TITLE: Do templates cause code bloat?


PROBLEM: Jan_Walter@mindlink.bc.ca (Jan Walter)

[...] You're telling me that if under compiler X that I have a
list template and do the following:

List<int> li;
List<Object> lo;
List<float> lf;

That 3x the amount of code is not going to be generated?  Or that 3x the
amount of code MAY be generated?  I would think that this is highly
dependent on the compiler, but have a feeling that, yes, 3x the amount of
code is going to be generated regardless.


RESPONSE: bs@alice.att.com (Bjarne Stroustrup), 21 Mar 94
          AT&T Bell Laboratories

That depends primarily of how List is defined. If List stores a COPY of its
argument, code replication is the obvious and often the best implementation
technique. There is no single best strategy for implementting such templates.
What is ``best'' depends critically on the definition of the template, the
use of the templates, the users needs in terms of time, space, re-compilation
overheads, etc. There was a nice PhD thesis from CMU demonstrating that about
ten years ago, but I don't recall the title.

However, if - as is not uncommon - the list stores a pointer to the argument,
no code replication is necessary. This point/problem was specifically
considered in the design of templates. The basic technique is to use
inheritance to eliminate the need for code replication.

See sec13.8 [ Interface Classes ] of

	Bjarne Stroustrup:
	The C++ Programming Language (2nd edition)
	Addison-Wesley ISBN 0-201-53992-6.
	1991. 

or sec15.5 of 

	Bjarne Stroustrup:
	The Design and Evolution of C++
	Addison Wesley, ISBN 0-201-54330-3.
	March 1994.