Re: [Boost-users] boost::apply_visitor(const& visitor, ...) - why const?

From: Yuval Ronen (ronen_yuval_at_[hidden])
Date: 2007-01-19 05:40:39

cdr_at_[hidden] wrote:
> Hello,
> 
> Why does boost::apply_visitor take a const reference to the visitor object? This forces me to write a
> boost::static_visitor-derived visitor that looks like this:
> 
> class my_visitor_t : public boost::static_visitor<>
> {
>     template <typename T> void operator()(T const& t) const
>     { // do something ... }
> 
> };
> 
> ... when what I would really like to do is something like this:
> 
> class my_visitor_t : public boost::static_visitor<>
> {
>     my_visitor(my_data & _data) : data(_data) {}
> 
>     void operator()(int const& i) // NOT const
>     { // data.blah.blah = // blah }
> 
>     template <typename T> void operator()(T const& t) // NOT const
>     { // data.foo() }
> 
>     my_data & data
> 
> };
> 
> So I might have several different instances of a given a my_variant_t:
> 
> my_variant_t v1;
> my_variant_t v2;
> my_variant_t v3;
> 
> my_data_t my_data;
> 
> my_visitor_t my_visitor(my_data);
> 
> boost::apply_visitor(my_visitor, v1);
> boost::apply_visitor(my_visitor, v2);
> // and so on...
> 
> ... BUT, the fact that I must make the visitor operator() const makes this impossible and is inconsistent with the way that I'm
> used to leveraging visitors in the BGL. Would someone be kind enough to comment on the rationale behind this? What am I missing
> here?
> 
> - Thanks
> 
> Chris

Accepting the visitor by non-const reference will make calling 
apply_visitor with a temporary visitor not compile, so there's a 
downside to that too. You can use the apply_visitor member function of 
variant:

v1.apply_visitor(my_visitor);
v2.apply_visitor(my_visitor);

which accepts the visitor by non-const reference.

Not that I'm saying that any if these solutions is best. IMO, a 
static_visitor is some kind of a functor, and as such should be accepted 
by value, making it possible to pass temporaries, and write code as you 
described. However, I couldn't convince about it...

Yuval