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

#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';
 }
}

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