apply_visitor_delayed_t 's operator() is not const correct

[zhuo.qiang@…]

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:

[anonymous]

The patch for adding the harmless const qualify for the apply_visitor_delayed_t

[anonymous]

The second code snipper should be this:

    
times_2_t times_2; 
function<int(V const&)> f = apply_visitor(times_2); 
int const result = boost::accumulate(
        all | boost::adaptors::transformed(f), 0);

[Steven Watanabe]

Simply making the overloads const is not harmless, because it will break code that uses function objects that aren't const. The right way is to add const overloads.

[Steven Watanabe]

Never mind. I'm obviously not looking at the code carefully enough.

[Steven Watanabe]

(In [67605]) Make operator() const. Fixes #4967