C# in Depth

Limitations of generics in C# and other languages109reference types. That lack of sharing does have its benefits, though—it allows typespecificoptimizations, such as inlining method calls for some type parameters but notothers, from the same template. It also means that overload resolution can be performedseparately for each set of type parameters, rather than just once based solelyon the limited knowledge the C# compiler has due to any constraints present.Don’t forget that with “normal” C++ there’s only one compilation involved, ratherthan the “compile to IL” then “JIT compile to native code” model of .NET. A programusing a standard template in ten different ways will include the code ten times in a C++program. A similar program in C# using a generic type from the framework in ten differentways won’t include the code for the generic type at all—it will refer to it, and theJIT will compile as many different versions as required (as described in section 3.4.2) atexecution time.One significant feature that C++ templates have over C# generics is that the templatearguments don’t have to be type names. Variable names, function names, and constantexpressions can be used as well. A common example of this is a buffer type that has thesize of the buffer as one of the template arguments—so a buffer will alwaysbe a buffer of 20 integers, and a buffer will always be a buffer of 35 doubles.This ability is crucial to template metaprogramming 15 —an 15 advanced C++ technique thevery idea of which scares me, but that can be very powerful in the hands of experts.C++ templates are more flexible in other ways, too. They don’t suffer from theproblem described in 3.6.2, and there are a few other restrictions that don’t exist inC++: you can derive a class from one of its type parameters, and you can specialize atemplate for a particular set of type arguments. The latter ability allows the templateauthor to write general code to be used when there’s no more knowledge availablebut specific (often highly optimized) code for particular types.The same variance issues of .NET generics exist in C++ templates as well—anexample given by Bjarne Stroustrup 16 is that there are no implicit conversionsbetween Vector and Vector with similar reasoning—in this case,it might allow you to put a square peg in a round hole.For further details of C++ templates, I recommend Stroustrup’s The C++Programming Language (Addison-Wesley, 1991). It’s not always the easiest book tofollow, but the templates chapter is fairly clear (once you get your mind around C++terminology and syntax). For more comparisons with .NET generics, look at the blogpost by the Visual C++ team on this topic: http://blogs.msdn.com/branbray/archive/2003/11/19/51023.aspx.The other obvious language to compare with C# in terms of generics is Java, whichintroduced the feature into the mainstream language for the 1.5 release, 17 severalyears after other projects had compilers for their Java-like languages.15 http://en.wikipedia.org/wiki/Template_metaprogramming16 The inventor of C++.17 Or 5.0, depending on which numbering system you use. Don’t get me started.Licensed to Rhona Hadida