Any of c что это

What is a «span» and when should I use one?

Recently I’ve gotten suggestions to use span ‘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 , and why (or when) is it a good idea to use it if it’s non-standard?

asked Aug 16, 2017 at 22:15
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@jww: span’s are quite usable with C++11. as gsl::span rather than std::span . See also my answer below.

Jan 24, 2018 at 6:39
Also documented on cppreference.com: en.cppreference.com/w/cpp/container/span
Mar 24, 2020 at 19:18
@KeithThompson: Not in 2017 it wasn’t.
Mar 24, 2020 at 21:02

@jww All compilers support std::span<> now in C++20 mode. And span is available from many 3rd party libs. You were right — it was years: 2 years to be precise.

Jun 20, 2020 at 7:43

4 Answers 4

What is it?

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 ), 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 (respectively, span ) 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);

void read_into(span 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.
```
int buffer[BUFFER_SIZE]; read_into(buffer, BUFFER_SIZE); 
```
becomes this:
```
int buffer[BUFFER_SIZE]; read_into(buffer); 
```
There’s even more motivation for using span s, 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 .
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:
- martinmoene/span-lite requires C++98 or later
- tcbrindle/span requires C++11 or later
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).

answered Aug 16, 2017 at 22:15
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It would make more sense to standardize a general range (supporting iterator+sentinel and iterator+length, maybe even iterator+sentinel+length) and make span a simple typedef. Because, you know, that’s more generic.

Aug 17, 2017 at 12:47

@Deduplicator: Ranges are coming to C++, but the current proposal (by Eric Niebler) requires support for Concepts. So not before C++20.

Aug 17, 2017 at 12:52

@HảiPhạmLê: Arrays don’t immediately decay into pointers. try doing std::cout
Aug 23, 2017 at 7:59
@Jim std::array is a container, it owns the values. span is non-owning
Dec 4, 2018 at 14:18

@Jim: std::array is a completely different beast. Its length is fixed at compile-time and it’s a value-type rather than a reference-type, as Caleth explained.

Dec 4, 2018 at 14:56
```
template struct span < T * ptr_to_array; // pointer to a contiguous C-style array of data // (which memory is NOT allocated nor deallocated // nor in any way managed by the span) std::size_t length; // number of elements of type `T` in the array // Plus a bunch of constructors and convenience accessor methods here >
```
It is a light-weight wrapper around a C-style array, preferred by C++ developers whenever they are using C libraries and want to wrap them with a C++-style data container for «type safety» and «C++-ishness» and «feelgoodery». 🙂

Note: I call the struct container defined above, known as a span, a «light-weight wrapper around a C-style array» because it points to a contiguous piece of memory, such as a C-style array, and wraps it with accessor methods and the array’s size. This is what I mean by «light-weight wrapper»: it is a wrapper around a pointer and a length variable, plus functions.

Unlike a std::vector<> and other C++ standard containers, however, which may also just have fixed class sizes and contain pointers which point to their storage memory, a span does not own the memory it points to, and will never delete it nor resize it nor allocate new memory automatically. Again, a container like a vector owns the memory it points to, and will manage (allocate, reallocate, etc.) it, but a span does not own the memory it points to, and therefore will not manage it.

Going further:

@einpoklum does a pretty good job of introducing what a span is in his answer here. However, even after reading his answer, it is easy for someone new to spans to still have a sequence of stream-of-thought questions which aren’t fully answered, such as the following:
1. How is a span different from a C array? Why not just use one of those? It seems like it’s just one of those with the size known as well.
2. Wait, that sounds like a std::array , how is a span different from that?
3. Oh, that reminds me, isn’t a std::vector like a std::array too?
4. I’m so confused. 🙁 What’s a span ?
So, here’s some additional clarity on that:

DIRECT QUOTE OF HIS ANSWER—WITH MY ADDITIONS and parenthetical comments IN BOLD and my emphasis in italics:

What is it?
- A very lightweight abstraction of a contiguous sequence of values of type T somewhere in memory.
- Basically a single struct < T * ptr; std::size_t length; >with a bunch of convenience methods. (Notice this is distinctly different from std::array<> because a span enables convenience accessor methods, comparable to std::array , via a pointer to type T and length (number of elements) of type T , whereas std::array is an actual container which holds one or more values of type T .)
- 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 .

Those bold parts are critical to one’s understanding, so don’t miss them or misread them! A span is NOT a C-array of structs, nor is it a struct of a C-array of type T plus the length of the array (this would be essentially what the std::array container is), NOR is it a C-array of structs of pointers to type T plus the length, but rather it is a single struct containing one single pointer to type T , and the length, which is the number of elements (of type T ) in the contiguous memory block that the pointer to type T points to! In this way, the only overhead you’ve added by using a span are the variables to store the pointer and length, and any convenience accessor functions you use which the span provides.

This is UNLIKE a std::array<> because the std::array<> actually allocates memory for the entire contiguous block, and it is UNLIKE std::vector<> because a std::vector is basically just a std::array that also does dynamic growing (usually doubling in size) each time it fills up and you try to add something else to it. A std::array is fixed in size, and a span doesn’t even manage the memory of the block it points to, it just points to the block of memory, knows how long the block of memory is, knows what data type is in a C-array in the memory, and provides convenience accessor functions to work with the elements in that contiguous memory.

It is part of the C++ standard:

std::span is part of the C++ standard as of C++20. You can read its documentation here: https://en.cppreference.com/w/cpp/container/span. To see how to use Google’s absl::Span(array, length) in C++11 or later today, see below.

Summary Descriptions, and Key References:
1. std::span ( Extent = «the number of elements in the sequence, or std::dynamic_extent if dynamic». A span just points to memory and makes it easy to access, but does NOT manage it!):
2. https://en.cppreference.com/w/cpp/container/span
3. std::array (notice it has a fixed size N !):
4. https://en.cppreference.com/w/cpp/container/array
5. http://www.cplusplus.com/reference/array/array/
6. std::vector (automatically dynamically grows in size as necessary):
7. https://en.cppreference.com/w/cpp/container/vector
8. http://www.cplusplus.com/reference/vector/vector/
How Can I Use span in C++11 or later today?

Google has open-sourced their internal C++11 libraries in the form of their «Abseil» library. This library is intended to provide C++14 to C++20 and beyond features which work in C++11 and later, so that you can use tomorrow’s features, today. They say:

Compatibility with the C++ Standard

Google has developed many abstractions that either match or closely match features incorporated into C++14, C++17, and beyond. Using the Abseil versions of these abstractions allows you to access these features now, even if your code is not yet ready for life in a post C++11 world.

Here are some key resources and links:
1. Main site: https://abseil.io/
2. https://abseil.io/docs/cpp/
3. GitHub repository: https://github.com/abseil/abseil-cpp
4. span.h header, and absl::Span(array, length) template class: https://github.com/abseil/abseil-cpp/blob/master/absl/types/span.h#L153
Other references:
1. Struct with template variables in C++
2. Wikipedia: C++ classes
3. default visibility of C++ class/struct members
Related:
1. [another one of my answers on templates and spans] How to make span of spans
Перевод «of C» на русский

Denote by fk and gk the number of k-gonal faces of the embedding that are inside and outside of C, respectively.

Обозначим через fk и gk число k-угольных граней вложения, которые находятся внутри и вне C соответственно.

Fourth, the ratio of C to C in the atmosphere is not constant.
Кроме того, соотношение 14C/ 12C в атмосфере не постоянно.
Thanks to the popularity of C++, the according CPP files are very well known as well.
Благодаря популярности С ++ файлы СРР также очень хорошо известны.
Because the Government of C won’t let us.
«И мы с правительства не слезем.
Turbo Vision: A set of C++ classes to create professional applications in DOS.
Turbo Vision — набор классов C++ для создания профессиональных приложений в DOS.

In the compositions of Ludwig van Beethoven, the key of C minor has been regarded by some as significant.

В произведениях Людвига ван Бетховена тональность до минор расценивается специалистами как особо важная.

Unlike Objective-C, Swift is a statically typed language that is not a strict extension of C.

В отличие от Objective-C, Swift — статически типизированный язык, который не является строгим расширением C.

French Utopian socialist and follower of C. Fourier.
Французский социалист-утопист, последователь Ш. Фурье.
Most of C Company moved all the way to the 25th Division positions south of the Naktong.
Большая часть роты С прошло весь путь до позиций 25-й дивизии к югу от реки Нактонган.
A Microsoft version of C++ with added Java-like functions.
Microsoft версии C + + с добавлением Java-подобных функций.
Accordingly, the map Δ is called the comultiplication (or coproduct) of C and ε is the counit of C.

Соответственно, отображение Δ называется коумножением (или копроизведением) в C, а ε является коединицей C.

Возможно неприемлемое содержание

Примеры предназначены только для помощи в переводе искомых слов и выражений в различных контекстах. Мы не выбираем и не утверждаем примеры, и они могут содержать неприемлемые слова или идеи. Пожалуйста, сообщайте нам о примерах, которые, на Ваш взгляд, необходимо исправить или удалить. Грубые или разговорные переводы обычно отмечены красным или оранжевым цветом.

Treatment — Hepatitis C

If the infection is diagnosed in the early stages, known as acute hepatitis, treatment may not need to begin straight away.

Instead, you may have another blood test after a few months to see if your body fights off the virus.

If the infection continues for several months, known as chronic hepatitis, treatment will usually be recommended.

Your treatment plan

Treatment for chronic hepatitis C (those infected for 6 months or more) involves:
- tablets to fight the virus
- a test to see if your liver is damaged
- lifestyle changes to prevent further damage
There are 6 main strains of the virus. In the UK, the most common strains are genotype 1 and genotype 3. You can be infected with more than 1 strain.

You’ll be offered the medicine most appropriate for your type of hepatitis C.

During treatment, you should have blood tests to check that your medicine is working.

If it’s not, you may be advised to try another medicine. This will only affect a small number of people.

Your doctor will also assess your liver for damage (scarring), either with a blood test or a scan called a fibroscan.

At the end of your treatment, you’ll have a blood test to see if the virus has been cleared and a second blood test 12 or 24 weeks after treatment has stopped.

If both tests show no sign of the virus, this means treatment has been successful.

Hepatitis C medicines

Hepatitis C is treated using direct-acting antiviral (DAA) tablets.

DAA tablets are the safest and most effective medicines for treating hepatitis C.

They’re highly effective at clearing the infection in more than 90% of people.

The tablets are taken for 8 to 12 weeks. The length of treatment will depend on which type of hepatitis C you have.

Some types of hepatitis C can be treated using more than 1 type of DAA.

NHS-approved hepatitis C medicines include:
- sofosbuvir
- a combination of ledipasvir and sofosbuvir
- a combination of ombitasvir, paritaprevir and ritonavir, taken with or without dasabuvir
- a combination of elbasvir and grazoprevir
- a combination of sofosbuvir and velpatasvir
- a combination of sofosbuvir, velpatasvir and voxilaprevir
- a combination of glecaprevir and pibrentasvir
- ribavirin
For more information, see the NICE guidelines on:
- sofosbuvir for treating chronic hepatitis C
- ledipasvir-sofosbuvir for treating chronic hepatitis C
- ombitasvir-paritaprevir-ritonavir with or without dasabuvir for treating chronic hepatitis C
- elbasvir-grazoprevir for treating chronic hepatitis C
- sofosbuvir-velpatasvir for treating chronic hepatitis C
- sofosbuvir-velpatasvir-voxilaprevir for treating chronic hepatitis C
- glecaprevir–pibrentasvir for treating chronic hepatitis C
Side effects of treatment

Treatments with direct-acting antivirals (DAAs) have very few side effects. Most people find DAA tablets very easy to take.

You may feel a little sick and have trouble sleeping to begin with, but this should soon settle down.

Your nurse or doctor should be able to suggest things to help ease any discomfort.

You need to complete the full course of treatment to ensure you clear the hepatitis C virus from your body.

If you have any problems with your medicines, speak to your doctor or nurse straight away.

Side effects for each type of treatment can vary from person to person.

For a very small number of people, more severe side effects from hepatitis C treatments may include:
- depression
- skin irritation
- anxiety
- problems sleeping (insomnia)
- anorexia
- tiredness caused by anaemia
- hair loss
- aggressive behaviour
How effective is treatment?

Direct-acting antivirals (DAAs) cure 9 out of 10 patients with hepatitis C.

Successful treatment does not give you any protection against another hepatitis C infection. You can still catch it again.

There’s no vaccine for hepatitis C.

If treatment does not work, it may be repeated, extended, or a different combination of medicines may be tried.

Your doctor or nurse will be able to advise you.

Things you can do during treatment for hepatitis C

There are some things you can do to help limit any damage to your liver and prevent the infection spreading to others.

These can include:
- eating a healthy, balanced diet
- exercising regularly
- cutting out alcohol or limiting how much you drink
- quitting smoking
- keeping personal items, such as toothbrushes or razors, for your own use
- not sharing any needles or syringes with others
- practising safer sex
- telling sexual partners who might need to get tested
Pregnancy and hepatitis C

The new hepatitis C medicines have not been tested in pregnancy.

You should not become pregnant while taking treatment as it could be harmful to unborn babies.

If you’re pregnant, you must delay treatment until after your baby is born.

Speak to your doctor before starting hepatitis C treatment if you’re planning to become pregnant in the near future.

You’ll need to wait several weeks after treatment has ended before trying to get pregnant.

Women taking ribavirin should use contraception during treatment and for another 4 months after the end of treatment.

Men taking ribavirin should use a condom during treatment and for another 7 months after the end of treatment. This is because semen can contain ribavirin.

If you become pregnant during treatment, speak to your doctor as soon as possible to discuss your treatment options.

Deciding against treatment

Some people with chronic hepatitis C decide against treatment.

This may be because they:
- do not have any symptoms
- are willing to live with the risk of cirrhosis at a later date
- do not feel the potential benefits of treatment outweigh the side effects some treatments can cause
Your care team can give you advice about this, but the final decision about treatment will be yours.

If you decide not to have treatment but then change your mind, you can ask to be treated at any point.

Page last reviewed: 27 October 2021
Next review due: 27 October 2024

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