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Showing content from https://en.cppreference.com/w/cpp/language/../keywords/../string/basic_string/substr.html below:

std::basic_string<CharT,Traits,Allocator>::substr - cppreference.com

(1)

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

(until C++23)
(constexpr since C++20)

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

(since C++23)

constexpr basic_string substr( size_type pos = 0, size_type count = npos ) &&;

(2) (since C++23)

Returns a substring [pos, pos + count). If the requested substring extends past the end of the string, i.e. the count is greater than size() - pos (e.g. if count == npos), the returned substring is [pos, size()).

1) Equivalent to return basic_string(*this, pos, count);.

2) Equivalent to return basic_string(std::move(*this), pos, count);.

[edit] Parameters pos - position of the first character to include count - length of the substring [edit] Return value

String containing the substring [pos, pos + count) or [pos, size()).

[edit] Exceptions

std::out_of_range if pos > size().

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

[edit] Complexity

Linear in count.

[edit] Notes

The allocator of the returned string is default-constructed: the new allocator might not be a copy of get_allocator().

[edit] Example

#include <iostream>
#include <string>
 
int main()
{
    std::string a = "0123456789abcdefghij";
 
    // count is npos, returns [pos, size())
    std::string sub1 = a.substr(10);
    std::cout << sub1 << '\n';
 
    // both pos and pos + count are within bounds, returns [pos, pos + count)
    std::string sub2 = a.substr(5, 3);
    std::cout << sub2 << '\n';
 
    // pos is within bounds, pos + count is not, returns [pos, size())
    std::string sub4 = a.substr(a.size() - 3, 50);
    // this is effectively equivalent to
    // std::string sub4 = a.substr(17, 3);
    // since a.size() == 20, pos == a.size() - 3 == 17, and a.size() - pos == 3
 
    std::cout << sub4 << '\n';
 
    try
    {
        // pos is out of bounds, throws
        std::string sub5 = a.substr(a.size() + 3, 50);
        std::cout << sub5 << '\n';
    }
    catch (const std::out_of_range& ex)
    {
        std::cout << ex.what() << '\n';
    }
}

Possible output:

abcdefghij
567
hij
basic_string::substr: __pos (which is 23) > this->size() (which is 20)

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior LWG 847 C++98 there was no exception safety guarantee added strong exception safety guarantee [edit] See also copies characters
(public member function) [edit] returns the number of characters
(public member function) [edit] finds the first occurrence of the given substring
(public member function) [edit] constexpr size_type npos [static] the special value size_type(-1), its exact meaning depends on the context[edit] returns a substring
(public member function of std::basic_string_view<CharT,Traits>) [edit]

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