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

From: Chris Russell (cdr_at_[hidden])
Date: 2007-01-18 20:37:31

Thank you for this work-around Felipe - that will get the job done. Still - 
it seems rather a convoluted way to accomplish a task that could be done 
with little fuss if boost::apply_visitor accepted a non-const reference to 
the visitor instance. I'll use your method to get my task complete but would 
still like to understand why it was done this way.

- Thanks

"Felipe Magno de Almeida" <felipe.m.almeida_at_[hidden]> wrote in message 
news:a2b17b60701181719k3da526a2p7030194abf588f03_at_mail.gmail.com...
> On 1/18/07, cdr_at_[hidden] <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?
>
>
> Assuming you dont want to return the results of the execution of the
> visitor into my_data, but to really use it in a non-const way, why
> dont you do something like this:
>
> struct my_visitor
> {
>  typedef void result_type;
>
>  void operator()(int const&, data&) const
>  {
>  }
>
>  template <typename T>
>  void operator()(T const&, data&) const
>  {
>  }
> };
>
> my_visitor v;
>
> boost::apply_visitor(boost::bind(v, my_data), v1);
>
> Now, if you want to return from the visitor, you should use:
>
> struct visitor
> {
>  typedef data result_type;
>
>  data operator()(int const&) const;
>  template <typename T>
>  data operator()(T const&) const;
> };
>
> data d = boost::apply_visitor(visitor(), v1);
>
> It have worked for me for a time.
>
>>
>> - Thanks
>>
>> Chris
>>
>
> Best regards,
> -- 
> Felipe Magno de Almeida