Why doesn\'t C support template covariance, and how can we address the resulting type safety issues when working with polymorphic templates?

Templates and Polymorphism in C

Consider the following class structure:

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

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

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

A constructor of some other class Bar is defined as:

<code class="cpp">Bar(const Container<Interface>& bar) {
  // ...
}</code>

However, when attempting to call the constructor as follows:

<code class="cpp">Container<Foo> container();

Bar *temp = new Bar(container);</code>

we encounter a "no matching function" error.

Polymorphism in Templates

The concept of polymorphism in templates, or template covariance, would imply that if class B inherits from class A, then T likewise inherits from T. However, this is not the case in C or other languages like Java or C#.

Reason for Lack of Template Covariance

The absence of template covariance is justified by the need to maintain type safety. Consider the following example:

<code class="cpp">// Class hierarchy
class Fruit {...};
class Apple : public Fruit {...};
class Orange : public Fruit {...};

// Template instantiation using std::vector
int main() {
  std::vector<Apple> apple_vec;
  apple_vec.push_back(Apple()); // Valid

  // With covariance, the following would be allowed
  std::vector<Fruit>& fruit_vec = apple_vec;

  // Adding an Orange to the vector
  fruit_vec.push_back(Orange());

  // Incorrect addition of an orange to an apple vector
}</code>

This demonstrates the potential for unsafe behavior if templates were covariant. Therefore, T and T are considered completely distinct types, regardless of the relationship between A and B.

Resolving the Issue

One approach to resolving the issue in Java and C# is to use bounded wildcards and constraints, respectively:

<code class="java">Bar(Container<? extends Interface) {...}

<code class="csharp">Bar<T>(Container<T> container) where T : Interface {...}</code>

In C , the Boost Concept Check library can provide similar functionality. However, using a simple static assert may be a more practical solution for the specific problem encountered:

<code class="cpp">static_assert(std::is_base_of<Interface, Foo>::value, "Container must hold objects of type Interface or its derived classes.");</code>

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