std::string_view constructors

std::basic_string_view class can be constructed in many different ways. Use the button in the top-right corner to navigate with arrows for convenience.

zanotuj

std::basic_string_view is a class template, with following type parameters, that are used within constructors:

pub	CharT	Character type
pub	Traits	CharTraits class specifying the operations on the character type. important Like for std::basic_string, Traits::char_type must name the same type as CharT or the program is ill-formed .

Default constructor

C++20

Simplified

basic_string_view();

• this constructor is constexpr and noexcept

Detailed

constexpr basic_string_view() noexcept;

Default constructor. Constructs an empty string view with:

• data() == nullptr
• size() == 0

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>
#include <iomanip>

int main() {
  // Default constructed string view
  std::string_view sv;

  std::cout << sv.size() << ' ' << std::quoted(sv);
}

Result (console)

0 ""

Defaulted copy constructor

C++20

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

Copy constructor. Constructs a view of the same content as other, which has:

• data() == other.data()
• size() == other.size()

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>
#include <iomanip>

int main() {
  std::string_view sv = "Hello";
  std::string_view sv2 = sv;

  std::cout << sv.size() << ' ' << std::quoted(sv) << '\n';
  std::cout << sv2.size() << ' ' << std::quoted(sv2);
}

Result (console)

5 "Hello"
5 "Hello"

Char pointer constructor (bound)

C++20

constexpr basic_string_view( const CharT* s, size_type count );

Constructs a view of the first count characters of the character array starting with the element pointed by s. The created view has:

• data() == s
• size() == count

Undefined Behavior

The behavior is undefined

if:

• [ s, s + count ) is not a valid range (even though the constructor may not access any of the elements of this range)

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s can contain null characters.

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>
#include <iomanip>

int main() {
  std::string_view sv("Hello World!", 5);

  std::cout << sv.size() << ' ' << std::quoted(sv) << '\n';
}

Result (console)

5 "Hello"

Char pointer constructor (unbound)

C++20

constexpr basic_string_view( const CharT* s );

• data() == s
• size() == Traits::length(s)

Undefined Behavior

The behavior is undefined

if:

• [ s, s + Traits::length(s) ) is not a valid range (even though the constructor may not access any of the elements of this range)

zanotuj

The length of the view is determined as if by Traits::length(s).

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>
#include <iomanip>

int main() {
  const char* string = "Hello\0 World!";
  std::string_view sv(string);

  std::cout << sv.size() << ' ' << std::quoted(sv) << '\n';
}

Result (console)

5 "Hello"

Copy range constructor (iterators)

C++20

template< class It, class End >
constexpr basic_string_view( It first, End last );

Constructs a string view over the range [ first, last ). The resulting view has:

• data() == std::to_address(first)
• size() == last - first

Undefined Behavior

The behavior is undefined

if:

• **[ first, last ) is not a valid range
• It does not model contiguous_iterator
  or
• End does not model sized_sentinel_for

Overload resolution

These overloads participate in overload resolution only if:

• It satisfies contiguous_iterator
• End satisfies sized_sentinel_for for It
• std::iter_value_t<It> and CharT are the same type
  and
• End is not convertible to std::size_t

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>

int main() {
  std::string s = "Hello World!";
  std::string_view sv(s.begin(), s.begin()+5);
  std::string_view sv2(s.begin() + 6, s.end());

  std::cout << sv << ' ' << sv2;
}

Result (console)

Hello World!

Copy range constructor (range)

C++23

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

Constructs a string view over the range r. The resulting view has:

• data() == ranges::data(r)
• size() == ranges::size(r)

Overload resolution

These overloads participate in overload resolution only if:

• std::remove_cvref_t<R> is not the same type as std::basic_string_view
• R models contiguous_range and sized_range
• ranges::range_value_t<R> and CharT are the same type
• R is not convertible to const CharT*
• let d be an lvalue of type std::remove_cvref_t<R>, d.operator ::std::basic_string_view<CharT, Traits>() is not a valid expression
  and
• if qualified-id std::remove_reference_t<R>::traits_type is valid and denotes a type, it is same as Traits

Complexity

Constant - O(1).

Example

#include <string_view>
#include <iostream>

int main() {
  std::string s = "Hello";
  std::string_view sv(s);

  std::cout << sv;
}

Result (console)

Hello

Deleted std::nullptr_t constructor

C++23

constexpr basic_string_view( std::nullptr_t ) = delete;

A deleted constructor that prohibits constructing a string view from a nullptr.

Example

#include <string_view>

int main() {
  // std::string_view sv(nullptr); // Won't compile!
}