Re: [boost] [proto]howto generate extended tree top down instead of bottom up?

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-08 10:42:24

Larry Evans wrote:
> On 07/07/2007 11:49 AM, Eric Niebler wrote:
> [snip]
>>      template<typename Expr>
>>      struct wrap
>>      {
>>          explicit wrap(Expr const &expr)
>>          {}
>>      };
>>
> Thanks very much Eric; however, I found that with the following:
> 
>       template<typename Expr>
>       struct wrap
>       #define INHERIT_EXPR
>       #ifdef INHERIT_EXPR
>       : Expr
>       #endif
>       {
>           explicit wrap(Expr const &expr)
>           {
>               std::cout<<"Wrap::CTOR\n";
>           }
>       };
> 
> the wrap CTOR is not called when:
> 
>           WrapItUp::call( _1 + _2, i, i );
> 
> is executed.  OTOH, if INHERIT_EXPR is undefined,
> then wrap CTOR *is* called.

This is construct<> being too clever by a half. The expr<> struct 
doesn't have a constructor. It uses brace initialization. The 
construct<> transform tries to accommodate that by using brace 
initialization for things that look like Proto expressions. But I see 
now that it can't really know, so I should break this up into separate 
construct<> and pod_construct<> transforms.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com