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Why doesn\'t C support template covariance, and how can we address the resulting type safety issues when working with polymorphic templates?
Why doesn\'t C support template covariance, and how can we address the resulting type safety issues when working with polymorphic templates?
Oct 28, 2024 am 08:14 AM
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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