# std::basic_string_view<CharT,Traits>::basic_string_view

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< cpp‎ | string‎ | basic string view

constexpr basic_string_view() noexcept; | (1) | (since C++17)
constexpr basic_string_view( const basic_string_view& other ) noexcept = default; | (2) | (since C++17)
constexpr basic_string_view( const CharT* s, size_type count ); | (3) | (since C++17)
constexpr basic_string_view( const CharT* s ); | (4) | (since C++17)
template< class It, class End > constexpr basic_string_view( It first, End last ); | (5) | (since C++20)
template< class R > constexpr explicit basic_string_view( R&& r ); | (6) | (since C++23)
constexpr basic_string_view( std::nullptr_t ) = delete; | (7) | (since C++23)

1) Default constructor. Constructs an empty `std::basic_string_view`. After construction, data() is equal to nullptr, and size() is equal to ​0​.
2) Copy constructor. Constructs a view of the same content as other. After construction, data() is equal to other.data(), and size() is equal to other.size().
3) Constructs a view of the first count characters of the character array starting with the element pointed by s. s can contain null characters. 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). After construction, data() is equal to s, and size() is equal to count.
4) Constructs a view of the null-terminated character string pointed to by s, not including the terminating null character. The length of the view is determined as if by Traits::length(s). The behavior is undefined if `[`s`,` s + Traits::length(s)`)` is not a valid range. After construction, data() is equal to s, and size() is equal to Traits::length(s).
5) Constructs a `std::basic_string_view` over the range `[`first`,` last`)`. The behavior is undefined if `[`first`,` last`)` is not a valid range, if `It` does not actually model `contiguous_iterator`, or if `End` does not actually model `sized_sentinel_for` for `It`. After construction, data() is equal to std::to_address(first), and size() is equal to last - first.

This overload participates 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.

6) Constructs a `std::basic_string_view` over the range r. After construction, data() is equal to ranges::data(r), and size() is equal to ranges::size(r).

This overload participates 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*, and
  - 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.

7) `std::basic_string_view` cannot be constructed from nullptr.

### Parameters

other | - | another view to initialize the view with
s | - | pointer to a character array or a C string to initialize the view with
count | - | number of characters to include in the view
first | - | iterator to the first character of the sequence
last | - | iterator past the last character of the sequence or another sentinel
r | - | a contiguous range that contains the sequence

### Complexity

1-3,5-6) Constant.
4) Linear in length of s.

### Example

Run this code

    #include <array>
    #include <iomanip>
    #include <iostream>
    #include <string>
    #include <string_view>

    int main()
    {
        std::string cppstr = "Foo";
        std::string_view cppstr_v(cppstr); // overload (2), after
                                           // std::string::operator string_view
        std::cout << "1) cppstr_v: " << std::quoted(cppstr_v) << '\n';

        char array[3] = {'B', 'a', 'r'};
        std::string_view array_v(array, std::size(array)); // overload (3)
        std::cout << "2) array_v: " << std::quoted(array_v) << '\n';

        const char* one_0_two = "One\0Two";

        std::string_view one_two_v{one_0_two, 7}; // overload (3)
        std::cout << "3) one_two_v: \"";
        for (char c : one_two_v)
            std::cout << (c != '\0' ? c : '?');
        std::cout << "\", one_two_v.size(): " << one_two_v.size() << '\n';

        std::string_view one_v{one_0_two}; // overload (4)
        std::cout << "4) one_v: " << std::quoted(one_v) << ", one_v.size(): "
                  << one_v.size() << '\n';

        constexpr std::wstring_view wcstr_v = L"xyzzy"; // overload (4)
        std::cout << "5) wcstr_v.size(): " << wcstr_v.size() << '\n';

        std::array ar = {'P', 'u', 'b'};
        std::string_view ar_v(ar.begin(), ar.end()); // overload (5), C++20
        std::cout << "6) ar_v: " << std::quoted(ar_v) << '\n';

    //  std::string_view ar_v2{ar}; // overload (6), OK in C++23
    //  std::cout << "ar_v2: " << std::quoted(ar_v2) << '\n'; // ar_v2: "Pub"

        [[maybe_unused]] auto zero = [] { /* ... */ return nullptr; };
    //  std::string_view s{zero()}; // overload (7), won't compile since C++23
    }

Output:

    1) cppstr_v: "Foo"
    2) array_v: "Bar"
    3) one_two_v: "One?Two", one_two_v.size(): 7
    4) one_v: "One", one_v.size(): 3
    5) wcstr_v.size(): 5
    6) ar_v: "Pub"

### See also

operator= (C++17) | assigns a view (public member function)
(constructor) | constructs a `basic_string` (public member function of `std::basic_string<CharT,Traits,Allocator>`)

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