Re: [boost] bug in boost::variant?

Subject: Re: [boost] bug in boost::variant?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-12-11 13:30:34

On 12/10/09 22:11, Zachary Turner wrote:
> I'm using boost 1.41.  I use a variant to store a symbolic representation of
> an algebraic expression, wrapped inside of a master "node" class.  Like
> this:
> 
> struct expr_node
> {
>     friend class constant;
>     friend class variable;
>     friend class arithmetic_op;
>     friend class exponent;
>     friend class special_func;
>     friend class negative;
> public:
>     template<class T>
>     expr_node(const T& expr)
>         : expr_(expr)
>     {
>     }
> 
>     expr_node& operator=(const expr_node& rhs)
>     {
>         expr_ = rhs;
>         return *this;
>     }
> 
> private:
>     typedef boost::variant<constant, variable, arithmetic_op, exponent,
> special_func, negative> node_type;

I think some of these variant args should have recursive_wrapper
wrapping them.  See the link in following Dec 3 post to this list:

http://sourceforge.net/mailarchive/message.php?msg_name=loom.20091203T005648-860%40post.gmane.org

However, in that code, I don't think there needs to be a

   recursive_wrapper<Node>

in the variant's components.

> 
>     node_type expr_;
> };
> 
> Of course, this is a recursive definition, because exponent, arithmetic_op,
> etc also contain expr_node members.
[snip]

HTH.

-regards,
Larry