Contra built-in library types

C++ has several guiding principles or mantras. The most famous one is probably “zero-cost abstraction,” but for my money the most important one is “C++ does not have a string type.”

Let me explain. In C++, the syntax "hello" gives you a value of type const char* (well, really const char[6], but never mind); this is required for C compatibility. Stroustrup couldn’t make "hello" give you an object of some built-in type str, as it does in Python. But clearly C++ needed a real string type! So we got std::string — a class type that behaves just like a string type should. This meant that the core language had to be flexible enough to handle all the things that we wanted strings to do, generically.

  • Because strings require memory management, we had to have destructors and RAII.

  • Because strings can be concatenated s + t and indexed s[42], we had to have operator overloading.

  • Because strings can be function parameters func_taking_string("hello"), we had to have implicit conversions.

(Yes, these are in order from best idea to worst idea. Why do you ask?)

Of course std::string isn’t the only reason, or even the primary reason, that we got all these flexibility-producing features in C++. But it’s a great showcase for them. And the mantra generalizes: C++ doesn’t have Python’s str, but you can build std::string as a class. C++ doesn’t have a built-in list type, but you can build both std::vector and std::list as classes and then pick the best one for the job. (And then build fixed_capacity_vector when std::vector turns out to still be wrong.) C++ doesn’t have C’s _Atomic T or _Complex T types, but you can build std::atomic<T> and std::complex<T> as classes. C++ doesn’t have a built-in regex matcher like Javascript, but you can build std::regex as a class. (And then build "regex"_pre when std::regex turns out to still be wrong.) C++ doesn’t have built-in reference counting like Objective-C, but you can build std::shared_ptr as a class.

So, being able to build our own software tools is a bedrock principle of C++. We don’t build “magic types” into the core language; we give programmers the tools, and the freedom, to build their own types from scratch. You can (and usually should!) use std::string in your own programs, but there is nothing special about std::string from the compiler’s point of view.

But there are some magic types

When C++ forgets this guiding mantra — “C++ does not have a string type” — it tends to fall off the rails badly.

auto x = typeid(42);  // core language syntax

This returns a reference to an object of the built-in type std::type_info, which has virtual functions and stuff.

struct A { virtual ~A(); } a;
struct B : A {};
auto& y = dynamic_cast<B&>(a);

This throws an exception of the built-in type std::bad_cast, which has virtual functions and stuff. And std::bad_cast also has a noexcept copy constructor, which means that it is not allowed to allocate heap memory during a copy, which means that its what() string must be either static or reference-counted. (In practice it will be reference-counted.) This is a lot of heavyweight code we’re pulling in, just to use a core language feature!

That reminds me: another famous C++ mantra is “You don’t pay for what you don’t use.” These magic built-in types are a perfect example of having to pay for things we’re not intending to use.

So, I suggest that a good guiding design principle for C++ is: Avoid magic built-in types.

Nuancing my position

But I have recently added some nuance to my views on “magic types”, following a breakfast discussion at the WG21 meeting in Albuquerque (November 2017). This discussion was about the then-proposed C++2a operator<=> (which is now solidly in the working draft). I didn’t like that it seemed to be adding new “magic” library types, which as we’ve seen is usually a terrible idea.

I asked, what if I wanted to define my own operator<=> for one of my own types, and I wanted it to return a custom result type; wouldn’t it be troublesome that my custom type would not play well with Herb’s magic baked-in library type?

We definitely have this problem today with the std::exception hierarchy (e.g. std::bad_cast above). Suppose I want all my thrown exceptions to inherit from MyCustomRootObject (say, an object whose constructor takes a stack trace). I cannot ever achieve that goal, unless I abandon the STL (out_of_range) and abandon typeid and dynamic_cast (bad_cast, bad_typeid) and abandon operator new (bad_alloc). All of which I should arguably be abandoning anyway, but still, this is a horrible state of affairs. In order not to bathe in dirty water, I must throw out the baby as well.

“C++: It should always be possible to separate the baby from the bathwater.”

So, I said at breakfast, if we let std::strong_ordering into the core language, we’re basically replicating the horrible state of affairs we have with std::exception — but now instead of abandoning all those crappy features like typeid, you’re going to make me abandon comparison operators? This seems insane, I said.

But I was successfully convinced otherwise! After all, the existence of std::nullptr_t in the library does not make me want to abandon null pointers. The existence of std::ptrdiff_t in the library does not make me abandon subtraction. Psychologically, we don’t even tend to think of these features as “library” features baked into the core language; instead, we think of them as “core language” features which are exposed via convenient library typedefs. The big difference between std::nullptr_t and std::exception is that std::nullptr_t is awesome enough, and cheap enough, that nobody ever wants to replace it. Therefore it is not a problem that std::nullptr_t is inflexibly a part of the core language. std::nullptr_t is clean bathwater.

I decided that there was a decent chance that std::strong_equality would probably be more like nullptr_t than like bad_alloc, and that I was willing to take that risk.

One (logically weak but psychologically compelling) argument in favor of std::strong_equality is that it’s possible to write a library typedef that “exposes” the core-language type. Just as we can imagine writing

using nullptr_t = decltype(nullptr);

(and in fact I do write things like bool operator==(decltype(nullptr)) const in my own code, in order to eliminate a header dependency; and I even find it more readable than the alternative) — we can also imagine writing

using strong_ordering = decltype(1 <=> 2);
using strong_equality = decltype(nullptr <=> nullptr);
using partial_ordering = decltype(1. <=> 2.);

(Now, horribly, this snippet is ill-formed unless you have already #included <compare>, which is absolutely unconscionable, but let that pass for now.)

Whereas it is not possible to write a similar library typedef for, let’s say, std::bad_alloc.

On the other hand, it is possible to write the typedef

template<class T> struct S;
template<class T> struct S<const T&> { using type = T; };
using type_info = typename S<decltype(typeid(1))>::type;

(again, ignoring the horrible requirement to #include <typeinfo> before doing so), and this should not be taken as compelling evidence that std::type_info is awesome!

Trivia: I notice that it is not currently possible to write a typedef for

using weak_ordering = ???;

because you cannot create a C++ type with that comparison category using only core-language features. It is also not currently possible to write a typedef for weak_equality; but there has been a suggestion of

union U { int a; int b; };
using weak_equality = decltype(&U::a <=> &U::b);

because it is guaranteed that U::a and U::b must be equal, yet they are not substitutable.

However, none of the “big three” compilers implement C++ pointers-to-union-members correctly, so I think it is likely that the Committee will end up just “fixing” pointers-to-union-members somehow so that they become sane, rather than trying to pretend that they are actually a good example of weak_equality.

A fun test program for your compiler of choice is:

#include <iostream>

union U { int a; int b; };

int main() {
    constexpr auto pa = &U::a;
    constexpr auto pb = &U::b;

    constexpr bool x = (pa == pb);
              bool y = (pa == pb);

    std::cout << std::boolalpha << x << ' ' << y << ' ' << (pa == pb) << std::endl;

GCC prints “false false true”; Clang and MSVC print “false true true”. Intel’s ICC compiler correctly prints “true true true”.

Posted 2018-04-15