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From: Joel de Guzman (joel_at_[hidden])
Date: 20071122 18:12:28
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 lvalues, 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 puremetafunction
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.boostconsulting.com http://spirit.sf.net
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