W3cubDocs

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

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

(since C++17)

Returns a view of the substring [[pos, pos + rcount), where rcount is the smaller of count and size() - pos.

Parameters

pos	-	position of the first character
count	-	requested length

Return value

View of the substring [pos, pos + rcount).

Exceptions

std::out_of_range if pos > size().

Complexity

Constant.

Example

#include <cstddef>
#include <iostream>
#include <stdexcept>
#include <string_view>
 
int main()
{
    typedef std::size_t count_t, pos_t;
 
    constexpr std::string_view data{"ABCDEF"};
 
    std::cout << data.substr() << '\n'; // ABCDEF, i.e. data[0, 5] that is [0, 6)
    std::cout << data.substr(pos_t(1)) << '\n'; // BCDEF, i.e. [1, 6)
    std::cout << data.substr(pos_t(2), count_t(3)) << '\n'; // CDE, i.e. [2, 2 + 3)
    std::cout << data.substr(pos_t(4), count_t(42)) << '\n'; // EF, i.e. [4, 6)
 
    try
    {
        data.substr(pos_t(666), count_t(1)); // throws: pos > size()
    }
    catch (std::out_of_range const& ex)
    {
        std::cout << ex.what() << '\n';
    }
}

Possible output:

ABCDEF
BCDEF
CDE
EF
basic_string_view::substr: __pos (which is 666) > __size (which is 6)