In this short article, we’ll compare and contrast how C++ and Rust handle dynamic polymorphism. A quick disclaimer, I’m not a Rust guru, but rather someone who’s more familiar with C++ (by the way, you can join our San Diego C++ meetup at
This is my first Medium.com story. After reading, tweeting, and bookmarking hundreds of stories, I’ve decided to create a simple one for those of you learning Rust after coming from C++, just like me.
In this short article, we’ll compare and contrast how C++ and Rust handle dynamic polymorphism. A quick disclaimer, I’m not a Rust guru, but rather someone who’s more familiar with C++ (by the way, you can join our San Diego C++ meetup at https://www.meetup.com/San-Diego-CPP/ )
Let’s start with the purpose of polymorphism: with dynamic, runtime polymorphism, we’re able to hold a pointer or reference to a type that is actually pointing to a more concrete, derived type. It’s the basic lesson that people learn when working with OO (Object Oriented) languages. For our example, we’ll use Animal *as the base, top level type, along with more concrete types, like *Cat and Dog.
Animal class type has a
The above is simple: we create two newclasses, Dog and Cat, use public inheritance from Animal, and implement the pure virtual function declared in the Animal *base class, *talk(). *The *final *keyword defines that the class will be a leaf type. You cannot subclass from *Dog/Cat.
Here is how we can use the class hierarchy demonstrated in “Modern C++”.
For this to compile, you will need to #include *the following headers: *,** and **.
In the above main function, we created a vector of pointers to the Animal *class type. We use *unique_ptr<> to manage the heap allocation and deallocation. This is a C++11 feature. make_unique<>() is a helper function from C++14 that allocates such concrete type instances.
Finally, we create the loop (C++11 feature — range for loop) to iterate over Animal and invoke talk() on each instance. So long as each type implements the talk() interface (override virtual function of *Animal *base class), the type of the concrete instance is irrelevant.
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