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Differences Between C++ Templates and C# Generics (C# Programming Guide)

C# Generics and C++ templates are both language features that provide support for parameterized types. However, there are many differences between the two. At the syntax level, C# generics are a simpler approach to parameterized types without the complexity of C++ templates. In addition, C# does not attempt to provide all of the functionality that C++ templates provide. At the implementation level, the primary difference is that C# generic type substitutions are performed at runtime and generic type information is thereby preserved for instantiated objects. For more information, see Generics in the Run Time (C# Programming Guide).

The following are the key differences between C# Generics and C++ templates:

  • C# generics do not provide the same amount of flexibility as C++ templates. For example, it is not possible to call arithmetic operators in a C# generic class, although it is possible to call user defined operators.

  • C# does not allow non-type template parameters, such as template C<int i> {}.

  • C# does not support explicit specialization; that is, a custom implementation of a template for a specific type.

  • C# does not support partial specialization: a custom implementation for a subset of the type arguments.

  • C# does not allow the type parameter to be used as the base class for the generic type.

  • C# does not allow type parameters to have default types.

  • In C#, a generic type parameter cannot itself be a generic, although constructed types can be used as generics. C++ does allow template parameters.

  • C++ allows code that might not be valid for all type parameters in the template, which is then checked for the specific type used as the type parameter. C# requires code in a class to be written in such a way that it will work with any type that satisfies the constraints. For example, in C++ it is possible to write a function that uses the arithmetic operators + and - on objects of the type parameter, which will produce an error at the time of instantiation of the template with a type that does not support these operators. C# disallows this; the only language constructs allowed are those that can be deduced from the constraints.

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