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c++c++20c++-faqcpp-core-guidelinesstd-span

What is a "span" and when should I use one?

Recently I've gotten suggestions to use span<T>'s in my code, or have seen some answers here on the site which use span's - supposedly some kind of container. But - I can't find anything like that in the C++17 standard library.

So what is this mysterious span<T>, and why (or when) is it a good idea to use it?

Solution

  • 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 spans:

    • Don't use a span in code that could just take any pair of start & end iterators (like std::sort, std::find_if, std::copy and other templated functions from <algorithm>), and also not in code that takes an arbitrary range (see The C++20 ranges library for information about those). A span has stricter requirements than a pair of iterators or a range: element contiguity and presence of the elements in memory.
    • Don't use a span if you have a standard library container (or a Boost container etc.) which you know is the right fit for your code. spans are not intended to supplant existing containers.

    Now for when to actually use a span:

    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; and with less opportunities to exceed array bounds.

    • 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 [12]. 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).

    There is also a multi-dimensional extension of spans: mdspan's; see this SO question.