Re: "Must instantiate controlled types at library level." Why?

["Peter C. Chapin" <pchapin@sover.net>]

Martin Krischik <krischik@users.sourceforge.net> wrote in 
news:13392802.3gDeTK7ybb@linux1.krischik.com:

> It is not quite complete:
> 
>> -----> cut <-----
>> #include <iostream>
>> 
>> class X {
>> public:
>>   virtual void f();
>> };
>> 
>> void X::f()
>> {
>>   std::cout << "I'm in X::f\n";
>> }
>> 
>> X *ptr;
>> 
>> void helper()
>> {
> 
> auto int Z := 5;
> 
>>   class Y : public X {
>>   public:
>>     virtual void f() { 
>>     std::cout << "I'm in Y::f\n";
> 
> std::cout << "and Z is " << Z << std:endl;
> 
>>     }  
>>   };
>> 
>>   ptr = new Y;
>> }
>> 
>> int main()
>> {
>>   helper();
>>   ptr->f();
>>   return 0;
>> }

Actually I tried this modification. In fact, the three C++ compilers I 
used *do* produce a compile time error on this code. In particular the use 
of an auto variable in the member function of a local class is 
specifically illegal. The relevant language from the C++ standard is in 
section section 9.8, paragraph 1: "Declarations in a local class can use 
only type names, static variables, extern variables and functions, and 
enumerators from the enclosing scope." The standard then goes on to give 
an example showing that the use of an auto variable is illegal.

So it appears that contrary to my earlier assumption, C++ does in fact try 
to do something sensible with this construction.

Peter