What is it?
A span<T> is:
- A very lightweight abstraction of a contiguous sequence of values of type
T somewhere in memory.
- Basically a
struct { T * ptr; std::size_t length; } with a bunch of convenience methods.
- A non-owning type (i.e. a "reference-type" rather than a "value type"): It never allocates nor deallocates anything and does not keep smart pointers alive.
It was formerly known as an array_view and even earlier as array_ref.
When should I use it?
First, when not to use it:
- Don't use it in code that could just take any pair of start & end iterators, like
std::sort, std::find_if, std::copy and all of those super-generic templated functions.
- Don't use it if you have a standard library container (or a Boost container etc.) which you know is the right fit for your code. It's not intended to supplant any of them.
Now for when to actually use it:
Use span<T> (respectively, span<const T>) instead of a free-standing T* (respectively const T*) when the allocated length or size also matter. So, replace functions like:
void read_into(int* buffer, size_t buffer_size);
with:
void read_into(span<int> buffer);
Why should I use it? Why is it a good thing?
Oh, spans are awesome! Using a span...
means that you can work with that pointer+length / start+end pointer combination like you would with a fancy, pimped-out standard library container, e.g.:
for (auto& x : my_span) { /* do stuff */ }std::find_if(my_span.cbegin(), my_span.cend(), some_predicate);std::ranges::find_if(my_span, some_predicate); (in C++20)
... but with absolutely none of the overhead most container classes incur.
lets the compiler do more work for you sometimes. For example, this:
int buffer[BUFFER_SIZE];
read_into(buffer, BUFFER_SIZE);
becomes this:
int buffer[BUFFER_SIZE];
read_into(buffer);
... which will do what you would want it to do. See also Guideline P.5.
is the reasonable alternative to passing const vector<T>& to functions when you expect your data to be contiguous in memory. No more getting scolded by high-and-mighty C++ gurus!
facilitates static analysis, so the compiler might be able to help you catch silly bugs.
allows for debug-compilation instrumentation for runtime bounds-checking (i.e. span's methods will have some bounds-checking code within #ifndef NDEBUG ... #endif)
indicates that your code (that's using the span) doesn't own the pointed-to memory.
There's even more motivation for using spans, which you could find in the C++ core guidelines - but you catch the drift.
But is it in the standard library?
edit: Yes, std::span was added to C++ with the C++20 version of the language!
Why only in C++20? Well, While the idea is not new - its current form was conceived in conjunction with the C++ core guidelines project, which only started taking shape in 2015. So it took a while.
So how do I use it if I'm writing C++17 or earlier?
It's part of the Core Guidelines's Support Library (GSL). Implementations:
- Microsoft / Neil Macintosh's GSL contains a standalone implementation:
gsl/span
- GSL-Lite is a single-header implementation of the whole GSL (it's not that big, don't worry), including
span<T>.
The GSL implementation does generally assume a platform that implements C++14 support [11]. These alternative single-header implementations do not depend on GSL facilities:
Note that these different span implementations have some differences in what methods/support functions they come with; and they may also differ somewhat from the version adopted into the standard library in C++20.
Further reading: You can find all the details and design considerations in the final official proposal before C++17, P0122R7: span: bounds-safe views for sequences of objects by Neal Macintosh and Stephan J. Lavavej. It's a bit long though. Also, in C++20, the span comparison semantics changed (following this short paper by Tony van Eerd).
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