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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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generic <typename T> where T : I ref class G { T t; public: G(T t) { // The handle syntax -> is used // even though T could be a value type. t->F(); } }; int main() { V v; R^ r = gcnew R(); G<V>^ gv = gcnew G<V>(v); G<R^>^ gr = gcnew G<R^>(r); } You might then wonder how you code for situations such as creating objects of the type parameter type inside the generic function or type. If you are to treat the unknown type as a handle, can you use gcnew to create the object, and if so, how In fact, using gcnew is not allowed in a generic type without specifying a constraint on the type parameter that only allows types that support a default constructor to be used. These could be reference types or value types, even though normally you wouldn t use gcnew to create objects of value type.
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The gcnew Constraint
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The gcnew constraint indicates that the type parameter must have a default constructor that takes no arguments. The constraint is used if you need to use gcnew on the type parameter in the definition of the generic type. The use of gcnew on an unknown type is limited to the default constructor with no arguments. The gcnew constraint is used with an empty pair of parentheses as a reminder that only the default constructor is allowed (see Listing 11-13). Types that are used must have a public default constructor, either an implicit one (as for all value types) or an explicitly declared default constructor with public accessibility. Listing 11-13. Using the gcnew Constraint // generic_gcnew.cpp using namespace System; generic <typename T> where T: gcnew() T CreateInstance() {
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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return gcnew T(); } ref class R { public: R() { } }; int main() { int i = CreateInstance<int>(); R^ r = CreateInstance<R^>(); } The gcnew constraint is useful, but you cannot specify a specific constructor other than the default constructor.
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Value Type Constraints
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Unconstrained type parameters can be either reference types or value types, and the same constructs will just work appropriately for either. When an assignment occurs in the generic type, if it s a value type, the value is copied, but if it s a reference type, the reference, not the value, is copied. In some cases, however, you will want to write an algorithm that assumes the semantics of one or the other. Perhaps your algorithm copies and then destroys or destructively modifies objects so it either wouldn t work or would corrupt any reference types that are used. It is possible to constrain a type parameter such that it may only be a value type, or only a reference type, if needed. The syntax is show in Listing 11-14. Listing 11-14. Using Value Type Constraints // valuetype_constraint.cpp generic <typename T> where T : value class ref class G { /* ... */ }; As an example, suppose the List generic class removes nodes. The question you face when creating a collection class that deletes nodes is what to do with the objects when the node is deleted. If the collection uses a reference type as an argument, and it merely references the objects, but doesn t own them, it s fine to remove the references in the nodes. If the collection is considered to own the object, it might need to delete the object to make sure the destructor is called, perhaps to free up some scarce resource such as a database connection. But if you do that, you have to worry about whether any other references to that object are being held so that you don t destroy an object that s being used. If the objects in the nodes are value types, they are automatically owned by the collection, and they will get destroyed when the containing
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