shunsuke wrote:
Hi,
(Now my question was simplified.) How can a sequence object be transformed into fusion::vector object, keeping mutability of elements?
For example,
When at_c return type Sequence of input is:
int&, char, double const&, ...
, a type Sequence I need is:
fusion::vector<result_of<F(int&)>::type, result_of<F(char)>::type, result_of<F(double const&)>::type, ...>
Note that the output I need is not a fusion::transform_view but a fusion::vector. as_vector doens't work, because as_vector makes use of value_of. Am I missing some predefined function?
I think you misunderstood some of my previous posts. value_of is only used to compute the desired result. There is actually no corresponding value_of function. It is only a metafunction, unlike /at/ which has both a result_of metafunction and a function. When the input sequence is actually traversed to generate the result, /at/ is the one being called. Hence, the sequence above is what is actually generated by as_vector. So, if you are sure that you have real l-values, the transform function: struct f { template <typename FunCall> struct result; template <typename Fun> struct result<Fun(int&)> { typedef int& type; }; template <typename Fun> struct result<Fun(int)> { typedef int& type; }; int& operator()(int& i) const { return i; } }; is perfectly safe. In fact, it won't compile if you do not have an lvalue. This won't compile if you give it an mpl::vector, for example (which doesn't have lvalue elements) because your operator() specifically asks for a reference. The second struct result<Fun(int)> is a pure-metafunction call called by value_at. You can read it as: the type will be an int& if the element type is an int -- it's a type only computation. It's unfortunate and a bit unintuitive to be using the result_of scheme to compute the value_at metafunction here. MPL uses the nested "apply" for this purpose. OTOH, it would be annoying to have to use "apply" for value_at and "result" for /at/. It's a compromise. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net