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std::basic_string<CharT,Traits,Allocator>::substr

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(1)
basic_string substr( size_type pos = 0, size_type count = npos ) const;
(until C++20)
constexpr basic_string
    substr( size_type pos = 0, size_type count = npos ) const;
(since C++20)
(until C++23)
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 [pospos + 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 [possize()).

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

Contents

[edit] Parameters

pos - position of the first character to include
count - length of the substring

[edit] Return value

String containing the substring [pospos + count) or [possize()).

[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]
[static]
special value. The exact meaning depends on the context
(public static member constant) [edit]
returns a substring
(public member function of std::basic_string_view<CharT,Traits>) [edit]