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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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Listing 11-23. A Simple ref class Template // managed_template.h template <typename T> public ref class CTemplate { T m_obj; public: CTemplate(T obj) { m_obj = obj; } property T InnerObject { T get() { return m_obj; } void set(T obj) { m_obj = obj; } } }; The template is instantiated just as we would normally instantiate a native template class. In Listing 11-24, we instantiate the type with an int and, separately, a String handle. Listing 11-24. Using a Template with Different Types // managed_templates.cpp #include "managed_template.h" using namespace System; int main() { CTemplate<int>^ ct_int; CTemplate<String^>^ ct_string; ct_int = gcnew CTemplate<int>(55); ct_string = gcnew CTemplate<String^>("test"); Console::WriteLine("{0} ", ct_int->InnerObject ); Console::WriteLine("{0} ", ct_string->InnerObject ); }
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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In a similar manner, you can use all the other features of templates on your managed reference types, interfaces, and value types. Some managed types cannot be templates: you cannot declare template enum classes or delegate types. Otherwise, you can use nontype template parameters, you can use template functions, you can use template arguments, partial specialization, and so on. However, let s perform a little experiment with the template class in Listing 11-24 to illustrate an important limitation of managed templates: let s compile the template in two different assemblies. We ll also create some functions that take arguments of the specialized template types, so you can see what happens when we try to pass these template class types over an assembly boundary. We ll create two assemblies that include the managed template header file. One, compiled from assembly1.cpp (see Listing 11-25), will expose a class, CBridge, with a public static function that takes the template as a parameter. First, compile assembly1.cpp as an executable, then compile it as a library; and compile assembly2.cpp, which references assembly1.dll and tries to call the public static method CBridge::F, passing assembly2.cpp s instantiation of the managed template. Listing 11-25. An Assembly That Uses a Template Type in Its Public Interface // assembly1.cpp #include "managed_template.h" using namespace System; public ref class CBridge { public: static void F(CTemplate<int>^ ct_int) { Console::WriteLine("{0} ", ct_int->InnerObject ); } }; int main() { CTemplate<int>^ ct_int; ct_int = gcnew CTemplate<int>(55); CBridge::F(ct_int); } Compile the code in Listing 11-25 as a DLL: cl /clr /LD assembly2.cpp Listing 11-26 is assembly2.cpp.
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Listing 11-26. Trying to Use the Template in Another Assembly // assembly2.cpp #include "managed_template.h" #using "assembly1.dll" int main() { CTemplate<int>^ ctemplate_int = gcnew CTemplate<int>(67); CBridge^ bridge = gcnew CBridge(); bridge->F(ctemplate_int); } If we try to compile assembly2.cpp in Listing 11-26 as follows: cl /clr assembly2.cpp we ll get an error similar to the following: assembly2.cpp assembly2.cpp(12) : error C2664: 'CBridge::F' : cannot convert parameter 1 from 'CTemplate<T> ^' to 'CTemplate<int> ^' with [ T=int ] No user-defined-conversion operator available, or Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast What s the problem You can plainly see that CTemplate<T> with T = int is the same as CTemplate<int>, right Well, no. The truth of the matter is that the CTemplate<int> compiled into the first assembly is not considered the same type as the CTemplate<int> compiled into the second assembly, because the runtime sees them as two different types. The compiler won t let you compile code that tries to do this. The bottom line is that you should confine your template code to intra-assembly code. Don t expose your template classes as public classes. If you want a parameterized type to use in the public classes and methods of an assembly, use a generic type. You may often find yourself defining a generic interface to a template class. You can then use the generic interface over the assembly boundary, and use the template classes freely within each assembly. Listing 11-27 shows how you would declare such a thing.
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