Problem of returning an object at friend operator and general function

Hello, everyone. I have a problem while programming. Please refer to the
code below:

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

class T
{
   public:
      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="<<cp.id<<"
this ID="<<id<<endl; id=cp.id+10; }
      T operator=(const T& data) { cout<<"T operator=! source
ID="<<data.id<<" this ID="<<id<<endl;
          this->id=data.id+100;
          return *this;}
      double ShowID(void) { return id; }
      operator double() { return id; }
      friend T operator+(const T& a,const T& b);
   private:
      static int count;
      double id;
};

int T::count=0;
T operator+(const T& a,const T& b)
{
    T c;
    c.id=a.id+b.id/10;
    return c;
}
T x(T t)
{
   cout<<"inside x(): t ID="<<t.ShowID()<<endl;
   return t;
}

int main(void)
{
    T a,b;
    cout<<(a+b).ShowID()<<endl;
    cout<<x(a)<<endl;
}
----------------------------------------------------

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

T constructor! ID=1
T constructor! ID=2
T constructor! ID=3
1.2
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
21
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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