Re: [boost] Re: basic guarantees in variant

From: E. Gladyshev (egladysh_at_[hidden])
Date: 2003-10-08 23:56:21

Sorry, cannot stop comming back to it. :)
I just realized that this solution seems 
to be able to provide strong guarantess 
for variant w/o memory allocation and double 
storage overhead?

Eugene

--- "E. Gladyshev" <egladysh_at_[hidden]> wrote:
> Please remember it was pseudo-code and please don't tell about threads. :)
> 
> Here is a few fixes anyway.
> 
>     stack_variant<T>& operator=( const T& l )
>     {
>         const int size = largest_size<T>::value;
>         char b1[size], b2[size];
>         
>         //save the current data
>         variant_copy( b1, data_, size );
>         
>         T *obj(0);
>         
>         try
>         {
>             //place new object in _data
>             obj = new(data_) T;
>             //copy data
>             *obj = l;
>             obj = 0;
>             //copy the new object to another storage
>             variant_copy( b2, data_, size );
>             //copy the original object back for destruction
>             variant_copy( data_, b1, size );
>             static_cast<T*>((void*)data_)->~T();
>             //put the new object back in place
>             variant_copy( data_, b2, size );
>         }
>         catch( ... )
>         {
>             if( obj ) obj->~T();
>             variant_copy( data_, b1, size );
>             throw;
>         }
>         return *this;
> 
> Eugene
> 
> 
> 
> 
> --- "E. Gladyshev" <egladysh_at_[hidden]> wrote:
> > 
> > --- David Abrahams <dave_at_[hidden]> wrote:
> > 
> > [...]
> > > Please elaborate.
> > 
> > Here is how I would do it.
> > 
> > Below is a working psedo-code. I tested it and it worked ok
> > including assignment exceptions.
> > The sample is a simple type holder that provides
> > basic guarantees during assignments.
> > 
> > template< typename T >
> > struct largest_size
> > {
> >    enum
> >    {
> >       value = sizeof(T)
> >    };
> > };
> > 
> > 
> > //doesn't throw
> > void variant_copy( char* dst, char* src, size_t size )
> > {
> >     for( size_t i = 0; i < size; ++i, ++dst, ++src )
> >     {
> >         *dst =  *src;
> >     }
> > }
> > 
> > 
> > template< typename T >
> > struct stack_variant
> > {
> >     char data_[largest_size<T>::value];
> >     
> >     stack_variant() { new(data_) T; }
> >     
> >     stack_variant<T>& operator=( const T& l )
> >     {
> >         const int size = largest_size<T>::value;
> >         char b1[size];
> >         char b2[size];
> >         
> >         //save the current data
> >         variant_copy( b1, data_, size );
> >         
> >         try
> >         {
> >             //place new object in _data
> >             T* obj = new(data_) T;
> >             //copy data
> >             *obj = l;
> >             //copy the new object to another storage
> >             variant_copy( b2, data_, size );
> >             //copy the original object back for destruction
> >             variant_copy( data_, b1, size );
> >             static_cast<T*>((void*)data_)->~T();
> >             //put the new object back in place
> >             variant_copy( data_, b2, size );
> >         }
> >         catch( ... )
> >         {
> >             variant_copy( data_, b1, size );
> >             throw;
> >         }
> >         return *this;
> >     }
> > };
> > 
> > 
> > Eugene
> > 
> > 
> > 
> > 
> > 
> > 
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> 
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