NAME

std::basic_string::substr - std::basic_string::substr

Synopsis

basic_string substr( size_type pos = 0, size_type count = npos ) (until C++20)
const;
constexpr basic_string substr( size_type pos = 0, size_type count = (since C++20)
npos ) const;

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()).

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()

Complexity

Linear in count

Notes

The returned string is constructed as if by basic_string(data()+pos, count), which
implies that the returned string's allocator will be default-constructed — the new
allocator might not be a copy of this->get_allocator().

Example

// Run this code

#include <string>
#include <iostream>

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