[Boost-users] Re: may it change ?

From: maxch (chenzero_at_[hidden])
Date: 2003-09-17 09:15:50

oh,  the reason is this :)   I see,
thanks for the details reply !

"Steve Horne" <steve_at_[hidden]>
??????:5.1.1.6.0.20030916202741.00ad9870_at_pop.freeserve.com...
> At 23:25 16/09/2003 +0800, you wrote:
> >Hi All,
> >I am a new user of Boost. but when I look the source of Boost, I found
> >that almost all the implementations is put in head files together, not as
> >usual to
> >put into CPP file. I know, it must be intended, however, could it
possible
> >to move the implementations into cpp file in future?
> >
> >Sorry if this question has been posted.
>
> Cool - a question that I can answer!
>
> The boost libraries make heavy use of templates. To be useful, the
> templates must be 'instantiated' by the library user - ie the template
> parameters supplied and the resulting object code generated. In a sense,
> this is rather like using a macro to generate the code.
>
> As the code has to be available when the users code instantiates it, it
has
> to be kept in header files. There is no resultant object code until the
> instantiation is compiled.
>
> For example, every cpp file where a particular template is instantiated
> with the same parameters will contain a duplicate copy of the resultant
> object code. This is handled by the linker, which discards duplicate
copies
> of that object code so they don't increase the size of the application.
>
> This approach to templates can result in 'bloat', however, if templates do
> not share code with other instantiations (instantiated with different
> parameters) when possible. This is normally solved by writing templates
> that provide typesafe wrappers to non-template underlying code and data
> structures. When this is done, the template instantiation itself generates
> very little object code.
>
> It is also not unusual for templates to not generate object code at all,
> but to effectively evaluate expressions at compile-time - so-called
> template metaprogramming.
>
> The point of all this is basically this - to the best of my (very limited)
> knowledge, Boost already uses CPP files wherever that would be
appropriate.