Re: [boost] BOOST_NO_EXTERN_TEMPLATE

Subject: Re: [boost] BOOST_NO_EXTERN_TEMPLATE
From: John Maddock (boost.regex_at_[hidden])
Date: 2010-05-27 04:40:51

> Boost.Config defines the macro BOOST_NO_EXTERN_TEMPLATE, which  probably 
> [1] refers to suppressing the instantiation of a template  using the 
> "extern" keyword.
> This is part of the C++0x draft but has long been implemented by  several 
> compilers, including MSVC[2] and GCC[3].
> BOOST_NO_EXTERN_TEMPLATE is still defined for both compilers, why is that?
> do those implementations differ from the proposed C++0x behaviour[4],  so 
> the macro is defined because it refers to a C++0x feature? Or is  this a 
> bug?

There was a bug in the original test case that was committed for this C++0x 
feature that resulted in the macro being set for all current compilers, that 
was recently fixed in Trunk, it just hasn't made it to the release branch 
yet.

> Is using "extern template" recommended for boost libraries (under 
> construction)?
> specifically, the library lends itself to seperate precompiled source  and 
> started out as such, but making it more configurable turned  seperate 
> source code into templates and increased compile time.
> the default use case (which was the only use case before) could still  be 
> compiled seperately if "extern template" is available.

Sure, Boost.Regex has done that for a long time.

FYI though the feature seems to be flaky with some compilers: I'm currently 
stuggling to make non trivial cases work with the Intel compiler for example 
:-(

> [1] the documentation says the macro indicates no support for explicit 
> instantiation. I assume this is a documentation bug, because of its  name 
> and it's listed as a C++0x feature:
> http://www.boost.org/doc/libs/1_43_0/libs/config/doc/html/boost_config/boost_macro_reference.html

Oh :-(

Will fix,

John.