Re: [Boost-users] [value_initialized] Non-default constructors

From: Edward Diener (eldiener_at_[hidden])
Date: 2008-05-20 08:14:22

Niels Dekker - mail address until 2008-12-31 wrote:
> Edward Diener wrote:
>> I have a class template, templated on type T, which I need to hold as
>> a data value in the class. I use value_initialized for this type T
>> effectively, and the value gets value initialized correctly when the
>> default constructor of my class is used.
>>
>> If, however, I add a second constructor to my class which takes a T,
>> for initializing my 'value_initialized<T> data' value, things do not
>> work as well. Obviously I can still value initialize my data value,
>> as in my default constructor, and then use 'boost::get(data) = arg;'
>> to set the value of my value initialized variable. But this will not
>> work if the type T is a top-level const, whereas I do want to
>> initialize the data value in this case to the T arg passed into my
>> second constructor.
> 
>> Never mind, the question was due to my unclear thinking about the
>> problem.
> 
> So... how did you solve the problem?

I solved it in my template by having the data value for T be of type T 
when T is top-level const, otherwise it is boost::value_initialized<T> 
when T is not a top-level const. The mpl::if_ metafunction and 
boost::is_const were used to generate the correct type, ala

mpl::if_<boost::is_const<T>,T,boost::value_initialized<T> >::type data;