std::basic_string<CharT,Traits,Allocator>::npos (3) - Linux Manuals

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


std::basic_string<CharT,Traits,Allocator>::npos - std::basic_string<CharT,Traits,Allocator>::npos


static const size_type npos = -1;

This is a special value equal to the maximum value representable by the type size_type. The exact meaning depends on context, but it is generally used either as end of string indicator by the functions that expect a string index or as the error indicator by the functions that return a string index.


Although the definition uses -1, size_type is an unsigned integer type, and the value of npos is the largest positive value it can hold, due to signed-to-unsigned_implicit_conversion. This is a portable way to specify the largest value of any unsigned type.


// Run this code

  #include <iostream>
  #include <bitset>
  #include <string>

  int main()
      // string search functions return npos if nothing is found
      std::string s = "test";
      if(s.find('a') == std::string::npos)
          std::cout << "no 'a' in 'test'\n";

      // functions that take string subsets as arguments
      // use npos as the "all the way to the end" indicator
      std::string s2(s, 2, std::string::npos);
      std::cout << s2 << '\n';

      std::bitset<5> b("aaabb", std::string::npos, 'a', 'b');
      std::cout << b << '\n';


  no 'a' in 'test'