Idioms and Patterns: Type Erasure

Type Erasure

Type Erasure enables you to use various concrete types through a single generic interface.

Type erasure is duck typing applied in C++

“When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.” (James Whitcomb Rileys)

Of course, you’ve already used type erasure in C++ or C. The C-ish way of type erasure is a void pointer; the C++ish way of type erasure is object-orientation.

Typical Use Case

Type erasure can be performed with void pointers, object-orientation, or templates.

Case 1: Void Pointers

void qsort(void *ptr, std::size_t count, std::size_t size, cmp);

Before the qsort(), consider using:

int cmp(const void *a, const void *b);

The comparison function cmp should return a

• negative integer: the first argument is less than the second
• zero: both arguments are equal
• positive integer: the first argument is greater than the second

Thanks to the void pointer, std::qsort is generally applicable but also quite error-prone.

Maybe you want to sort a std::vector<int>, but you used a comparator for C-strings. The compiler cannot catch this error because the type of information was removed and we end up with undefined behavior.

In C++ we can do better.

Case 2: Object-Orientation

Having a class hierarchy and using the BaseClass pointer instead of concrete types is one way to enable type erasure

std::vector<const BaseClass*> vec;

The vec object has a pointer to a constant BaseClasses.

Case 3: std::function

std::function as a polymorphic function wrapper is a nice example of type erasure in C++. std::function can accept everything, which behaves like a function. To be more precise, this can be any callable such as a function, a function object, a function object created by std::bind, or just a lambda function.

Let’s have a look at the difference between the three types listed above in the following table:

Technique	Each Datatype	Type-safe Interface	Easy to implment	Common Base Class
void*	Yes	No	Yes	No
Object-Orientation	No	Yes	Yes	Yes
Templates	Yes	Yes	No	No