How Does Polymorphism Work with Templates in C ?

Polymorphism in C Templates

In C , polymorphism is a fundamental concept that allows derived classes to inherit and override methods of their base classes. However, when it comes to templates, polymorphism behaves in a non-traditional manner.

The Template Covariance Issue

Consider the following code:

<code class="cpp">class Interface {
  // ...
};

class Foo : public Interface {
  // ...
};

template <class T>
class Container {
  // ...
};

Bar(const Container<Interface>& bar){
  // ...
}</code>

If you attempt to construct Bar using a Container, you will encounter a "no matching function" error. This is because C templates are not covariant, meaning that Container is not implicitly convertible to Container.

Reasoning Behind Invariance

Template invariance exists for an important reason. Consider a class template vector that stores objects of type T. If templates were covariant, it would be possible to assign a vector to a vector and subsequently add objects of type Orange (derived from Fruit) to the vector. This would violate type safety, as vector is intended to store fruits and not oranges.

Solutions

To address this issue, several solutions are available:

• Assert Invariance: You can use a static assert to verify that the template argument is the expected type.
• Use Bounded Wildcards (Java): Java offers bounded wildcards that allow you to specify upper or lower bounds on template arguments, ensuring compatibility with other types.
• Use Constraints (C#): C# provides constraints that enable you to impose restrictions on template arguments based on their inheritance or interface implementations.
• Concepts (C 1x): The upcoming C 1x standard will introduce Concepts, a more powerful mechanism for enforcing requirements on template parameters.

Conclusion

While templates in C are not covariant, various solutions exist to ensure type safety and compatibility between classes and templates. Understanding the reasoning behind template invariance is crucial for writing robust and correct C code.

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