Added sections about extended friend declarations

git-svn-id: https://cppannotations.svn.sourceforge.net/svnroot/cppannotations/trunk@620 f6dd340e-d3f9-0310-b409-bdd246841980

yo/classtemplates.yo

@@ -101,6 +101,9 @@ includefile(classtemplates/friends)
     subsect(Unbound templates as friends)
     includefile(classtemplates/unboundfriends)
 
+    subsect(Extended friend declarations (C++11, 4.7))
+    includefile(classtemplates/extended)
+
 lsect(DERIVEDTEMPCLASS)(Class template derivation)
 includefile(classtemplates/derived)
 

yo/classtemplates/extended.yo

@@ -0,0 +1,35 @@
+The C++11 standard defines emi(extended friend) em(declarations). Extended
+friend declarations are also available for class templates.
+
+Extended friend declarations for class templates allow us to use template type
+parameters as friend declarations. A template type argument, however, does not
+necessarily have to be a type for which the keyword tt(friend) makes sense,
+like tt(int). In those cases the friend declaration is simply ignored.
+
+Consider the following class template, declaring tt(Friend) as a friend:
+        verb(
+    template <typename Friend>
+    class Class
+    {
+        friend Friend;
+        void msg();             // private, displays some message
+    };
+        )
+    Now, an actual tt(Friend) class may access all of tt(Class)'s members
+        verb(
+    class Concrete
+    {
+        Class<Concrete> d_class;
+        Class<std::string> d_string;
+ 
+       public:
+            void msg()
+            {
+                d_class.msg();    // OK: calls private Class<Concrete>::msg()
+                //d_string.msg(); // fails to compile: msg() is private
+            }
+    };
+        )
+    A declaration like tt(Class<int> intClass) is also OK, but here the friend
+declaration is simply ignored. After all, there are no `int members' to access
+tt(Class<int>)'s private members.

yo/friends.yo

@@ -5,5 +5,5 @@ includefile(friends/intro)
     lsect(FriendsFriendfun)(Friend functions)
     includefile(friends/friendfun)
 
-    sect(Extended friend declarations (C++11, 4.7))     TODO
+    sect(Extended friend declarations (C++11, 4.7))
     includefile(friends/extended)

yo/friends/extended.yo

@@ -0,0 +1,29 @@
+C++11 simplifies tt(friend) declarations by adding 
+    emi(extended friend) em(declarations) 
+    hi(friend: extended declaration)
+        to the language. When a class is declared as a friend, then the
+tt(class) keyword no longer has to be provided. E.g.,
+        verb(
+    class Friend;                   // declare a class
+    typedef Friend FriendType;      // and a typedef for it
+    using FName = Friend;           // and a using declaration
+
+    class Class1
+    {
+        friend Friend;              // FriendType and FNaem: also OK
+    };
+        )
+In the pre-C++11 standards the friend declaration required an explicit
+tt(class); e.g., tt(friend class Friend). 
+    
+The explicit use of tt(class) remains required if the compiler hasn't seen
+the friend's name yet. E.g.,
+        verb(
+    class Class1
+    {
+        // friend Unseen;           // fails to compile: Unseen unknown.
+        friend class Unseen;        // OK
+    };
+    
+Section ref(TEMPFRIENDS) covers the use of extended friend declarations in
+class templates.