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_::type data;

Niels Dekker - mail address until 2008-12-31 wrote:

Edward Diener wrote:

...

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_::type data;

Okay, so your data is either a T, or a value_initialized<T>. I was wondering, do you use the fact that value_initialized<T> implicitly converts to reference-to-T?

If your data is value_initialized<T>, you do boost::get(data) = arg. Would it be helpful to you to have an extra (explicit) boost::value_initialized<T> constructor, having const-reference to T as argument?

I have moved away from solving my problem based on const. This is because in C++ there is the notion of 'read-only' ( const ) and 'read-write' ( no const ) whereas what I am designing also needs the notion of 'write-only'. Therefore I have a second template parameter, defaulting to 'read-write' to control that and I can specialize on that parameter. I may revisit the const issue in my design again, but for now I am giving a compile time error when my T has top level const.