# Prefer core-language features over library facilities

There are many places in C++ where the working programmer has a choice between doing something via a standard-library facility, or via a similar (often newer) core-language feature. I keep expanding my mental list of these, so let’s write them all down in one place.

First, the caveats: This post’s title is expressed as advice to working programmers, not language designers. One of the coolest things about C++ is that “C++ doesn’t have a string type”; things that other languages must do in the core language, C and C++ can and often will do in the library instead. (Another classic example is C’s printf.) If you’re designing a language, I think it’s a fantastic idea to make a small core language that’s powerful enough to implement most things in the library instead. The Committee should indeed prefer to extend the library where possible, rather than keep adding to the core language. But as a working programmer, you don’t need to worry about what the core language should provide; you just need to know when a standard-library facility has been obsoleted by expansions to the core language. Here are some examples.

These examples deliberately include some that will make you say “Duh, everyone knows that one,” and some that will make some people say “Whoa, I disagree there!” However, these are all cases that fall into the same category for me personally: places where the core-language feature should be preferred.

## Prefer alignof over std::alignment_of

static_assert(std::alignment_of_v<Widget> == 8);  // worse
static_assert(alignof(Widget) == 8);  // better


Training-course students regularly ask me why <type_traits> provides std::alignment_of at all, if alignof exists. The answer (as far as I know) is that the C++11 cycle took eight years. So they brought in std::alignment_of from Boost, and then at some later point — maybe years later — they invented the alignof keyword. This made std::alignment_of redundant; but it’s a lot harder to remove something than to add it originally, especially since people were already using std::alignment_of in real “C++0x” code.

The same situation, even worse, applies to…

## Prefer alignas over std::aligned_storage_t

std::aligned_storage<sizeof(Widget)> data;  // utterly wrong
std::aligned_storage_t<sizeof(Widget)> data;  // still kind of wrong
std::aligned_storage_t<sizeof(Widget), alignof(Widget)> data;  // correct but bad
alignas(Widget) char data[sizeof(Widget)];  // correct and better


std::aligned_storage was also brought in from Boost during the C++11 cycle, before they invented the alignas keyword. It has many problems: The actual class type aligned_storage<T> doesn’t provide storage; instead, you must be careful to use aligned_storage<T>::type a.k.a. aligned_storage_t<T>. The template takes a second parameter that’s defaulted and thus easy to forget; if you omit that parameter, you might get storage that’s not aligned. Or worse, overaligned and thus bigger than expected!

The core-language alignas(T) attribute has none of these problems. It Just Works.

## Prefer char over std::byte

This is a controversial one, but I stick to it dogmatically. std::byte is a library facility that arrived in C++17 specifically as an enumeration type for talking about octets. To use it, you must include <cstddef>. It’s not an integral type (good?), but on the other hand it still supports all the integral bitwise operations such as &, |, <<, which (because it is an enum type instead of an integral type) it must provide via overloaded operators in namespace std. It’s just so heavyweight for what it is… which, again, is a verbose reinvention of the built-in primitive type char.

std::byte data[100];  // worse
char data[100];  // better


Similarly…

## Prefer void* over std::byte*

I believe this example is less controversial than the preceding one. Suppose you’re writing a function to copy some dumb bytes from point A to point B. How should you write its parameter types? I’ve heard at least one person say “I’ll pass a std::byte*, because my function deals in bytes.” But that requires casting at every call-site! Instead, we should use void*, which is the C++ vocabulary type for “pointer to untyped memory” (i.e., just a bunch of dumb bytes).

int data[10];

// worse
void mycopy(std::byte *dst, const std::byte *src, size_t n);
mycopy((std::byte*)data, (std::byte*)(data+5), 5 * sizeof(int));

// better
void mycopy(void *dst, const void *src, size_t n);
mycopy(data, data+5, 5 * sizeof(int));


C++ inherited from (post-K&R) C the notion of void* as a vocabulary type. K&R C didn’t use void; it just passed around char* all over the place (e.g., in K&R1’s char *alloc()). This was rightly seen as a mistake and the C standardization committee quickly invented void* to replace char*. As a bonus, C++ permits any object pointer to implicitly convert to void*, whereas it’s rightly more difficult to create a char* or int* or std::byte*.

See also: “Pointer to raw memory? T* (2018-06-08).

## Prefer lambdas over std::bind

Yet another example of C++11 first taking a C++98-flavored thing (std::bind) from Boost into the library, and later taking another thing into the core language in a way that essentially obsoleted the library facility. However, std::bind didn’t become truly obsolete until C++14 gave us init-capture syntax.

auto f = std::bind(&Widget::serve, this, std::placeholders::_1, 10);  // worse
auto f = [this](int timeoutMs) { this->serve(timeoutMs, 10); };  // better


The lambda version is better in so many ways. It doesn’t require including <functional>. It gives us a place to document both the type of x and its purpose (via naming). It constrains f to accept only one parameter, whereas the std::bind closure object will happily accept any number of arguments and silently ignore the excess. It gives us a place to specify the return type of f, in case we want it to be different from the return type of serve (or, as in this case, to be void). And std::bind has unintuitive behavior when one closure is nested as an argument to another, whereas lambdas compose straightforwardly.

auto h = std::bind(f, std::bind(g, _1));  // worse
auto h = [](const auto& x) { return f(g(x)); };  // better

auto j = std::bind(std::plus<>(), std::bind(f, _1), 1);  // worse
auto j = [](const auto& x) { return f(x) + 1; };  // better


## Prefer ranged for over std::for_each

std::for_each(v.begin(), v.end(), [](int& x) { ++x; });  // worse
for (int& x : v) { ++x; }  // better


But the std::for_each algorithm can actually be the most natural way to express your intent if you’re writing an STL-style algorithm where you actually started with an iterator-pair [first, last). In that case, you don’t have to use the library to break down v.begin(), v.end(); you’d actually have to use the library to build first, last back up into a for-friendly range!

for (auto&& x : std::ranges::subrange(first, last)) { myCallable(x); }  // worse
std::for_each(first, last, myCallable);  // better


A particularly outlandish example of library misuse is the std::accumulate in Conor Hoekstra’s “Structure and Interpretation of Computer Programs” (CppCon 2020), where he’s basically reinventing a for loop but having to track all of his mutable bits through the elements of a std::pair accumulator he manually created, instead of getting the compiler to do it automatically via local variables on a stack frame.

// worse
auto [biggest, secondbiggest] = std::accumulate(
v.begin(), v.end(),
std::make_pair(0, 0),
[](std::pair<int, int> acc, int x) {
auto [biggest, secondbiggest] = acc;
if (x > biggest) return std::make_pair(x, biggest);
if (x > secondbiggest) return std::make_pair(biggest, x);
return acc;
}
);

// better
int biggest = 0;
int secondbiggest = 0;
for (int x : v) {
if (x > biggest) {
secondbiggest = std::exchange(biggest, x);
} else if (x > secondbiggest) {
secondbiggest = x;
}
}


See also: “The STL is more than std::accumulate (2020-12-14).

## Prefer struct over std::tuple

There are reasonable reasons to prefer std::variant over union, because std::variant adds type-safety (it throws if you access the wrong alternative at runtime, instead of just having UB). But there is no such reason to prefer std::tuple over struct.

// worse
auto info = getHostIP();
std::cout << std::get<0>(info);
auto [hostname, addr, ttl] = info;

// better
struct HostInfo {
std::string hostname;
double ttlMs;  // "time to live"
};
HostInfo getHostIP();
auto info = getHostIP();
std::cout << info.hostname;
auto [hostname, addr, ttl] = info;  // still works!


The struct version has many advantages over the std::tuple version: It gives us a strong type class HostInfo instead of a vague tuple of data. It gives us a single declaration for each member, so we have a place to document its purpose (via naming and/or code comments). It gives us names for each member: info.hostname is less error-prone and more readable than std::get<0>(info).

std::tuple also generates really long mangled names, so switching to named class types might lighten the load on your linker. Compare:

void f(std::tuple<std::string, IPAddress, double>);

void f(HostInfo);
// _Z1f8HostInfo


So why does std::tuple exist at all, you ask? Mainly, variadic templates. You’re allowed to create a std::tuple<Ts...> with a variadic number of elements, but (as of 2022) not allowed to create a struct S { Ts... ts; }.

## Prefer core-language arrays over std::array

std::array<int, 3> a = {1, 2, 3};  // worse
std::array<int, 3> a = {{1, 2, 3}};  // even worse
int a[] = {1, 2, 3};  // better


See “The array size constant antipattern” (2020-08-06).

The trick with the “even worse” line above is that std::array is technically an aggregate, and the paper standard doesn’t specify what its members actually are. So initializing an array with {1, 2, 3} is guaranteed to work because of brace elision, but initializing it with {{1, 2, 3}} could conceivably (on a hypothetical perverse implementation) fail to compile.

## Prefer C++20 co_yield over manual frame management

In the std::accumulate example above, we were using a temporary “accumulator” to hold data in a library data structure, when it really should have been in local variables on a stack frame (so that those variables could be seen and understood by the compiler’s optimizer; by the debugger; by the IDE; and so on, not to mention by our own coworkers). Another place you sometimes see people managing their own “stack frame” data structures is in coroutines.

For example, here’s a hand-coded generator of Pythagorean triples; see “Four versions of Eric’s Famous Pythagorean Triples Code” (2019-03-06).

struct TriplesGenerator {
int x_ = 1;
int y_ = 1;
int z_ = 1;
int n_;
explicit TriplesGenerator(int n) : n_(n) {}
std::optional<Triple> next() {
if (n_-- == 0) {
return std::nullopt;
}
for (; true; ++z_) {
for (; x_ < z_; ++x_) {
for (; y_ < z_; ++y_) {
if (x_*x_ + y_*y_ == z_*z_) {
return Triple{ x_, y_++, z_ };
}
}
y_ = x_ + 1;
}
x_ = 1;
}
}
};

auto tg = TriplesGenerator(5);
while (auto opt = tg.next()) {
auto [x,y,z] = *opt;
printf("%d %d %d\n", x, y, z);
}


The programmer of TriplesGenerator is having to manage the lifetimes of x_, y_, z_, and n_ by hand; and even worse, having to reason about the state of those variables each time the next() function is exited and re-entered from the top.

Contrast the above manual code with this C++20 Coroutines code, which uses a std::generator<T> class similar to the one currently slated for C++23.

“But wait, I thought you said library facilities were bad!”

Not at all. I said that some library facilities can be replaced with core-language features. Other library facilities complement core-language features in a good way. The core-language feature we’re going to use in the following code is co_yield, so that we can stop using data members of TriplesGenerator and start using plain old local variables on a stack frame (well, a coroutine frame) that the compiler will manage for us. Godbolt:

auto triples(int n) -> generator<Triple> {
for (int z = 1; true; ++z) {
for (int x = 1; x < z; ++x) {
for (int y = x; y < z; ++y) {
if (x*x + y*y == z*z) {
co_yield { x, y, z };
if (--n == 0) co_return;
}
}
}
}
}

for (auto [x,y,z] : triples(5)) {
printf("%d %d %d\n", x, y, z);
}


Notice on Godbolt that GCC can optimize the hand-coded TriplesGenerator version down to almost nothing, whereas the co_yield-based version has a lot more machine instructions. On Clang the two versions are more coequal; that’s because Clang does a much worse job optimizing TriplesGenerator, not because it does any better at optimizing co_yield.

If I had to maintain one of these two snippets in a real codebase, long-term (and I didn’t have any qualms about the long-term stability of Coroutines support), I’d definitely prefer to maintain the shorter simpler co_yield-based version. Let the compiler deal with allocating variables on frames — computers are really good at that! Save the programmer’s brain cells for other things (like dangling-reference bugs).

## Prefer ::new (p) T(args...) over std::construct_at

This is another controversial one. There’s no observable difference between a call to the library function std::construct_at and a manual call to ::new (p) Widget, so why should we prefer one over the other? For me, it’s partly (unintuitively?) about readability: I find the line involving Widget(x, y) looks more like a constructor call than the line involving (Widget*)data, x, y. And partly it’s consistent application of the mantra of this blog post: When we can accomplish something using a shorter syntax known to the compiler, or a longer syntax that requires library support, as a general rule we should prefer the core-language syntax.

alignas(Widget) char data[sizeof(Widget)];

Widget *pw = std::construct_at((Widget*)data, x, y);  // worse
Widget *pw = ::new ((void*)data) Widget(x, y);  // better


Ditto for std::destroy_at versus p->~T():

std::destroy_at(pw);  // worse
pw->~Widget();  // better


(Observe that std::destroy_at can also destroy arrays, starting in C++20; whereas a direct call to ~T() cannot. But if you’re in a situation where T might be an array type, and you actually want to support that, I’m okay with forcing you to break this guideline.)

(C++20 also begins a weird window of historical time where std::construct_at and std::allocator<T>::allocate are required to be constexpr-friendly where ::new and new are not; so constexpr code may need to use the library facilities for now, in the same way that C++0x code might have needed to use std::alignment_of or std::bind before the core language caught up and obsoleted them again. I predict this current window will close again by C++26 at the latest.)

## Prefer C++20 requires over std::enable_if

I don’t use C++20 Concepts syntax in my day-to-day programming yet, but if you’re writing C++20 code, then you should almost certainly prefer requires-clauses over the more old-school (and backward-compatible) SFINAE techniques.

template<class T, std::enable_if_t<std::is_polymorphic_v<T>, int> = 0>
void foo(T t);  // worse

template<class T>
requires std::is_polymorphic_v<T>
void foo(T t);  // better


It’s more readable, and might even compile faster.

## Prefer assignment over .reset

auto p = std::make_unique<Widget>();
p.reset();  // worse
p = nullptr;  // better


There are valid use-cases for .reset; for example, when you’re using unique_ptr<T, Deleter> as an implementation detail.

std::unique_ptr<FILE, FileCloser> fp = nullptr;
if (should_open) {
fp.reset(fopen("input.txt", "r"));
}


But in general, .reset (and also .get, and even the rarely-used .release) is a code smell that’s worth investigating closely.

## Prefer '\n' over std::endl

std::cout << "Hello world" << std::endl;  // worse
std::cout << "Hello world\n";  // better


On modern platforms, the standard output is almost always at least line-buffered, so flushing the stream immediately after a newline (as endl does) is redundant. std::endl (“newline and flush”) can always be reduced to '\n' (“newline and let the OS do its thing, just like you would in any other language”).

See this StackOverflow question for various more or less interesting differences between endl and '\n'. One interesting tidbit is that std::endl is a function template! Thus decltype(std::endl) is ill-formed, and std::cout << std::endl works only thanks to the long-despised rule [temp.deduct.funcaddr], the same rule that permits things like

template<class T> T f(T);
int (*p1)(int) = f;                      // OK
std::ostream& (*p2)(std::ostream&) = f;  // OK


If it weren’t for iomanipulators like endl and flush, maybe we wouldn’t need that rule in the core language?

## Finally, some near misses

Regular readers of this blog will know that I advise strongly against CTAD, which means that I strongly prefer the library syntax std::make_pair(x, y) over std::pair(x, y), std::make_reverse_iterator(it) over std::reverse_iterator(it), and so on. Of course if std::pair(x, y) and std::make_pair(x, y) did the same thing, it wouldn’t matter which you used, and you might well prefer the shorter, “corer” syntax. But they don’t do the same thing, and in fact CTAD is so pitfally that I just prefer to ban it outright.

Either CTAD is the exception that proves the rule, or the explanation is that both syntaxes involve standard library facilities: my preferred syntax involves a standard library function, and my disfavored syntax involves a set of deduction guides (some implicitly generated) associated with a standard library type. So we have a choice between two different library facilities, and we should choose the one that’s less likely to lead to a bug.

Also note that in contexts where you get to choose between std::make_pair(x, y) and a simple braced initializer {x, y}, I’d often recommend the latter.

Regular readers of this blog will also know that I tend to prefer T(x) over static_cast<T>(x), despite some caveats. Both of those are core-language syntaxes. But it’s probably no coincidence that I prefer the shorter, less angle-brackety of the two.

Posted 2022-10-16