Removing Overhead of dynamic_cast Codegen with Minimum Boilerplate
Tags: c, templates, multipleinheritance, dynamiccast
Published: 2023-08-16 12:06:37

Dynamic casting is a powerful feature in C++ that allows you to safely convert pointers or references of one class to another class within an inheritance hierarchy. However, it comes with a cost. The overhead of dynamic\_cast can negatively impact the performance of your code. In this article, we will explore how to remove this overhead and minimize the boilerplate associated with dynamic\_cast code generation.

Before we dive into the solution, let's understand why dynamic\_cast can be slow. When you use dynamic\_cast, the compiler generates type information at runtime to perform the cast. This type information includes a table called the vtable, which contains information about the class hierarchy. This process introduces some runtime overhead, especially if you have a large inheritance hierarchy.

One way to mitigate this overhead is to use a technique called static polymorphism. This technique leverages templates to perform type checking at compile-time rather than runtime. By doing so, we can eliminate the need for dynamic_cast and its associated overhead.

So how do we implement static polymorphism? The answer lies in the CRTP (Curiously Recurring Template Pattern). This pattern allows us to define a base class template that takes a derived class as a template parameter. By doing so, we can access the derived class's members and perform type-safe operations without any runtime overhead.

`template class Base { public: void doSomething() { static_cast(this)->doSomethingImpl(); } }; class Derived : public Base { public: void doSomethingImpl() { // Implementation goes here } };`

In the code snippet above, the Base class template takes the Derived class as a template parameter. The doSomething function in the Base class uses static_cast to downcast the this pointer to the Derived class and calls the doSomethingImpl function. This allows us to call the appropriate implementation at compile-time without any runtime overhead.

By using static polymorphism, we can remove the overhead of dynamic\_cast and achieve better performance in our code. Additionally, this approach reduces the boilerplate associated with dynamic\_cast code generation.

In conclusion, dynamic\_cast is a powerful feature in C++, but it can introduce runtime overhead. By leveraging static polymorphism and the CRTP pattern, we can eliminate this overhead and minimize the boilerplate associated with dynamic\_cast code generation. So next time you find yourself reaching for dynamic_cast, consider using static polymorphism instead.

References:

*   [https://en.cppreference.com/w/cpp/language/dynamic_cast](https://en.cppreference.com/w/cpp/language/dynamic_cast)
*   [https://en.wikipedia.org/wiki/Curiously\_recurring\_template_pattern](https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern)

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