23 Strings library [strings]

23.3 String view classes [string.view]

23.3.1 General [string.view.general]

The class template basic_string_view describes an object that can refer to a constant contiguous sequence of char-like ([strings.general]) objects with the first element of the sequence at position zero.

In the rest of [string.view], the type of the char-like objects held in a basic_string_view object is designated by charT.

[Note 1:

The library provides implicit conversions from const charT* and std::basic_string<charT, ...> to std::basic_string_view<charT, ...> so that user code can accept just std::basic_string_view<charT> as a non-templated parameter wherever a sequence of characters is expected.

User-defined types can define their own implicit conversions to std::basic_string_view<charT> in order to interoperate with these functions.

— end note]

23.3.2 Header <string_view> synopsis [string.view.synop]

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// mostly freestanding #include <compare> // see [compare.syn] namespace std { // [string.view.template], class template basic_string_view template<class charT, class traits = char_traits<charT>> class basic_string_view; // partially freestanding template<class charT, class traits> constexpr bool ranges::enable_view<basic_string_view<charT, traits>> = true; template<class charT, class traits> constexpr bool ranges::enable_borrowed_range<basic_string_view<charT, traits>> = true; // [string.view.comparison], non-member comparison functions template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> x, basic_string_view<charT, traits> y) noexcept; template<class charT, class traits> constexpr see below operator<=>(basic_string_view<charT, traits> x, basic_string_view<charT, traits> y) noexcept; // see [string.view.comparison], sufficient additional overloads of comparison functions // [string.view.io], inserters and extractors template<class charT, class traits> basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, basic_string_view<charT, traits> str); // hosted // basic_string_view typedef-names using string_view = basic_string_view<char>; using u8string_view = basic_string_view<char8_t>; using u16string_view = basic_string_view<char16_t>; using u32string_view = basic_string_view<char32_t>; using wstring_view = basic_string_view<wchar_t>; // [string.view.hash], hash support template<class T> struct hash; template<> struct hash<string_view>; template<> struct hash<u8string_view>; template<> struct hash<u16string_view>; template<> struct hash<u32string_view>; template<> struct hash<wstring_view>; inline namespace literals { inline namespace string_view_literals { // [string.view.literals], suffix for basic_string_view literals constexpr string_view operator""sv(const char* str, size_t len) noexcept; constexpr u8string_view operator""sv(const char8_t* str, size_t len) noexcept; constexpr u16string_view operator""sv(const char16_t* str, size_t len) noexcept; constexpr u32string_view operator""sv(const char32_t* str, size_t len) noexcept; constexpr wstring_view operator""sv(const wchar_t* str, size_t len) noexcept; } } }

The function templates defined in [utility.swap] and [iterator.range] are available when <string_view> is included.

23.3.3 Class template basic_string_view [string.view.template]

23.3.3.1 General [string.view.template.general]

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namespace std { template<class charT, class traits = char_traits<charT>> class basic_string_view { public: // types using traits_type = traits; using value_type = charT; using pointer = value_type*; using const_pointer = const value_type*; using reference = value_type&; using const_reference = const value_type&; using const_iterator = implementation-defined; // see [string.view.iterators] using iterator = const_iterator;205 using const_reverse_iterator = reverse_iterator<const_iterator>; using reverse_iterator = const_reverse_iterator; using size_type = size_t; using difference_type = ptrdiff_t; static constexpr size_type npos = size_type(-1); // [string.view.cons], construction and assignment constexpr basic_string_view() noexcept; constexpr basic_string_view(const basic_string_view&) noexcept = default; constexpr basic_string_view& operator=(const basic_string_view&) noexcept = default; constexpr basic_string_view(const charT* str); basic_string_view(nullptr_t) = delete; constexpr basic_string_view(const charT* str, size_type len); template<class It, class End> constexpr basic_string_view(It begin, End end); template<class R> constexpr explicit basic_string_view(R&& r); // [string.view.iterators], iterator support constexpr const_iterator begin() const noexcept; constexpr const_iterator end() const noexcept; constexpr const_iterator cbegin() const noexcept; constexpr const_iterator cend() const noexcept; constexpr const_reverse_iterator rbegin() const noexcept; constexpr const_reverse_iterator rend() const noexcept; constexpr const_reverse_iterator crbegin() const noexcept; constexpr const_reverse_iterator crend() const noexcept; // [string.view.capacity], capacity constexpr size_type size() const noexcept; constexpr size_type length() const noexcept; constexpr size_type max_size() const noexcept; [[nodiscard]] constexpr bool empty() const noexcept; // [string.view.access], element access constexpr const_reference operator[](size_type pos) const; constexpr const_reference at(size_type pos) const; // freestanding-deleted constexpr const_reference front() const; constexpr const_reference back() const; constexpr const_pointer data() const noexcept; // [string.view.modifiers], modifiers constexpr void remove_prefix(size_type n); constexpr void remove_suffix(size_type n); constexpr void swap(basic_string_view& s) noexcept; // [string.view.ops], string operations constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const; // freestanding-deleted constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const; // freestanding-deleted constexpr int compare(basic_string_view s) const noexcept; constexpr int compare(size_type pos1, size_type n1, basic_string_view s) const; // freestanding-deleted constexpr int compare(size_type pos1, size_type n1, basic_string_view s, size_type pos2, size_type n2) const; // freestanding-deleted constexpr int compare(const charT* s) const; constexpr int compare(size_type pos1, size_type n1, const charT* s) const; // freestanding-deleted constexpr int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const; // freestanding-deleted constexpr bool starts_with(basic_string_view x) const noexcept; constexpr bool starts_with(charT x) const noexcept; constexpr bool starts_with(const charT* x) const; constexpr bool ends_with(basic_string_view x) const noexcept; constexpr bool ends_with(charT x) const noexcept; constexpr bool ends_with(const charT* x) const; constexpr bool contains(basic_string_view x) const noexcept; constexpr bool contains(charT x) const noexcept; constexpr bool contains(const charT* x) const; // [string.view.find], searching constexpr size_type find(basic_string_view s, size_type pos = 0) const noexcept; constexpr size_type find(charT c, size_type pos = 0) const noexcept; constexpr size_type find(const charT* s, size_type pos, size_type n) const; constexpr size_type find(const charT* s, size_type pos = 0) const; constexpr size_type rfind(basic_string_view s, size_type pos = npos) const noexcept; constexpr size_type rfind(charT c, size_type pos = npos) const noexcept; constexpr size_type rfind(const charT* s, size_type pos, size_type n) const; constexpr size_type rfind(const charT* s, size_type pos = npos) const; constexpr size_type find_first_of(basic_string_view s, size_type pos = 0) const noexcept; constexpr size_type find_first_of(charT c, size_type pos = 0) const noexcept; constexpr size_type find_first_of(const charT* s, size_type pos, size_type n) const; constexpr size_type find_first_of(const charT* s, size_type pos = 0) const; constexpr size_type find_last_of(basic_string_view s, size_type pos = npos) const noexcept; constexpr size_type find_last_of(charT c, size_type pos = npos) const noexcept; constexpr size_type find_last_of(const charT* s, size_type pos, size_type n) const; constexpr size_type find_last_of(const charT* s, size_type pos = npos) const; constexpr size_type find_first_not_of(basic_string_view s, size_type pos = 0) const noexcept; constexpr size_type find_first_not_of(charT c, size_type pos = 0) const noexcept; constexpr size_type find_first_not_of(const charT* s, size_type pos, size_type n) const; constexpr size_type find_first_not_of(const charT* s, size_type pos = 0) const; constexpr size_type find_last_not_of(basic_string_view s, size_type pos = npos) const noexcept; constexpr size_type find_last_not_of(charT c, size_type pos = npos) const noexcept; constexpr size_type find_last_not_of(const charT* s, size_type pos, size_type n) const; constexpr size_type find_last_not_of(const charT* s, size_type pos = npos) const; private: const_pointer data_; // exposition only size_type size_; // exposition only }; // [string.view.deduct], deduction guides template<class It, class End> basic_string_view(It, End) -> basic_string_view<iter_value_t<It>>; template<class R> basic_string_view(R&&) -> basic_string_view<ranges::range_value_t<R>>; }

In every specialization basic_string_view<charT, traits>, the type traits shall meet the character traits requirements ([char.traits]).

[Note 1:

The program is ill-formed if traits::char_type is not the same type as charT.

— end note]

For a basic_string_view str, any operation that invalidates a pointer in the range [str.data(), str.data() + str.size()) invalidates pointers, iterators, and references to elements of str.

The complexity of basic_string_view member functions is

O (1)

unless otherwise specified.

basic_string_view<charT, traits> is a trivially copyable type ([basic.types.general]).

205)205)

Because basic_string_view refers to a constant sequence, iterator and const_iterator are the same type.

23.3.3.2 Construction and assignment [string.view.cons]

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constexpr basic_string_view() noexcept;

Postconditions: size_ == 0 and data_ == nullptr.

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constexpr basic_string_view(const charT* str);

Preconditions: [str, str + traits::length(str)) is a valid range.

Effects: Constructs a basic_string_view, initializing data_ with str and initializing size_ with traits::length(str).

Complexity:

O (traits::length(str))

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constexpr basic_string_view(const charT* str, size_type len);

Preconditions: [str, str + len) is a valid range.

Effects: Constructs a basic_string_view, initializing data_ with str and initializing size_ with len.

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template<class It, class End>
  constexpr basic_string_view(It begin, End end);

Constraints:

(7.1)
It satisfies contiguous_iterator .
(7.2)
End satisfies sized_sentinel_for<It>.
(7.3)
is_same_v<iter_value_t<It>, charT> is true.
(7.4)
is_convertible_v<End, size_type> is false.

Preconditions:

(8.1)
[begin, end) is a valid range.
(8.2)
It models contiguous_iterator .
(8.3)
End models sized_sentinel_for<It>.

Effects: Initializes data_ with to_address(begin) and initializes size_ with end - begin.

Throws: When and what end - begin throws.

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template<class R>
  constexpr explicit basic_string_view(R&& r);

Let d be an lvalue of type remove_cvref_t<R>.

Constraints:

(12.1)
remove_cvref_t<R> is not the same type as basic_string_view,
(12.2)
R models ranges::contiguous_range and ranges::sized_range,
(12.3)
is_same_v<ranges::range_value_t<R>, charT> is true,
(12.4)
is_convertible_v<R, const charT*> is false, and
(12.5)
d.operator ::std::basic_string_view<charT, traits>() is not a valid expression.

Effects: Initializes data_ with ranges::data(r) and size_ with ranges::size(r).

Throws: Any exception thrown by ranges::data(r) and ranges::size(r).

23.3.3.3 Deduction guides [string.view.deduct]

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template<class It, class End>
  basic_string_view(It, End) -> basic_string_view<iter_value_t<It>>;

Constraints:

(1.1)
It satisfies contiguous_iterator .
(1.2)
End satisfies sized_sentinel_for<It>.

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template<class R>
  basic_string_view(R&&) -> basic_string_view<ranges::range_value_t<R>>;

Constraints: R satisfies ranges::contiguous_range.

23.3.3.4 Iterator support [string.view.iterators]

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using const_iterator = implementation-defined;

A type that meets the requirements of a constant Cpp17RandomAccessIterator ([random.access.iterators]), models contiguous_iterator ([iterator.concept.contiguous]), and meets the constexpr iterator requirements ([iterator.requirements.general]), whose value_type is the template parameter charT.

All requirements on container iterators ([container.requirements]) apply to basic_string_view::const_iterator as well.

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constexpr const_iterator begin() const noexcept;
constexpr const_iterator cbegin() const noexcept;

Returns: An iterator such that

(3.1)
if !empty(), addressof(*begin()) == data_,
(3.2)
otherwise, an unspecified value such that [begin(), end()) is a valid range.

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constexpr const_iterator end() const noexcept;
constexpr const_iterator cend() const noexcept;

Returns: begin() + size().

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constexpr const_reverse_iterator rbegin() const noexcept;
constexpr const_reverse_iterator crbegin() const noexcept;

Returns: const_reverse_iterator(end()).

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constexpr const_reverse_iterator rend() const noexcept;
constexpr const_reverse_iterator crend() const noexcept;

Returns: const_reverse_iterator(begin()).

23.3.3.5 Capacity [string.view.capacity]

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constexpr size_type size() const noexcept;
constexpr size_type length() const noexcept;

Returns: size_.

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constexpr size_type max_size() const noexcept;

Returns: The largest possible number of char-like objects that can be referred to by a basic_string_view.

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[[nodiscard]] constexpr bool empty() const noexcept;

Returns: size_ == 0.

23.3.3.6 Element access [string.view.access]

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constexpr const_reference operator[](size_type pos) const;

Preconditions: pos < size().

Returns: data_[pos].

Throws: Nothing.

[Note 1:

Unlike basic_string::operator[], basic_string_view::operator[](size()) has undefined behavior instead of returning charT().

— end note]

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constexpr const_reference at(size_type pos) const;

Returns: data_[pos].

Throws: out_of_range if pos >= size().

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constexpr const_reference front() const;

Preconditions: !empty().

Returns: data_[0].

Throws: Nothing.

🔗

constexpr const_reference back() const;

Preconditions: !empty().

Returns: data_[size() - 1].

Throws: Nothing.

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constexpr const_pointer data() const noexcept;

Returns: data_.

[Note 2:

Unlike basic_string::data() and string-literals, data() can return a pointer to a buffer that is not null-terminated.

Therefore it is typically a mistake to pass data() to a function that takes just a const charT* and expects a null-terminated string.

— end note]

23.3.3.7 Modifiers [string.view.modifiers]

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constexpr void remove_prefix(size_type n);

Preconditions: n <= size().

Effects: Equivalent to: data_ += n; size_ -= n;

🔗

constexpr void remove_suffix(size_type n);

Preconditions: n <= size().

Effects: Equivalent to: size_ -= n;

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constexpr void swap(basic_string_view& s) noexcept;

Effects: Exchanges the values of *this and s.

23.3.3.8 String operations [string.view.ops]

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constexpr size_type copy(charT* s, size_type n, size_type pos = 0) const;

Let rlen be the smaller of n and size() - pos.

Preconditions: [s, s + rlen) is a valid range.

Effects: Equivalent to traits::copy(s, data() + pos, rlen).

Returns: rlen.

Throws: out_of_range if pos > size().

Complexity:

O (rlen)

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constexpr basic_string_view substr(size_type pos = 0, size_type n = npos) const;

Let rlen be the smaller of n and size() - pos.

Effects: Determines rlen, the effective length of the string to reference.

Returns: basic_string_view(data() + pos, rlen).

Throws: out_of_range if pos > size().

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constexpr int compare(basic_string_view str) const noexcept;

Let rlen be the smaller of size() and str.size().

Effects: Determines rlen, the effective length of the strings to compare.

The function then compares the two strings by calling traits::compare(data(), str.data(), rlen).

Returns: The nonzero result if the result of the comparison is nonzero.

Otherwise, returns a value as indicated in Table 80 .

Table 80: compare() results [tab:string.view.compare]

🔗	Condition	Return Value
🔗	size() < str.size()	< 0
🔗	size() == str.size()	0
🔗	size() > str.size()	> 0

Complexity:

O (rlen)

🔗

constexpr int compare(size_type pos1, size_type n1, basic_string_view str) const;

Effects: Equivalent to: return substr(pos1, n1).compare(str);

🔗

constexpr int compare(size_type pos1, size_type n1, basic_string_view str,
                      size_type pos2, size_type n2) const;

Effects: Equivalent to: return substr(pos1, n1).compare(str.substr(pos2, n2));

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constexpr int compare(const charT* s) const;

Effects: Equivalent to: return compare(basic_string_view(s));

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constexpr int compare(size_type pos1, size_type n1, const charT* s) const;

Effects: Equivalent to: return substr(pos1, n1).compare(basic_string_view(s));

🔗

constexpr int compare(size_type pos1, size_type n1, const charT* s, size_type n2) const;

Effects: Equivalent to: return substr(pos1, n1).compare(basic_string_view(s, n2));

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constexpr bool starts_with(basic_string_view x) const noexcept;

Let rlen be the smaller of size() and x.size().

Effects: Equivalent to: return basic_string_view(data(), rlen) == x;

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constexpr bool starts_with(charT x) const noexcept;

Effects: Equivalent to: return !empty() && traits::eq(front(), x);

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constexpr bool starts_with(const charT* x) const;

Effects: Equivalent to: return starts_with(basic_string_view(x));

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constexpr bool ends_with(basic_string_view x) const noexcept;

Let rlen be the smaller of size() and x.size().

Effects: Equivalent to: return basic_string_view(data() + (size() - rlen), rlen) == x;

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constexpr bool ends_with(charT x) const noexcept;

Effects: Equivalent to: return !empty() && traits::eq(back(), x);

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constexpr bool ends_with(const charT* x) const;

Effects: Equivalent to: return ends_with(basic_string_view(x));

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constexpr bool contains(basic_string_view x) const noexcept;
constexpr bool contains(charT x) const noexcept;
constexpr bool contains(const charT* x) const;

Effects: Equivalent to: return find(x) != npos;

23.3.3.9 Searching [string.view.find]

Member functions in this subclause have complexity

O (size() * str.size())

at worst, although implementations should do better.

Let F be one of find, rfind, find_first_of, find_last_of, find_first_not_of, and find_last_not_of.

(2.1)
Each member function of the form constexpr return-type F(const charT* s, size_type pos) const; has effects equivalent to: return F(basic_string_view(s), pos);
(2.2)
Each member function of the form constexpr return-type F(const charT* s, size_type pos, size_type n) const; has effects equivalent to: return F(basic_string_view(s, n), pos);
(2.3)
Each member function of the form constexpr return-type F(charT c, size_type pos) const noexcept; has effects equivalent to: return F(basic_string_view(addressof(c), 1), pos);

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constexpr size_type find(basic_string_view str, size_type pos = 0) const noexcept;

Let xpos be the lowest position, if possible, such that the following conditions hold:

(3.1)
pos <= xpos
(3.2)
xpos + str.size() <= size()
(3.3)
traits::eq(data_[xpos + I], str[I]) for all elements I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

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constexpr size_type rfind(basic_string_view str, size_type pos = npos) const noexcept;

Let xpos be the highest position, if possible, such that the following conditions hold:

(6.1)
xpos <= pos
(6.2)
xpos + str.size() <= size()
(6.3)
traits::eq(data_[xpos + I], str[I]) for all elements I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

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constexpr size_type find_first_of(basic_string_view str, size_type pos = 0) const noexcept;

Let xpos be the lowest position, if possible, such that the following conditions hold:

(9.1)
pos <= xpos
(9.2)
xpos < size()
(9.3)
traits::eq(data_[xpos], str[I]) for some element I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

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constexpr size_type find_last_of(basic_string_view str, size_type pos = npos) const noexcept;

Let xpos be the highest position, if possible, such that the following conditions hold:

(12.1)
xpos <= pos
(12.2)
xpos < size()
(12.3)
traits::eq(data_[xpos], str[I]) for some element I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

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constexpr size_type find_first_not_of(basic_string_view str, size_type pos = 0) const noexcept;

Let xpos be the lowest position, if possible, such that the following conditions hold:

(15.1)
pos <= xpos
(15.2)
xpos < size()
(15.3)
traits::eq(data_[xpos], str[I]) for no element I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

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constexpr size_type find_last_not_of(basic_string_view str, size_type pos = npos) const noexcept;

Let xpos be the highest position, if possible, such that the following conditions hold:

(18.1)
xpos <= pos
(18.2)
xpos < size()
(18.3)
traits::eq(data_[xpos], str[I]) for no element I of the string referenced by str.

Effects: Determines xpos.

Returns: xpos if the function can determine such a value for xpos.

Otherwise, returns npos.

23.3.4 Non-member comparison functions [string.view.comparison]

Let S be basic_string_view<charT, traits>, and sv be an instance of S.

Implementations shall provide sufficient additional overloads marked constexpr and noexcept so that an object t with an implicit conversion to S can be compared according to Table 81 .

Table 81: Additional basic_string_view comparison overloads [tab:string.view.comparison.overloads]

🔗	Expression	Equivalent to
🔗	t == sv	S(t) == sv
🔗	sv == t	sv == S(t)
🔗	t != sv	S(t) != sv
🔗	sv != t	sv != S(t)
🔗	t < sv	S(t) < sv
🔗	sv < t	sv < S(t)
🔗	t > sv	S(t) > sv
🔗	sv > t	sv > S(t)
🔗	t <= sv	S(t) <= sv
🔗	sv <= t	sv <= S(t)
🔗	t >= sv	S(t) >= sv
🔗	sv >= t	sv >= S(t)
🔗	t <=> sv	S(t) <=> sv
🔗	sv <=> t	sv <=> S(t)

[Example 1:

A sample conforming implementation for operator== would be: template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs) noexcept { return lhs.compare(rhs) == 0; } template<class charT, class traits> constexpr bool operator==(basic_string_view<charT, traits> lhs, type_identity_t<basic_string_view<charT, traits>> rhs) noexcept { return lhs.compare(rhs) == 0; }

— end example]

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template<class charT, class traits>
  constexpr bool operator==(basic_string_view<charT, traits> lhs,
                            basic_string_view<charT, traits> rhs) noexcept;

Returns: lhs.compare(rhs) == 0.

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template<class charT, class traits>
  constexpr see below operator<=>(basic_string_view<charT, traits> lhs,
                                  basic_string_view<charT, traits> rhs) noexcept;

Let R denote the type traits::comparison_category if that qualified-id is valid and denotes a type ([temp.deduct]), otherwise R is weak_ordering.

Mandates: R denotes a comparison category type ([cmp.categories]).

Returns: static_cast<R>(lhs.compare(rhs) <=> 0).

23.3.5 Inserters and extractors [string.view.io]

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template<class charT, class traits>
  basic_ostream<charT, traits>&
    operator<<(basic_ostream<charT, traits>& os, basic_string_view<charT, traits> str);

Effects: Behaves as a formatted output function ([ostream.formatted.reqmts]) of os.

Forms a character sequence seq, initially consisting of the elements defined by the range [str.begin(), str.end()).

Determines padding for seq as described in [ostream.formatted.reqmts].

Then inserts seq as if by calling os.rdbuf()->sputn(seq, n), where n is the larger of os.width() and str.size(); then calls os.width(0).

Returns: os

23.3.6 Hash support [string.view.hash]

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template<> struct hash<string_view>;
template<> struct hash<u8string_view>;
template<> struct hash<u16string_view>;
template<> struct hash<u32string_view>;
template<> struct hash<wstring_view>;

The specialization is enabled ([unord.hash]).

[Note 1:

The hash value of a string view object is equal to the hash value of the corresponding string object ([basic.string.hash]).

— end note]

23.3.7 Suffix for basic_string_view literals [string.view.literals]

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constexpr string_view operator""sv(const char* str, size_t len) noexcept;

Returns: string_view{str, len}.

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constexpr u8string_view operator""sv(const char8_t* str, size_t len) noexcept;

Returns: u8string_view{str, len}.

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constexpr u16string_view operator""sv(const char16_t* str, size_t len) noexcept;

Returns: u16string_view{str, len}.

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constexpr u32string_view operator""sv(const char32_t* str, size_t len) noexcept;

Returns: u32string_view{str, len}.

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constexpr wstring_view operator""sv(const wchar_t* str, size_t len) noexcept;

Returns: wstring_view{str, len}.