Template recompilation issue in Sun C++ 5.8 Patch 121017-02 2006/04/19

Question

I am having problems with recompiling programs that use templates for the above version. The problem manifests itself in the following case.
Suppose f1.cpp contains the implementation of template class t1. t1 is instantiated in f1.cpp as well as other files. I change f1.cpp and recompile. Only file f1.cpp is recompiled and the program is relinked. When I run the new program the code in f1.cpp still refers to the old implementation of t1. It's as if an old expansion of the template, present in some other .o is being used instead of the newly created implementation. If this is a 'design feature' of my compiler then how should I change my makefile to correct the dependencies?
Much obliged,
Adam

Answer 1

Your makefile must reflect the actual file dependencies. If more than one file depends on the file containing the definition of t1, you need to express each dependency on that file.
But your symptoms sound like you have violated the C++ One-Defintiion Rule. If different modules in a program see a different definition of what is supposed to be the same template instance, the results are undefined. No makefile change can fix that problem.
Example:
t1.h
template<class T> class t1 { ... };
f1.cpp
#include " t1.h"
template<> class t1<int> { ... }; // explicit specialization
f2.cpp
#include "t1.h"
t1<int> t; // implicit specialization
In this example, file f1 creates an explicit specialization t1<int> with its own definition. File f2 uses an implicit specialization based on the primvary template, because the explicit specialiation is not declared. The two modules have different ideas about what t1<int> means. To satisy the One-Definition Rule, the explicit specializaiton t1<int> must be declared or defined identically in every module that uses it. Usually you put at least the declaration in the header associated with t1.
Versions of Sun C++ since Sun Studio 8 (C++ 5.5) use the "comdat" model of template instantiation. Every module that needs to create a template instance does so. At link time, all but one instance is discarded. Since the One-Definition Rule requires that all instances be identical, it does not matter which instance is retained.
In your case, it sounds like you modified the definition of an instance in one file without affecting the definition of that instance in other files. Apparently the modified instance was discarded.