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

From: cdr_at_[hidden]
Date: 2007-01-18 19:54:29

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