16.3.1 Template Instantiation
The problem with template instantiation exists because of a number of
- The compiler should only generate an instance of a template once,
to speed the compilation process.
- The linker needs to be smart about where to locate the object code for
instantiations produced by the compiler.
This problem is exacerbated by separate compilation--that is, the method
List<T> may be located in a header file or in a
separate compilation unit. These files may even be in a different
directory than the current directory!
Life is easy for the compiler when the template definition appears in
the same compilation unit as the site of the instantiation--everything
that is needed is known:
template <class T> class List
This becomes significantly more difficult when the site of a template
instantiation and the template definition is split between two different
compilation units. In Linkers and Loaders, Levine describes in
detail how the compiler driver deals with this by iteratively attempting
to link a final executable and noting, from `undefined symbol'
errors produced by the linker, which template instantiations must be
performed to successfully link the program.
In large projects where templates may be instantiated in multiple
locations, the compiler may generate instantiations multiple times for
the same type. Not only does this slow down compilation, but it can
result in some difficult problems for linkers which refuse to link
object files containing duplicate symbols. Suppose there is the
following directory layout:
| `--- core.cxx
| `--- http.cxx
If the compiler generates `core.o' in the `core' directory and
`libhttp.a' in the `http' directory, the final link may fail
because `libhttp.a' and the final executable may contain duplicate
symbols--those symbols generated as a result of both `http.cxx' and
`core.cxx' instantiating, say, a
Stack<int>. Linkers, such
as that provided with AIX will allow duplicate symbols during a
link, but many will not.
Some compilers have solved this problem by maintaining a template
repository of template instantiations. Usually, the entire template
definition is expanded with the specified type parameters and compiled
into the repository, leaving the linker to collect the required object
files at link time.
The main concerns about non-portability with repositories center around
getting your compiler to do the right thing about maintaining a single
repository across your entire project. This often requires a
vendor-specific command line option to the compiler, which can detract
from portability. It is conceivable that Libtool could come to the
rescue here in the future.