With the external instances method, all instances are placed within the template repository. The compiler ensures that exactly one consistent template instance exists; instances are neither undefined nor multiply defined. Templates are reinstantiated only when necessary. For non-debug code, the total size of all object files (including any within the template cache) may be smaller with -instances=extern than with -instances=global.
Template instances receive global linkage in the repository. Instances are referenced from the current compilation unit with external linkage.
The disadvantage of this method is that the cache must be cleared whenever you change programs or make significant program changes. The cache is a bottleneck for parallel compilation, as when using dmake because access to the cache must be restricted to one compilation at a time. Also, you can build only one program within a directory.
Determining whether a valid template instance is already in the cache can take longer than just creating the instance in the main object file and discarding it later if needed.
Specify external linkage with the –instances=extern option.
Because instances are stored within the template repository, you must use the CC command to link C++ objects that use external instances into programs.
If you wish to create a library that contains all the template instances that it uses, compile with the-xar option. Do not use the ar command. For example:
example% CC -xar -instances=extern -o libmain.a a.o b.o c.o