Joel de Guzman wrote:
shunsuke wrote:
I tend to think `value_at` of transform_view should be the same as `result_of::deref` of transform_view. I'm not sure, though.
value_at (as well as value_of) returns (essentially):
f(value_at<s>)
while at (as well as deref) returns (essentially):
f(at<s>)
alas, this uncovers a bug in the implementation where value_at does not follow above.
What is a bug? Is there any pitfall if `value_at` is the same as `at` under transform_view?
You got me thinking though. It could very well be that you are correct! Perhaps we need the same behavior for both at/deref and value_at/value_of. My thinking, OTOH, is that both should get the value_at of the underlying sequence. Hence, transform(vector<int, int&>, f) will trigger f this way:
int ---> f::result<f(int)> int& ---> f::result<f(int&)>
No, when vector<int, int&> is mutable lvalue, f should be able to access the mutable int. Hence, int --> f::result<f(int&)> int& --> f::result<f(int&)> The current deref works like this unless as_vector is used. It's ok. Again, #include <boost/fusion/include/as_vector.hpp> #include <boost/fusion/include/transform_view.hpp> #include <boost/fusion/include/vector.hpp> #include <boost/fusion/include/at.hpp> #include <boost/fusion/include/begin.hpp> struct identity { template<class FunCall> struct result; template<class Fun> struct result<Fun(int&)> { typedef int& type; }; int& operator()(int& i) const { return i; } }; void test() { // This compiles fine. typedef boost::fusion::vector<int, int> from_t; from_t from(10, 20); boost::fusion::transform_view<from_t, ::identity> v(from, ::identity()); *boost::fusion::begin(v) = 999; BOOST_CHECK(boost::fusion::at_c<0>(from) == 999); //boost::fusion::as_vector(v); // error! } Here, as_vector calls value_of. Hence, int --> f::result<f(int)> My poor identity doesn't compile. IMHO, as_vector should be able to return vector<int&, int&>. Once as_vector intervenes, FunctionObject can't access mutable elements in a tuple. For example, mutable STL iterator can't be returned from std::string in a tuple. Regards, -- Shunsuke Sogame