Re: [boost] bug in type_traits/is_virtual_base_of ??

Subject: Re: [boost] bug in type_traits/is_virtual_base_of ??
From: Daniel Larimer (dlarimer_at_[hidden])
Date: 2010-02-11 16:00:09

Here is my fix that attempts to account for the potential case of the compiler padding the structure size when the derived class adds members and/or virtual functions.    I made the destructor of X and Y virtual and added an extra data-field that causes both X and Y to experience the same shift in padding and thus the only remaining difference is the virtual Base vs non-virtual Base.

What do you think?  Any potential problems / side effects?  

template<typename Base, typename Derived>
struct is_virtual_base_of_impl<Base, Derived, mpl::true_>
{
    struct X : Derived, virtual Base
    {
       X();
       X(const X&);
       X& operator=(const X&);
       virtual ~X();
       char x;
    };
    struct Y : Derived
    {
       Y();
       Y(const Y&);
       Y& operator=(const Y&);
       virtual ~Y();
       char x;
    };
    BOOST_STATIC_CONSTANT(bool, value = (sizeof(X)==sizeof(Y)));
};

On Feb 11, 2010, at 12:59 PM, John Maddock wrote:

>> cast the bool to an int, still does not work.    It works for most cases, for example this works:
>> 
>> struct C : public A, public B
>> {
>> virtual test(); //
>> }
>> 
>> struct C : public A, public B
>> {
>> virtual test();
>> float    parameter; // DOESN"T WORK ANY MORE
>> };
>> 
> 
> Which specific compiler version?  I need something that I can reproduce here... John.
> 
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