Heavy use of templates for mobile platforms

Question

I've been flicking through the book Modern C++ Design by Andrei Alexandrescu and it seems interesting stuff. However it makes very extensive use of templates and I would like to find out if this should be avoided if using C++ for mobile platform development (Brew MP, WebOS, iOS etc.) due to size considerations.

In Symbian OS C++ the standard use of templates is discouraged, the Symbian OS itself uses them but using an idiom known as thin templates where the underlying implementation is done in a C style using void* pointers with a thin template layered on top of this to achieve type safety. The reason they use this idiom as opposed to regular use of templates is specifically to avoid code bloating.

So what are opinions (or facts) on the use of templates when developing applications for mobile platforms.

Answer 1

Go ahead and use templates wherever they make your code easier to understand and to maintain. Avoidance of templates on mobile platforms can be categorized as "premature optimization".

If you run into executable-size issues, then redesign if necessary, but don't start with the assumption that templates will cause problems before you see any actual problems.

A lot of the stuff in "Modern C++ Design" and similar books is not going to lead to bloated code, because so much of it is really designed to ensure type safety and do compile-time metaprogramming magic, rather than to generate code.

Templates can be used to do a lot of different things. They can generate more code than you expect, but that's not a reason to ban their use. It wasn't so long ago that various authorities recommended avoiding exceptions, virtual functions, floating-point math, and even classes due to concerns about code size and performance, but people did those things, and somehow everything worked out fine.

Answer 2

Templates don't necessarily lead to code bloat. If you write a function or class template and instantiate it for a dozen different types then yes, you get a lot of duplicate code generated (probably, anyway. Some compilers can merge identical instantiations back together).

But if a template is instantiated for one type only, then there is zero cost in code size. If you instantiate it a couple of times, you pay a certain cost, but you'd also end up paying if you used any of the other ways to achieve the same thing. Dynamic polymorphism (virtual functions and inheritance) isn't free either. You pay for that in terms of vtables, code generated to facilitate all the type casts and conversions necessary, and simply because of code that can't be inlined or optimized away.

Taking std::vector as an example, then yes, if you use both vector<int> and vector<float>, you get two copies of some of the code. But with templates, only the code that is actually used gets compiled. The member functions that you never call won't generate any code, and even in the functions that are compiled, the compiler may be able to eliminate a lot of code. For example, for certain types, exception handling code may be unnecessary, so the compiler can eliminate it, yielding smaller code than if you'd used dynamic polymorphism, where the compiler would've been unable to make any assumptions about the type being stored. So in this made-up example, you'd get some code generated for both vector<int> and vector<float>, but each of them is going to be a lot smaller than a polymorphic vector as you might find in Java, for example.

The main problem with using templates is that it requires a compiler which supports it. On a PC, that's no problem. On any other platform which has a mature C++ compiler available, it's no problem.

But not all platforms have a modern heavy-duty C++ compiler available. Some don't support a number of advanced features, and some are just not good enough at the optimizations required to make template code work (templates tend to require a lot of inlining, for example). So on some platforms, it may be best to avoid templates. Not because of any concern for code size, but because the compiler may be unable to handle it.

Answer 3

In my personal experience using (and even abusing) templates very rarely result in large code bloat, and compiling with -Os will help a lot.

It's not that common to see huge template classes duplicated (instantiated) many times, both because rarely classes are huge, and because in most cases you only instantiate templates with a few different arguments, not hundreds. Besides it's easy to reuse some common code in your biggest template classes/functions, and compiler will help you in doing this.

Usually size of data (graphics, audio, ...) is orders of magnitude bigger than the code. So I wouldn't worry.

Of course there could be exceptions to what I said, but I guess they'll mostly be about advanced (special / weird / complicated) stuff, not with the most common everyday classes.

Summarizing my suggestion: use templates as much as you want, if something will go wrong you'll find that out by profiling, and you will easily be able to optimize the size.