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The Guidelines Support Library (GSL) interface is very lightweight and exposed via a header-only library. This document attempts to document all of the headers and their exposed classes and functions.

Types and functions are exported in the namespace gsl.

See GSL: Guidelines support library

Headers

<algorithms>
<assert>
<byte>
<gsl>
<narrow>
<pointers>
<span>
<span_ext>
<zstring>
<util>

`<algorithms>`

This header contains some common algorithms that have been wrapped in GSL safety features.

gsl::copy

`gsl::copy`

template <class SrcElementType, std::size_t SrcExtent, class DestElementType,
          std::size_t DestExtent>
void copy(span<SrcElementType, SrcExtent> src, span<DestElementType, DestExtent> dest);

This function copies the content from the src span to the dest span. It Expects that the destination span is at least as large as the source span.

`<assert>`

This header contains some macros used for contract checking and suppressing code analysis warnings.

See GSL.assert: Assertions

GSL_SUPPRESS
Expects
Ensures

`GSL_SUPPRESS`

This macro can be used to suppress a code analysis warning.

The core guidelines request tools that check for the rules to respect suppressing a rule by writing [[gsl::suppress(tag)]] or [[gsl::suppress(tag, justification: "message")]].

Clang does not use exactly that syntax, but requires tag to be put in double quotes [[gsl::suppress("tag")]].

For portable code you can use GSL_SUPPRESS(tag).

See In.force: Enforcement.

`Expects`

This macro can be used for expressing a precondition. If the precondition is not held, then std::terminate will be called.

See I.6: Prefer Expects() for expressing preconditions

`Ensures`

This macro can be used for expressing a postcondition. If the postcondition is not held, then std::terminate will be called.

See I.8: Prefer Ensures() for expressing postconditions

`<byte>`

This header contains the definition of a byte type, implementing std::byte before it was standardized into C++17.

gsl::byte

`gsl::byte`

If GSL_USE_STD_BYTE is defined to be 1, then gsl::byte will be an alias to std::byte.
If GSL_USE_STD_BYTE is defined to be 0, then gsl::byte will be a distinct type that implements the concept of byte.
If GSL_USE_STD_BYTE is not defined, then the header file will check if std::byte is available (C++17 or higher). If yes, gsl::byte will be an alias to std::byte, otherwise gsl::byte will be a distinct type that implements the concept of byte.

⚠ Take care when linking projects that were compiled with different language standards (before C++17 and C++17 or higher). If you do so, you might want to #define GSL_USE_STD_BYTE 0 to a fixed value to be sure that both projects use exactly the same type. Otherwise you might get linker errors.

See SL.str.5: Use std::byte to refer to byte values that do not necessarily represent characters

Non-member functions

template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
constexpr byte& operator<<=(byte& b, IntegerType shift) noexcept;

template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
constexpr byte operator<<(byte b, IntegerType shift) noexcept;

template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
constexpr byte& operator>>=(byte& b, IntegerType shift) noexcept;

template <class IntegerType, class = std::enable_if_t<std::is_integral<IntegerType>::value>>
constexpr byte operator>>(byte b, IntegerType shift) noexcept;

Left or right shift a byte by a given number of bits.

constexpr byte& operator|=(byte& l, byte r) noexcept;
constexpr byte operator|(byte l, byte r) noexcept;

Bitwise "or" of two bytes.

constexpr byte& operator&=(byte& l, byte r) noexcept;
constexpr byte operator&(byte l, byte r) noexcept;

Bitwise "and" of two bytes.

constexpr byte& operator^=(byte& l, byte r) noexcept;
constexpr byte operator^(byte l, byte r) noexcept;

Bitwise xor of two bytes.

constexpr byte operator~(byte b) noexcept;

Bitwise negation of a byte. Flips all bits. Zeroes become ones, ones become zeroes.

template <typename T>
constexpr byte to_byte(T t) noexcept;

Convert the given value to a byte. The template requires T to be an unsigned char so that no data loss can occur. If you want to convert an integer constant to a byte you probably want to call to_byte<integer constant>().

template <int I>
constexpr byte to_byte() noexcept;

Convert the given value I to a byte. The template requires I to be in the valid range 0..255 for a gsl::byte.

`<gsl>`

This header is a convenience header that includes all other GSL headers. Since <narrow> requires exceptions, it will only be included if exceptions are enabled.

`<narrow>`

This header contains utility functions and classes, for narrowing casts, which require exceptions. The narrowing-related utilities that don't require exceptions are found inside util.

See GSL.util: Utilities

gsl::narrowing_error
gsl::narrow

`gsl::narrowing_error`

gsl::narrowing_error is the exception thrown by gsl::narrow when a narrowing conversion fails. It is derived from std::exception.

`gsl::narrow`

gsl::narrow<T>(x) is a named cast that does a static_cast<T>(x) for narrowing conversions with no signedness promotions. If the argument x cannot be represented in the target type T, then the function throws a gsl::narrowing_error (e.g., narrow<unsigned>(-42) and narrow<char>(300) throw).

Note: compare gsl::narrow_cast in header util.

See ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions and ES.49: If you must use a cast, use a named cast

`<pointers>`

This header contains some pointer types.

See GSL.view

gsl::unique_ptr
gsl::shared_ptr
gsl::owner
gsl::not_null
gsl::strict_not_null

`gsl::unique_ptr`

gsl::unique_ptr is an alias to std::unique_ptr.

See GSL.owner: Ownership pointers

`gsl::shared_ptr`

gsl::shared_ptr is an alias to std::shared_ptr.

See GSL.owner: Ownership pointers

`gsl::owner`

gsl::owner<T> is designed as a safety mechanism for code that must deal directly with raw pointers that own memory. Ideally such code should be restricted to the implementation of low-level abstractions. gsl::owner can also be used as a stepping point in converting legacy code to use more modern RAII constructs such as smart pointers. T must be a pointer type (std::is_pointer<T>).

A gsl::owner<T> is a typedef to T. It adds no runtime overhead whatsoever, as it is purely syntactic and does not add any runtime checks. Instead, it serves as an annotation for static analysis tools which check for memory safety, and as a code comprehension guide for human readers.

See Enforcement section of C.31: All resources acquired by a class must be released by the class’s destructor.

`gsl::not_null`

gsl::not_null<T> restricts a pointer or smart pointer to only hold non-null values. It has no size overhead over T.

The checks for ensuring that the pointer is not null are done in the constructor. There is no overhead when retrieving or dereferencing the checked pointer. When a nullptr check fails, std::terminate is called.

See F.23: Use a not_null<T> to indicate that “null” is not a valid value

Member functions

Construct/Copy

template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
constexpr not_null(U&& u);

template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
constexpr not_null(T u);

Constructs a gsl_owner<T> from a pointer that is convertible to T or that is a T. It Expects that the provided pointer is not == nullptr.

template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
constexpr not_null(const not_null<U>& other);

Constructs a gsl_owner<T> from another gsl_owner where the other pointer is convertible to T. It Expects that the provided pointer is not == nullptr.

not_null(const not_null& other) = default;
not_null& operator=(const not_null& other) = default;

Copy construction and assignment.

not_null(std::nullptr_t) = delete;
not_null& operator=(std::nullptr_t) = delete;

Construction from std::nullptr_t and assignment of std::nullptr_t are explicitly deleted.

Modifiers

not_null& operator++() = delete;
not_null& operator--() = delete;
not_null operator++(int) = delete;
not_null operator--(int) = delete;
not_null& operator+=(std::ptrdiff_t) = delete;
not_null& operator-=(std::ptrdiff_t) = delete;

Explicitly deleted operators. Pointers point to single objects (I.13: Do not pass an array as a single pointer), so don't allow these operators.

Observers

constexpr details::value_or_reference_return_t<T> get() const;
constexpr operator T() const { return get(); }

Get the underlying pointer.

constexpr decltype(auto) operator->() const { return get(); }
constexpr decltype(auto) operator*() const { return *get(); }

Dereference the underlying pointer.

void operator[](std::ptrdiff_t) const = delete;

Array index operator is explicitly deleted. Pointers point to single objects (I.13: Do not pass an array as a single pointer), so don't allow treating them as an array.

Non-member functions

template <class T>
auto make_not_null(T&& t) noexcept;

Creates a gsl::not_null object, deducing the target type from the type of the argument.

template <class T, class U>
auto operator==(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() == rhs.get()))
    -> decltype(lhs.get() == rhs.get());
template <class T, class U>
auto operator!=(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() != rhs.get()))
    -> decltype(lhs.get() != rhs.get());
template <class T, class U>
auto operator<(const not_null<T>& lhs,
               const not_null<U>& rhs) noexcept(noexcept(lhs.get() < rhs.get()))
    -> decltype(lhs.get() < rhs.get());
template <class T, class U>
auto operator<=(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() <= rhs.get()))
    -> decltype(lhs.get() <= rhs.get());
template <class T, class U>
auto operator>(const not_null<T>& lhs,
               const not_null<U>& rhs) noexcept(noexcept(lhs.get() > rhs.get()))
    -> decltype(lhs.get() > rhs.get());
template <class T, class U>
auto operator>=(const not_null<T>& lhs,
                const not_null<U>& rhs) noexcept(noexcept(lhs.get() >= rhs.get()))
    -> decltype(lhs.get() >= rhs.get());

Comparison of pointers that are convertible to each other.

Input/Output

template <class T>
std::ostream& operator<<(std::ostream& os, const not_null<T>& val);

Performs stream output on a not_null pointer, invoking os << val.get(). This function is only available when GSL_NO_IOSTREAMS is not defined.

Modifiers

template <class T, class U>
std::ptrdiff_t operator-(const not_null<T>&, const not_null<U>&) = delete;
template <class T>
not_null<T> operator-(const not_null<T>&, std::ptrdiff_t) = delete;
template <class T>
not_null<T> operator+(const not_null<T>&, std::ptrdiff_t) = delete;
template <class T>
not_null<T> operator+(std::ptrdiff_t, const not_null<T>&) = delete;

Addition and subtraction are explicitly deleted. Pointers point to single objects (I.13: Do not pass an array as a single pointer), so don't allow these operators.

STL integration

template <class T>
struct std::hash<gsl::not_null<T>> { ... };

Specialization of std::hash for gsl::not_null.

`gsl::strict_not_null`

strict_not_null is the same as not_null except that the constructors are explicit.

The free function that deduces the target type from the type of the argument and creates a gsl::strict_not_null object is gsl::make_strict_not_null.

`<span>`

This header file exports the class gsl::span, a bounds-checked implementation of std::span.

gsl::span

`gsl::span`

template <class ElementType, std::size_t Extent>
class span;

gsl::span is a view over memory. It does not own the memory and is only a way to access contiguous sequences of objects. The extent can be either a fixed size or gsl::dynamic_extent.

The gsl::span is based on the standardized version of std::span which was added to C++20. Originally, the plan was to deprecate gsl::span when std::span finished standardization, however that plan changed when the runtime bounds checking was removed from std::span's design.

The only difference between gsl::span and std::span is that gsl::span strictly enforces runtime bounds checking. Any violations of the bounds check results in termination of the program. Like gsl::span, gsl::span's iterators also differ from std::span's iterator in that all access operations are bounds checked.

Which version of span should I use?

Use `gsl::span` if

you want to guarantee bounds safety in your project.
- All data accessing operations use bounds checking to ensure you are only accessing valid memory.
your project uses C++14 or C++17.
- std::span is not available as it was not introduced into the STL until C++20.

Use `std::span` if

your project is C++20 and you need the performance offered by std::span.

Types

using element_type = ElementType;
using value_type = std::remove_cv_t<ElementType>;
using size_type = std::size_t;
using pointer = element_type*;
using const_pointer = const element_type*;
using reference = element_type&;
using const_reference = const element_type&;
using difference_type = std::ptrdiff_t;

using iterator = details::span_iterator<ElementType>;
using reverse_iterator = std::reverse_iterator<iterator>;

Member functions

constexpr span() noexcept;

Constructs an empty span. This constructor is only available if Extent is 0 or gsl::dynamic_extent. span::data() will return nullptr.

constexpr explicit(Extent != gsl::dynamic_extent) span(pointer ptr, size_type count) noexcept;

Constructs a span from a pointer and a size. If Extent is not gsl::dynamic_extent, then the constructor Expects that count == Extent.

constexpr explicit(Extent != gsl::dynamic_extent) span(pointer firstElem, pointer lastElem) noexcept;

Constructs a span from a pointer to the begin and the end of the data. If Extent is not gsl::dynamic_extent, then the constructor Expects that lastElem - firstElem == Extent.

template <std::size_t N>
constexpr span(element_type (&arr)[N]) noexcept;

Constructs a span from a C style array. This overload is available if Extent ==gsl::dynamic_extent or N == Extent.

template <class T, std::size_t N>
constexpr span(std::array<T, N>& arr) noexcept;

template <class T, std::size_t N>
constexpr span(const std::array<T, N>& arr) noexcept;

Constructs a span from a std::array. These overloads are available if Extent ==gsl::dynamic_extent or N == Extent, and if the array can be interpreted as a ElementType array.

template <class Container>
constexpr explicit(Extent != gsl::dynamic_extent) span(Container& cont) noexcept;

template <class Container>
constexpr explicit(Extent != gsl::dynamic_extent) span(const Container& cont) noexcept;

Constructs a span from a container. These overloads are available if Extent ==gsl::dynamic_extent or N == Extent, and if the container can be interpreted as a contiguous ElementType array.

constexpr span(const span& other) noexcept = default;

Copy constructor.

template <class OtherElementType, std::size_t OtherExtent>
explicit(Extent != gsl::dynamic_extent && OtherExtent == dynamic_extent)
constexpr span(const span<OtherElementType, OtherExtent>& other) noexcept;

Constructs a span from another span. This constructor is available if OtherExtent == Extent || Extent ==gsl::dynamic_extent || OtherExtent ==gsl::dynamic_extent and if ElementType and OtherElementType are compatible.

If Extent !=gsl::dynamic_extent and OtherExtent ==gsl::dynamic_extent, then the constructor Expects that other.size() == Extent.

constexpr span& operator=(const span& other) noexcept = default;

Copy assignment

template <std::size_t Count>
constexpr span<element_type, Count> first() const noexcept;

constexpr span<element_type, dynamic_extent> first(size_type count) const noexcept;

template <std::size_t Count>
constexpr span<element_type, Count> last() const noexcept;

constexpr span<element_type, dynamic_extent> last(size_type count) const noexcept;

Return a subspan of the first/last Count elements. Expects that Count does not exceed the span's size.

template <std::size_t offset, std::size_t count = dynamic_extent>
constexpr auto subspan() const noexcept;

constexpr span<element_type, dynamic_extent>
subspan(size_type offset, size_type count = dynamic_extent) const noexcept;

Return a subspan starting at offset and having size count. Expects that offset does not exceed the span's size, and that offset == gsl::dynamic_extent or offset + count does not exceed the span's size. If count is gsl::dynamic_extent, the number of elements in the subspan is size() - offset.

constexpr size_type size() const noexcept;

constexpr size_type size_bytes() const noexcept;

Returns the size respective the size in bytes of the span.

constexpr bool empty() const noexcept;

Is the span empty?

constexpr reference operator[](size_type idx) const noexcept;

Returns a reference to the element at the given index. Expects that idx is less than the span's size.

constexpr reference front() const noexcept;
constexpr reference back() const noexcept;

Returns a reference to the first/last element in the span. Expects that the span is not empty.

constexpr pointer data() const noexcept;

Returns a pointer to the beginning of the contained data.

constexpr iterator begin() const noexcept;
constexpr iterator end() const noexcept;
constexpr reverse_iterator rbegin() const noexcept;
constexpr reverse_iterator rend() const noexcept;

Returns an iterator to the first/last normal/reverse iterator.

template <class Type, std::size_t Extent>
span(Type (&)[Extent]) -> span<Type, Extent>;

template <class Type, std::size_t Size>
span(std::array<Type, Size>&) -> span<Type, Size>;

template <class Type, std::size_t Size>
span(const std::array<Type, Size>&) -> span<const Type, Size>;

template <class Container,
          class Element = std::remove_pointer_t<decltype(std::declval<Container&>().data())>>
span(Container&) -> span<Element>;

template <class Container,
          class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>>
span(const Container&) -> span<Element>;

Deduction guides.

template <class ElementType, std::size_t Extent>
span<const byte, details::calculate_byte_size<ElementType, Extent>::value>
as_bytes(span<ElementType, Extent> s) noexcept;

template <class ElementType, std::size_t Extent>
span<byte, details::calculate_byte_size<ElementType, Extent>::value>
as_writable_bytes(span<ElementType, Extent> s) noexcept;

Converts a span into a span of bytes.

as_writable_bytes will only be available for non-const ElementTypes.

`<span_ext>`

This file is a companion for and included by <gsl/span>, and should not be used on its own. It contains useful features that aren't part of the std::span API as found inside the STL <span> header (with the exception of gsl::dynamic_extent, which is included here due to implementation constraints).

gsl::dynamic_extent
gsl::span
gsl::span comparison operators
gsl::make_span
gsl::at
gsl::ssize
gsl::span iterator functions

`gsl::dynamic_extent`

Defines the extent value to be used by all gsl::span with dynamic extent.

Note: std::dynamic_extent is exposed by the STL <span> header and so ideally gsl::dynamic_extent would be under <gsl/span>, but to avoid cyclic dependency issues it is under <span_ext> instead.

`gsl::span`

template <class ElementType, std::size_t Extent = dynamic_extent>
class span;

Forward declaration of gsl::span.

`gsl::span` comparison operators

template <class ElementType, std::size_t FirstExtent, std::size_t SecondExtent>
constexpr bool operator==(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r);
template <class ElementType, std::size_t FirstExtent, std::size_t SecondExtent>
constexpr bool operator!=(span<ElementType, FirstExtent> l, span<ElementType, SecondExtent> r);
template <class ElementType, std::size_t Extent>
constexpr bool operator<(span<ElementType, Extent> l, span<ElementType, Extent> r);
template <class ElementType, std::size_t Extent>
constexpr bool operator<=(span<ElementType, Extent> l, span<ElementType, Extent> r);
template <class ElementType, std::size_t Extent>
constexpr bool operator>(span<ElementType, Extent> l, span<ElementType, Extent> r);
template <class ElementType, std::size_t Extent>
constexpr bool operator>=(span<ElementType, Extent> l, span<ElementType, Extent> r);

The comparison operators for two spans lexicographically compare the elements in the spans.

`gsl::make_span`

template <class ElementType>
constexpr span<ElementType> make_span(ElementType* ptr, typename span<ElementType>::size_type count);
template <class ElementType>
constexpr span<ElementType> make_span(ElementType* firstElem, ElementType* lastElem);
template <class ElementType, std::size_t N>
constexpr span<ElementType, N> make_span(ElementType (&arr)[N]) noexcept;
template <class Container>
constexpr span<typename Container::value_type> make_span(Container& cont);
template <class Container>
constexpr span<const typename Container::value_type> make_span(const Container& cont);
template <class Ptr>
constexpr span<typename Ptr::element_type> make_span(Ptr& cont, std::size_t count);
template <class Ptr>
constexpr span<typename Ptr::element_type> make_span(Ptr& cont);

Utility function for creating a span with gsl::dynamic_extent from

pointer and length,
pointer to start and pointer to end,
a C style array, or
a container.

`gsl::at`

template <class ElementType, std::size_t Extent>
constexpr ElementType& at(span<ElementType, Extent> s, index i);

The function gsl::at offers a safe way to access data with index bounds checking.

This is the specialization of gsl::at for span. It returns a reference to the ith element and Expects that the provided index is within the bounds of the span.

Note: gsl::at supports indexes up to PTRDIFF_MAX.

`gsl::ssize`

template <class ElementType, std::size_t Extent>
constexpr std::ptrdiff_t ssize(const span<ElementType, Extent>& s) noexcept;

Return the size of a span as a ptrdiff_t.

`gsl::span` iterator functions

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::iterator
begin(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent = dynamic_extent>
constexpr typename span<ElementType, Extent>::iterator
end(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator
rbegin(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator
rend(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::iterator
cbegin(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent = dynamic_extent>
constexpr typename span<ElementType, Extent>::iterator
cend(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator
crbegin(const span<ElementType, Extent>& s) noexcept;

template <class ElementType, std::size_t Extent>
constexpr typename span<ElementType, Extent>::reverse_iterator
crend(const span<ElementType, Extent>& s) noexcept;

Free functions for getting a non-const/const begin/end normal/reverse iterator for a span.

`<zstring>`

This header exports a family of *zstring types.

A gsl::XXzstring<T> is a typedef to T. It adds no checks whatsoever, it is just for having a syntax to describe that a pointer points to a zero terminated C style string. This helps static code analysis, and it helps human readers.

basic_zstring is a pointer to a C-string (zero-terminated array) with a templated char type. Used to implement the rest of the *zstring family.
zstring is a zero terminated char string.
czstring is a const zero terminated char string.
wzstring is a zero terminated wchar_t string.
cwzstring is a const zero terminated wchar_t string.
u16zstring is a zero terminated char16_t string.
cu16zstring is a const zero terminated char16_t string.
u32zstring is a zero terminated char32_t string.
cu32zstring is a const zero terminated char32_t string.

See GSL.view and SL.str.3: Use zstring or czstring to refer to a C-style, zero-terminated, sequence of characters.

`<util>`

This header contains utility functions and classes. This header works without exceptions being available. The parts that require exceptions being available are in their own header file narrow.

See GSL.util: Utilities

gsl::narrow_cast
gsl::final_action
gsl::at

`gsl::index`

An alias to std::ptrdiff_t. It serves as the index type for all container indexes/subscripts/sizes.

`gsl::narrow_cast`

gsl::narrow_cast<T>(x) is a named cast that is identical to a static_cast<T>(x). It exists to make clear to static code analysis tools and to human readers that a lossy conversion is acceptable.

Note: compare the throwing version gsl::narrow in header narrow.

See ES.46: Avoid lossy (narrowing, truncating) arithmetic conversions and ES.49: If you must use a cast, use a named cast

`gsl::final_action`

template <class F>
class final_action { ... };

final_action allows you to ensure something gets run at the end of a scope.

See E.19: Use a final_action object to express cleanup if no suitable resource handle is available

Member functions

explicit final_action(const F& ff) noexcept;
explicit final_action(F&& ff) noexcept;

Construct an object with the action to invoke in the destructor.

~final_action() noexcept;

The destructor will call the action that was passed in the constructor.

final_action(final_action&& other) noexcept;
final_action(const final_action&)   = delete;
void operator=(const final_action&) = delete;
void operator=(final_action&&)      = delete;

Move construction is allowed. Copy construction is deleted. Copy and move assignment are also explicitely deleted.

Non-member functions

template <class F>
auto finally(F&& f) noexcept;

Creates a gsl::final_action object, deducing the template argument type from the type of the argument.

`gsl::at`

The function gsl::at offers a safe way to access data with index bounds checking.

Note: gsl::at supports indexes up to PTRDIFF_MAX.

See ES.42: Keep use of pointers simple and straightforward

template <class T, std::size_t N>
constexpr T& at(T (&arr)[N], const index i);

This overload returns a reference to the is element of a C style array arr. It Expects that the provided index is within the bounds of the array.

template <class Cont>
constexpr auto at(Cont& cont, const index i) -> decltype(cont[cont.size()]);

This overload returns a reference to the is element of the container cont. It Expects that the provided index is within the bounds of the array.

template <class T>
constexpr T at(const std::initializer_list<T> cont, const index i);

This overload returns a reference to the is element of the initializer list cont. It Expects that the provided index is within the bounds of the array.

template <class T, std::size_t extent = std::dynamic_extent>
constexpr auto at(std::span<T, extent> sp, const index i) -> decltype(sp[sp.size()]);

This overload returns a reference to the is element of the std::span sp. It Expects that the provided index is within the bounds of the array.

For gsl::at for gsl::span see header span_ext.

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Which version of span should I use?

Use gsl::span if

Use std::span if

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Use `gsl::span` if

Use `std::span` if