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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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ref class C : I { public: virtual void f() { // ... } }; int main() { R<C^>^ r = gcnew R<C^>(gcnew C()); }
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Class Constraints
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A class constraint on a type parameter indicates that the type used must be derived from a specified type. When you specify a class constraint, you may then be sure that the members on that type are available, and you may use those members in the definition of the generic type (see Listing 11-10). Listing 11-10. Specifying Class Constraints // class_constraint.cpp using namespace System; ref class B { public: virtual void f() {} }; generic <typename T> where T : B ref class G { T t; public: G(T t_in) : t(t_in) { // For this example, C::f is // called. t->f(); } };
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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ref class C : B { public: virtual void f() override { Console::WriteLine("C::f"); } }; int main() { G<C^>^ r = gcnew G<C^>(gcnew C()); } Here is the output of Listing 11-10:
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Any class in the hierarchy under C can be used as the type argument for the generic type G in Listing 11-10. There are other types of constraints, but before you proceed to them, let s look at reference types and value types in generic types and functions, which will give a better idea of why the other constraint types are needed.
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Reference Types and Value Types As Type Parameters
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Although the type parameter is written without a handle or any other adornment, when a type argument is supplied, it will either be a handle to a reference type or a value type. The same generic collection will work with both with the same syntax. The same constructs are interpreted differently depending on whether the type parameter is a value type or a reference type. Thus, the MyList class shown in Listing 11-8 works as well with a handle to a ref class, as demonstrated in Listing 11-11, as with the value type int used in Listing 11-8. Listing 11-11. Using a Generic List for Strings ref class R { String^ name; public: R(String^ n) : name(n) {}
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
virtual String^ ToString() override { return name; } }; int main() { MyList<R^>^ R_list = gcnew MyList<R^>(); R_list->Add(gcnew R("test1")); R_list->Add(gcnew R("test2")); R_list->Add(gcnew R("test3")); for each (ListNode<R^>^ node in R_list) { Console::WriteLine(node->item); } } You cannot use a naked reference type (as opposed to a handle type) as a type parameter: List<R>^ R_list = gcnew List<R>(); // illegal You can make it work by either making R a value type or using a handle to R as the generic type argument. When writing a generic class that can take either value types or handles, you need to understand something that may be surprising, especially if you re familiar with templates. And that is that regardless of the type argument, you code your generic class with the assumption that the unknown type is a handle. For example, you use the -> operator vs. the . operator for member access, as in Listing 11-12. You wouldn t expect to be able to do this with a pointer or a nonpointer type with the same template class, because different syntax would be required for each, but for generics, the unknown type is treated as if it were a handle, even if the type substituted is a nonhandle type. If the type argument is a value type, you could read the code as if the type parameter were a boxed value type. The actual implementation of the generic doesn t incur the overhead of boxing the value type unless a real boxing operation is needed, for example, if the type parameter is converted to Object^ or a method on Object is accessed. Listing 11-12. Assuming an Unknown Type Is a Handle // generic_reference_syntax.cpp interface class I { void F(); }; value struct V : I { virtual void F() {} }; ref struct R : I { virtual void F() {} };
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