Why does the C + + copy constructor parameter have to be a reference? Must the assignment constructor argument also be a reference?

Prior to writing the copy constructor, the parameter is referenced and not passed for the value, just to reduce the memory copy. Today, however, I see an article that finds that it is wrong to understand the parameters of the copy construction. The argument is a reference and is not passed as a value to prevent infinite recursion of the copy constructor, resulting in a stack overflow.
Let's look at an example:

  
 

   
  

  
class test
   
  

  
{
   
  

  
public:
   
  

  
test()
   
  

  
{
   
  

  
cout << "constructor with argument\n"; 
   
  

  
}
   
  

  
~test()
   
  

  
{
   
  

  

   
  

  
}
   
  

  
test(test& t)
   
  

  
{
   
  

  
 cout << "copy constructor\n"; 
   
  

  
}
   
  

  
test&operator=(const test&e)
   
  

  
{
   
  

  
cout << "assignment operator\n"; 
   
  

  
return *this;
   
  

  
}
   
  

  
};
   
  

  
int _tmain(int argc, _TCHAR* argv[])
   
  

  
{
   
  

  
test ort;
   
  

  
test a(ort);
   
  

  
test b = ort ;
   
  

  
 a = b;
   
  

  
return 0;
   
  

  
}
  
 

Output: If you can understand this knowledge. Let's explain why value passing is infinitely recursive! If the copy constructor is this:

 
   
  

  
    
   
test(test t);
 
   
  

We call

  
 

   
  

  
test ort;
   
  

  
test a   ( ort     -->   test   a   ( test t  =  ort   test    a   ( test t   ( ort    
   
  

  
 -->test.a(test t(test t = ort))
   
  

  
 ==test.a(test t(test t(ort)))
   
  

  
 -->test.a(test t(test t(test t=ort)))
   
  

  
...
   
  

  
    就这样会一直无限递归下去。
  
 

Here we also understand why the parameters of a copy constructor must be a reference and cannot be passed for a value.
Next, we test the parameters of the assignment constructor, and if we change its parameters to a value pass, do a test.

  
 

   
  

  
class test
   
  

  
{
   
  

  
public:
   
  

  
test()
   
  

  
{
   
  

  
cout << "constructor with argument\n"; 
   
  

  
}
   
  

  
~test()
   
  

  
{
   
  

  

   
  

  
}
   
  

  
test(test& t)
   
  

  
{
   
  

  
 cout << "copy constructor\n"; 
   
  

  
}
   
  

  
test&operator=(test e)
   
  

  
{
   
  

  
cout << "assignment operator\n"; 
   
  

  
return *this;
   
  

  
}
   
  

  
};
   
  

  
int _tmain(int argc, _TCHAR* argv[])
   
  

  
{
   
  

  
test ort;
   
  

  
test a(ort);
   
  

  
test b = ort ;
   
  

  
 a = b;
   
  

  
return 0;
   
  

  
}
  
 

Output: Assignment constructor If it is passed as a value, it is only one copy more, and does not have infinite recursion.
Summary: The parameters of the copy constructor must be references. An assignment constructor parameter can be either a reference or a value pass, and a value pass will be copied more than once. Therefore, it is recommended that the assignment constructor also be written as a reference type. (Ckk see just now I understand still have deviation: Left and right value is not the key, reduce copy number increase assignment efficiency is the focus)

From for notes (Wiz)

Why does the C + + copy constructor parameter have to be a reference? Must the assignment constructor argument also be a reference?