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

	(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 [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);.

Parameters

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

Return value

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

Exceptions

std::out_of_range if pos > size().

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

Complexity

Linear in count.

Notes

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

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& e)
    {
        std::cout << "pos exceeds string size\n";
    }
}

Output:

abcdefghij
567
hij
pos exceeds string size

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

copy	copies characters (public member function)
sizelength	returns the number of characters (public member function)
find	finds the first occurrence of the given substring (public member function)
npos [static]	special value. The exact meaning depends on the context (public static member constant)