Copy constructor and value assignment operator details

C ++ has always remembered that "if a class does not define a copy constructor or a value assignment operator, the compiler will generate a copy constructor and a value assignment operator for it ."
This statement is actually vague. I was surprised to see an example when I was reading a book today ".
# Include <stdio. h>
Struct X {X (){};
Template <typename T> X (const T &) {printf ("templated X copy constructor .");}
Template <typename T> X & operator = (const T &) {printf ("templated X assignment operator .");}};
Int main (int argc, char ** argv) {X x1; X x2 (x1); X x3; x3 = x1 ;}
After this program is compiled, what is the execution result? Answer: No! Neither of the two function templates is called.
The reason is not very complex, but it can be seen that I am still not clear about the details of C ++. Okay, now the details are coming:
C ++ standards (ISO/IEC 14882: 2003 (E) 12.8.2 are clearly written: A non-template constructor for class X is a copy constructor if its first parameter is of type X &, const X &, volatile X & or const volatile X &, and either there are no other parameters or else all other parameters have default arguments (8.3.6 ).
Key Point: A function template with template parameters is not a constructor.
Therefore, the template <typename T> X (const T &) declared in the preceding example is not a copy constructor.
In addition, according to the definition of the copy constructor, the class definition above does not explicitly define any copy constructor.
C ++ standards: If the class definition does not explicitly declare a copy constructor, one is declared implicitly. This is basically something that people who have learned C ++ know. Therefore, the compiler must generate an implicit copy constructor for the above class.
The implicit copy constructor generated by the compiler performs a shortest copy on all the members of the class without any output.
The assignment operator is similar. Standard definition: A user-declared copy assignment operator X: operator = is a non-static non-template member function of class X with exactly one parameter of type X, X &, const X &, volatile X & or const volatile X &. that is, the value assignment operator must be a non-template, non-static member function, and can only have one parameter. This is different from the copy constructor. A copy constructor can have multiple parameters, as long as the first one is X & or cv-qualified X &, and all subsequent parameters have default values.
When the program is actually executed, neither of the two function templates is called, so there is no output.