Several situations of the C ++ class template membership statement

According to section 16.4 of the third edition of C ++ primer, the C ++ class template can be divided into the following situations:
1. Non-template friends or friends functions.
Here is an example:
Class Foo {
Void bar ();
};
Template <class T>
Class queueitem {
Friend class foobar;
Friend void Foo ();
Friend void FOO: bar ();
//....
};
It is very simple. It is no different from non-template classes. Note that if you want to set the function and class life as a friend, you do not need to declare or define it before. However, if you want to declare a class member function as a friend, you must first define the class (not declaration, but definition), becauseA class member can only be introduced by the class definition.
.

2. Bind a membership template or function template.
Example:
Template <class type>
Class foobar {...};

Template <class type>
Void Foo (queueitem <type> );

Template <class type>
Class queue {
Void bar ();
//...
};

Template <class type>
Class queueitem {
Friend class foobar<Type>
;
Friend void foo<Type>
(Queueitem <type> );
Friend void queue<Type>
: Bar ();
//...
};
Pay attention to the following two points:
A. Unlike non-template functions or classes, before the template functions or classes are declared as friendsMust have been declared earlier
Otherwise, compilation fails.
B. Pay attention to the red-letter part. The types should not be less. For example, if <type> is missing for function Foo, the compiler treats it as a non-template function, that is, for queueitem <int>, the compiler looks for void Foo (queueitem <int>) and turns a blind eye to template <class T> void Foo (queueitem <t>). If no non-template function is found, an error is returned.

3. Non-bind user meta templates or function templates.
Example:
Template <class type>
Class queueitem {
Template <class T>
Friend class foobar;

Template <class T>
Friend void Foo (queueitem <t> );

Template <class T>
Friend void queue <t >:: bar ();
//...
};