Semi-Explicit Instances

When you use the semi-explicit instances method, instances are generated only for templates that are explicitly instantiated or implicitly instantiated within the body of a template. Implicit instantiations in the mainline code are not satisfied. Instances are placed within the current compilation unit. As a consequence, templates are reinstantiated during each recompilation; they are not saved to the template repository.

Explicit instances receive global linkage. These instances are visible and usable outside the current compilation unit. Multiple explicit instantiations within a program result in multiple symbol definition errors during linking.The semi-explicit instances method is therefore suitable only when you know that explicit instances will not be repeated, such as when you construct libraries with explicit instantiation.

Implicit instances used from within the bodies of explicit instances receive static linkage. These instances are not visible outside the current compilation unit. As a result, templates can have identical instantiations in several object files. This has two undesirable consequences.

• Multiple instances produce unnecessarily large programs. (Semi-explicit instance linkage is therefore suitable only for programs where template bodies do not cause multiple instantiations.)

• Templates that contain static variables have many copies of the variable; this is an unavoidable violation of the C++ standard. Therefore, use of the semi-explicit instances method is not supported with static variables within templates.

Specify semi-explicit instances with the -instances=semiexplicit option.

You can use semi-explicit instance linkage only with the definitions-included template organization. The compiler does not search for definitions.