Re: [boost] New libraries should provide forward declarations?

From: Simonson, Lucanus J (lucanus.j.simonson_at_[hidden])
Date: 2008-08-20 12:29:25

OP wrote:
>>Would it practical to add a rule of thumb to the developer guidelines
that
>>library developers should offer a fwd declaration header for their
library
>>when possible?

Benoit wrote:
>I second this question. In fact, i am wondering whether it would not
>be even more effective to suggest the separation of each (public)
>template class into 3 different header files : one containing a
>forward declaration header, one containing the template class
>definition and the last one with the definition of its member
>functions.

I have found in my template programming that it is highly beneficial to
separate the template class declaration from the definition of its
member functions.  This leads to a much more readable set of header
files similar to what people are used to reading when definitions are
contained within a cpp file.  I find the boost code somewhat hard to
read because the header files contain all the implementation details
that could have been put in a separate definitions hpp.  At the very
least definitions can be moved to the bottom of the same header file.
This does have the drawback, however, that the compiler will complain
about cyclic dependencies that it would not otherwise.

>That would make it possible to easily separate the compilation of
>boost classes (or at least their methods) from that of user-defined
>classes.

No, unfortunately, it wouldn't, because you have to pull in the
definitions to use the template class, what you want is precompiled
headers.  It would, however, allow a header file that makes declarations
that include boost references to not have to pull in all the related
boost implementation details and slow down the compile time of every
compilation unit that depends on it.  I consider this benefit secondary
to the readability benefit.

I third the question, should we make separating template declarations
from their definitions a preferred practice?

Luke