TITLE: template separate compilation and specialization

[ There is an interesting article on templates in Glen's
  newsletter. For brevity, I removed the other articles.
  See the trailer for complete subscription info. -adc ]

Issue #026
August, 1997


Contents:

Dynamic Casts
Introduction to Object-oriented Design Part 5 - Representation Hiding
Notes From ANSI/ISO - Template Separate Compilation and Specialization
Introduction to STL Part 13 - Accumulating


DYNAMIC CASTS

[snip]

INTRODUCTION TO OBJECT-ORIENTED DESIGN PART 5 - REPRESENTATION HIDING

[snip]

NOTES FROM ANSI/ISO - TEMPLATE SEPARATE COMPILATION AND SPECIALIZATION
Jonathan Schilling, jls@sco.com

In C++ Newsletter #017 the new template separate compilation model was
described.  Since its introduction the ANSI/ISO standards committee has
been dealing with sorting out the loose ends from this addition to the
language.

Take the following case, for instance:  

file1.h:
        template<class T> class A { };
        template<class T> void f(T);

file1.C:
        #include "file1.h"
        export template<class T> void f(T) { A<T> a; }

These files introduce a template class, and a template function whose 
definition depends upon the template class.  The keyword "export" says 
that the template function definition doesn't have to be included into 
translation units that call it; rather it can be separately compiled.

file2.C:
        #include "file1.h"
        template<> class A<int> { };    // line 2
        void g() ( f(1); }              // line 3

Line 2 of this file declares a specialization (see C++ Newsletter #012)
of the class template for type int, and then line 3 instantiates the 
function template for type int, which in turns instantiates the class 
template for type int.  But the function template definition in file1.C 
cannot see the specialization in file2.C.  So how does this work?  Does 
the specialization not get used?  Or does compilation of file1.C get 
deferred somehow until the specialization is seen?

At the recent standards meeting in London, the committee decided that
when an explicit specialization is not visible in an instantiation
context, yet would affect the instantiation (such as in this case), the
program is ill-formed, but that no diagnostic is required.  

This ruling falls into the same general behavior as the One Definition 
Rule (see C++ Newsletter #015).  As such, it makes things easier for 
compiler implementors but harder for language users.  For instance, the 
writer of file2.C may not look at the implementation of f(), and thus
may not realize that specialization of class A will cause (possibly
silent) undefined behavior.

As we've editorialized before, the One Definition Rule is a weak area 
of C++, especially when compared to languages such as Ada and Java which
stay well-defined across separate compilation boundaries.  The problems
mostly result from the use of #include files as the mechanism for 
modularity; improving this was something occasionally discussed in C++ 
standardization circles but never really tackled.  Maybe in C++ 0X!


INTRODUCTION TO STL PART 13 - ACCUMULATING

[snip]


ACKNOWLEDGEMENTS

Thanks to Nathan Myers, Eric Nagler, David Nelson, and Jonathan
Schilling for help with proofreading.


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-------------------------

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