[pooma-dev] signed == signed - sizeof(...), and other warnings... (non-POOMA C++ questions)

[Jeffrey Oldham]

On Tue, May 01, 2001 at 01:07:07PM -0700, James Crotinger wrote:
> 
> Mark> Normally, that warning fires on code like:
> Mark> 
> Mark>   class C {
> Mark>     ~C ();
> Mark>   };
> Mark> 
> Mark> Such a class isn't very useful, since you can't create an object of
> Mark> this type.  (Why?  Because when you construct an object, the
> Mark> destructor has to be accessible at the point of construction; so
> Mark> sayeth the standard.)
> 
> GCC flags "C c;" as an error, but is happy with "C *pc = new C;". This 
> is good or my code below wouldn't work, but I'm not sure I see the 
> difference (for this simple case). 
> 
> Mark> However, the case you bring up (an overloaded operator `delete')
> Mark> should probably be an exception to the warning.
> 
> Actually, I'm not overloading delete. Here's a short example code:
> 
>     class IA { public: virtual int release() = 0; };
> 
>     class A : public IA
>     {
>     public:
>       A() : count_m(0) { }
> 
>       int release()
>       {
>          --count_m;
>          if (count_m == 0) { delete this; return 0; }
>          return count_m;
>       }
> 
>     private:
>       ~A() { }
>       int count_m;
>     };
> 
> When I compile this:
> 
>   $ g++ -Wall -c bar.cpp
>   bar.cpp:17: warning: \
>   `class A' only defines a private destructor and has no friends
>   bar.cpp:17: warning: \
>   `class A' has virtual functions but non-virtual destructor
> 
> You are right in that I can't do:
> 
>   int main()
>   {
>     A a;
>     return 0;
>   }
> 
> But that is actually one of the reasons that I want to have the 
> destructor private - I want to disallow this. You can however do:
> 
>   int main()
>   {
>     IA *pa = new A;
>     pa->release();
>     return 0;
>   }
> 
> This compiles and works just fine. The proper destructor is called
> since it is called through the virtual function release(). But I still
> get the warning.

To eliminate the warnings,
1) add "virtual" in front of the destructor and
2) use the command line option -Wno-ctor-dtor-privacy when compiling.

Thanks,
Jeffrey D. Oldham
oldham at codesourcery.com