id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
4967	apply_visitor_delayed_t 's operator() is not const correct	zhuo.qiang@…	ebf	"The operator() overload in Boost.Variant's boost::apply_visitor_delayed_t is not const qualified which I think could be safely add.

The benefit of adding const is as following:

There are some generic function need to store and call the apply_visitor_delayed_t instance, transform_iterator & transform_range, for example. The following code will not compile without the const quality:

----------------------------------------------------
using namespace boost;
using namespace std;

typedef variant<int, string> V;

struct  times_2_t : public static_visitor<int>
{
    template<class T>
    int operator()(T const& t) const
    {
        return 2 * t;
    };

    int operator()(string const& s) const
    {
        return 2 * s.size();
    };
};

BOOST_AUTO_TEST_CASE(Test)
{
    vector<V> all;
    all.push_back(1);
    all.push_back(2);
    
    times_2_t times_2;
    int const result = boost::accumulate(
        all | boost::adaptors::transformed(apply_visitor(times_2)),
        0);

    BOOST_CHECK_EQUAL(6, result);
}

---------------------------------------

One way to fix this is using a boost::funciton to hold the apply_visitor_delayed_t instance and pass the wrapper to transformed:
---------------------------------------------------------
    times_2_t times_2;
    function<int(V const&)> f = apply_visitor(times_2);
    int const result = boost::accumulate(
        all | boost::adaptors::transformed(apply_visitor(f)),
        0);
-------------------------------------------------------
However, this will make simple things complex and have runtime cost.

So a more direct fix would be add the harmless const qualify to the operator() for apply_visitor_delayed_t

The attached is a simply patch:

"	Patches	closed	To Be Determined	variant	Boost 1.45.0	Problem	fixed		prabhu.swain@…