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Unlike the generic interface, the template is declared and defined in a header file, as shown in Listing 11-28. Listing 11-28. Declaring a Generic Interface for a Template // template_with_generic_interface.h #using "generic_interface.dll" template <typename T> ref class CTemplate : IGInterface<T> { T m_obj; public: CTemplate(T obj) { m_obj = obj; } virtual property T InnerObject { T get() { return m_obj; } void set(T obj) { m_obj = obj; } } }; Now the CBridge::F function can be rewritten to use the generic interface handle instead of the template class directly (see Listing 11-29). Listing 11-29. Using a Generic Interface Instead of a Template // template_with_generic_interface.cpp #include "template_with_generic_interface.h" using namespace System; public ref class CBridge { public: static void F(IGInterface<int>^ ct_int) { Console::WriteLine("{0} ", ct_int->InnerObject ); } };
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CHAPTER 11 PARA METERIZE D FUNC TIONS AND TYPES
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And the second assembly can now call the CBridge::F function. It will include the template using #include and reference the generic interface (as well as the other assembly containing CBridge::F) with #using, as in Listing 11-30. Listing 11-30. Successfully Using a Template from Another Assembly // assembly2_with_generic.cpp #using "generic_interface.dll" #using "template_with_generic_interface.dll" #include "template_with_generic_interface.h" int main() { CTemplate<int>^ ctemplate_int = gcnew CTemplate<int>(67); CBridge^ bridge = gcnew CBridge(); bridge->F(ctemplate_int); } The conversion from the template to the generic parameter of F is implicit, since it amounts to a simple derived class to base interface conversion. The presence of both generics and templates in the language can be confusing. If you remember nothing else, remember that templates are good for use within assemblies, but that generics should be used for any interassembly functionality, and also for any cross-language functionality. The language you are interoperating with must also support consuming generics, which VB and C# do. You might also wonder, Why use managed templates at all There are some limitations to the usefulness of generics, especially for those who are used to the full expressive power of templates in C++. Many features of templates are not available with generics, as described here: Templates support nontype template parameters; generics don t. Templates support specialization and partial specialization; generics don t. Templates work better with mathematical operations; unconstrained generics don t allow the use of mathematical operators on the unknown type parameter, and there are no viable constraints for families of primitive types (e.g., int, double, etc.). Generic types cannot inherit from the type parameter, as is possible with templates. Generics have no equivalent of template metaprogramming, that is, using template expansion by the compiler to perform operations. Templates are compiled at the time of instantiation; generics are compiled at the point of definition. The last point bears some further explanation, since it has far-reaching implications in terms of what code is allowed in a generic class. The basic rule is that a generic class may not include any code that is not ensured to compile with any type argument. Think about the fact
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that the compiler will not even know what types might eventually be used as type arguments. You could compile G<T> today and deploy it somewhere, and years later someone could instantiate it with a type that never even existed when G<T> was compiled. This would not be possible if just any code were allowed to compile. That s the reason why constraints are so important in generic classes. In order to call a method on a type parameter, the compiler must be certain that that method is in fact available for every allowable type argument that may be used. The runtime must also be equally forceful in insisting that only types that meet the constraints are allowed to be used as type arguments. Contrast this with templates, in which you can make all kinds of unstated assumptions about the type (such as assuming the type has certain methods, operators, and so on) that might be used as a type parameter, without any worries because you know that when someone tries to instantiate your template, the compiler will check the template with the actual type that is being used. You don t have to constrain the template type parameter because the type never remains unknown at runtime. To drive home the point, consider a template class that works with mathematical entities and assumes the existence of a + operator on the type, as in Listing 11-31. Listing 11-31. Assuming the Existence of an Operator template <class T> ref class A { // assumes T supports the + operator T add(T t1, T t2) { return t1 + t2; } }; If you want a generic class that does this, you probably need to define an interface constraint and add that interface to any types that are to be used as a type argument, as in Listing 11-32. Listing 11-32. Using a Constraint to Guarantee the Existence of an Operator interface class IAddition { static IAddition^ operator+(IAddition^, IAddition^); }; generic <typename T> where T : IAddition ref class G { IAddition^ add(T t1, T t2) { return t1 + t2; } }; The problems arise when you try to use the primitive types, since, although they might have a + operator, they don t implement IAddition. Using templates, you can just use the + operator without the constraint, and if someone tries to instantiate the template with a type that is incompatible, it simply won t compile, but the template would work with int as well as with your types that define the + operator. There are certainly other examples of when you would want to use templates instead of generics. It is a trade-off, since the additional expressive power of templates does come at the cost of only having access to the templates with a single assembly, apart from generic interfaces you might set up for interassembly communication.
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