Re: Template instantiation in libraries

[Paul Pluzhnikov]

Garrett Kajmowicz <garrett@garrett.dyndns.biz> writes:

> However, I am unable to figure out how to get the compiler or
> (ideally) my library code to specify that in the binary that if a version
> of the function is available in the library it is being linked against
> that it should not be kept as a weak symbol.
> 
> Is there any easy way to do this after-the-fact with binutils?

Since the "undesired" functions each reside in a separate easily
identifiable section, couldn't you e.g.
  objcopy --remove-section 
.gnu.linkonce.t._ZNSt9basic_iosIcSt11char_traitsIcEED1Ev
on the objects, before linking the application?

> I have found a partial solution.  For cases where I want to provide
> certain versions of a template, I define a specialized version of the
> template in the header file, but put the definition in a .cpp file and
> compile it into the library.  This works and prevents instantiation when
> compiling.  However, it results in my having to have duplicate copies of
> the code kicking around, which is bad engineering and bad for
> maintenance, etc.

Here is another possible approach. Define all template functions
is a separate .cpp file, and let the user control where they are
instantiated. For example, let's assume you have

  /// "include/string"
  template <typename T> class basic_string { 
      basic_string(); // body in separate string.cpp
      ...
  };
  typedef basic_string<char> string;

The user who simply '#include <string>' and uses only 'std::string',
doesn't have to do anything: his objects will have
'std::basic_string<char>()::basic_string()' undefined, and your
library will provide a definition at link time.

The (presumably rare) user who uses 'basic_string<wchar_t>' will
get unresoved symbols (assuming you do not have basic_string<wchar_t>
compiled into your library). So you tell that user that he has to
#include <string.cpp> into one of his source files that uses the
"non-standard" instances, and you are done.

Cheers,
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