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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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Listing 11-5. Explicitly Specifying a Type Argument // generic_return_value.cpp using namespace System; generic <typename T> T f() { return T(); } int main() { int i = f<int>(); // OK String^ s = f<String^>(); // OK double d = f(); // Error! Can't deduce type. }
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Generic Types
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Like generic functions, the declaration of a generic type differs from a nongeneric declaration by the appearance of the contextual keyword generic followed by the type parameter list. The type parameter may then be used in the generic definition wherever a type is used, for example, as a field, in a method signature as an argument type or return value, or as the type of a property, as shown in Listing 11-6. Listing 11-6. Using a Generic Type // generic_class1.cpp using namespace System; generic <typename T> ref class R { T t; public: R() {} property T InnerValue { T get() { return t; } void set(T value) { t = value; } } };
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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int main() { double d = 0.01; int n = 12; // Create an object with T equal to double. R<double>^ r_double = gcnew R<double>(); // Create an object with T equal to int. R<int>^ r_int = gcnew R<int>(); r_double->InnerValue = d; r_int->InnerValue = n; Console::WriteLine( r_double->InnerValue ); Console::WriteLine( r_int->InnerValue ); } The types created from a generic type, such as R<double> and R<int> in Listing 11-6, are referred to as constructed types. Two or more types constructed from the same generic type are considered to be unique, unrelated types. Thus, you cannot convert from R<double> to R<int>. When a generic class or function is compiled, a generic version of that function or class is inserted into the assembly or module created for that source code. At runtime, constructed types are created on demand. Thus, it is not necessary to know at compile time all the possible types that might be used as type parameters. However, this freedom also means that the compiletime restrictions must be greater; otherwise, you would risk adding an incompatible type in at runtime, which might not have all the features required. When the compiler interprets the code for a generic class, it only allows methods, properties and other constructs to be called on the unknown type that are certain to be available. This ensures the type safety of generic types, since otherwise it would be possible to create a generic type that compiled but failed at runtime when the method used was not available. This restriction imposes constraints on the code you can use in your generic functions and types. For example, the code in Listing 11-7 won t compile. Listing 11-7. Compiler Restrictions on Generic Types // invalid_use_of_type_param.cpp generic <typename T> ref class G { T t; public:
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CHAPTER 11 PARAMETERIZED FUNCTIONS AND TYPES
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G() { t = gcnew T("abc", 100); // Error: T may not have // a compatible constructor. t->F(); // Error: T may not have F. } }; Listing 11-7 will produce the compiler error: invalid_use_of_type_param.cpp(12) : error C3227: 'T' : cannot use 'gcnew' to allocate a generic type invalid_use_of_type_param.cpp(14) : error C2039: 'F' : is not a member of 'System::Object' c:\windows\microsoft.net\framework\v2.0.50727\mscorlib.dll : see declaration of 'System::Object' As you can see, the first complaint is that gcnew is not available on a generic type parameter; the second error occurs because the compiler is only willing to allow methods that are available on System::Object. There is a way to get around these restrictions. If you need to use specific features of a type, you must constrain the generic so that only types with those features are allowed to be used as type arguments. You ll see how to do that in the section Using Constraints. But first, let s look at a typical generic class implementing a simple collection.
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