Inheritance is for sharing an interface (and so is overloading)

By Arthur O’Dwyer

This blog post was inspired by my answer to a question on Code Review StackExchange. To utterly oversimplify the question: When is it appropriate to use classical inheritance?

struct Cat { void meow();
};
struct Dog { void bark();
}; // Should I write a struct AbstractAnimal?

Here’s what the same code would look like, with an AbstractAnimal base class (and omitting the NVI for pedagogical reasons):

struct AbstractAnimal { virtual void do_speak() = 0;
}; struct Cat : public AbstractAnimal { void meow();
private: void do_speak() override { meow(); }
}; struct Dog : public AbstractAnimal { void bark();
private: void do_speak() override { bark(); }
};

The only reason to use a base class here is in order to write polymorphic code that operates on (a pointer or reference to) that base class.

// If all my code looks like this, I don't need a base class.
void operateOnCat(Cat *c) { c->meow(); }
operateOnCat(myCat); // I need the base class only if I want to write code like this.
void operateOnAnything(AbstractAnimal *a) { a->do_speak(); }
operateOnAnything(myCat);
operateOnAnything(myDog);

If you don’t need to write operateOnAnything, then you shouldn’t write AbstractAnimal. Abstraction is a tool for solving problems — and, as such, it is also a signal to the reader that a problem is being solved! If your code doesn’t actually need polymorphism in order to work, then you should avoid making an inheritance hierarchy.

Keep it simple, stupid.

The design principles of polymorphic programming apply equally to generic programming…

When should I name two methods the same?

Notice that in our original Cat/Dog code, I named the two methods Cat::meow() and Dog::bark(). It’s very tempting to give them the same name:

struct Cat { void talk(); // meows
};
struct Dog { void talk(); // barks
};

However, this is once again a signal to the reader that something indispensable is going on here! Meowing and barking are vaguely similar at an abstract level, yes; but abstraction is a tool for solving problems, and so we should avoid it unless a problem exists to be solved.

The only reason to use the same name for meowing and barking, here, is in order to write generic code that operates on an object of unknown type T.

// If all my code looks like this, I don't need to reuse the
// same name in both classes.
void operateOnCat(Cat& c) { c.meow(); }
operateOnCat(myCat); // I need the same name only if I want to write code like this.
template<class T>
void operateOnAnything(T& t) { t.talk(); }
operateOnAnything(myCat);
operateOnAnything(myDog);

Notice that another way to ask “When should I name two methods the same?” is to ask “When should I define a C++20 concept modeled by both these classes?” The rules of classical OOP are isomorphic to the rules of generic programming with concepts: just replace “base class” with “concept” and “derives from” with “models.”

Let’s go deeper.

When should I put two functions into an overload set?

The previous section was about reusing the same name outside of an overload set: Cat::talk and Dog::talk aren’t “overloads” per se. But what about something like this?

struct Snake { void eatPellet(const Pellet&); void eatCat(const Cat&);
};

Should we rename both methods to just Snake::eat, creating an overload set?

The only reason to use the same name for eating pellets and eating cats, here, is in order to write generic code that passes arguments of unknown type T.

// If all my code looks like this, I don't need to create
// an overload set.
void consumePellet(Pellet& p) { mySnake.eatPellet(p); }
consumePellet(myPellet); // I must create an overload set only if I want to write code like this.
template<class T>
void consumeAnything(T& t) { mySnake.eat(t); }
consumeAnything(myPellet);
consumeAnything(myCat);

The major place we see overload sets being used for their intended purpose in C++ is in constructors and assignment operators. Constructor overload sets help us to write generic functions like std::make_shared<X>(args...) and vector<X>::emplace(args...) which forward args... along to some constructor in the overload set (and we don’t have to care which one). Assignment-operator overload sets help us to use generic STL functions like std::fill.

The major place we see overload sets being misused in C++ is the STL containers’ erase method:

v.erase(first, last); // erase a range of elements
v.erase(first); // oops, erase just one element

I see this pitfall in real code more often than I want to. And the worst part is that the overload set on erase serves no purpose! Nobody is writing generic code like

template<class... Args>
void eraseAnyhow(Args... args { myVec.erase(args...); }
eraseAnyhow(it);
eraseAnyhow(first, last);

The overload set is not motivated by genericity; therefore it should not exist. Imagine how many fewer bugs we’d have had over the past 20 years if the original STL containers had given these two different facilities two different names:

v.erase(first, last);
v.erase_at(it);

Wait, really?

I wrote:

The only reason to use the same name for eating pellets and eating cats, here, is in order to write generic code that passes arguments of unknown type T.

Aren’t there any other valid reasons to do function overloading? Well, here’s one: refactoring. If you’re changing the type of the edibles in your game from class Pellet to class Fruit, it might be perfectly reasonable to add the Snake::eat(Fruit) overload before removing Snake::eat(Pellet). This is kind of the same logic as in my quotation above: you’re not writing “generic code” per se, but you’re still dealing with code where the code’s author didn’t know what type they were dealing with. When they wrote the code, they were passing Pellet; but now they’re passing Fruit. Refactoring is kind of like generic programming with templates, turned 90 degrees through the temporal axis.

The other reason to create an overload set in C++ is because conceptually you have only one function, but for technical reasons (usually performance) you have to implement it as an overload set. For example, it’s common to overload push_back(const Foo&) and push_back(Foo&&) — conceptually one single facility, but implemented as an overload set for performance. In C++14 it was common to overload foo(const std::string&) and foo(const char*) — conceptually one facility, but implemented as an overload set for performance. In C++17 we were finally able to ditch that overload set in favor of a single function foo(std::string_view).

Titus Winters calls the overload set “the atom of C++ API design” — functions themselves being maybe the protons or something. So putting two functions in the same overload set is saying that they’re so closely related that nobody ever needs to know which of them is being called. I’m merely turning that logic around: putting two functions in the same overload set is tantamount to saying that your codebase contains at least one place in which somebody cannot afford to know which of them is being called! If you’re not in that situation, don’t overload.

Conclusions

  • Keep it simple, stupid.

  • Inheritance is for interface compliance. (Anyone got a less clunky version of this mantra?)

  • Use a single name for a single entity. Never use two names where one is appropriate. Even more importantly, never use one where two would be more appropriate.

  • Inheritance, genericity, and overloading are abstractions. Abstraction is a tool to solve problems. If you apply the tool in a situation where there is no problem to be solved, you’re implicitly lying to your reader, and may confuse them.