
T destructor! ID=11
T destructor! ID=2
T destructor! ID=1

It revealed the fact that although both operator+() and x() returned an
object, the latter called the copy-constructor while the prior did not. Why
I get such result? And why C++ is designed in this way? Thanks a lot for
your answer!

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