From cppreference.com
staticconstsize_typenpos=-1;This is a special value equal to the maximum value representable by the 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.
Note
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.
Example
Run this code
#include<bitset>#include<iostream>#include<string>intmain(){// string search functions return npos if nothing is foundstd::strings="test";if(s.find('a')==s.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" indicatorstd::strings2(s,2,std::string::npos);std::cout<<s2<<'\n';std::bitset<5>b("aaabb",std::string::npos,'a','b');std::cout<<b<<'\n';}Output:
no 'a' in 'test' st 00011 See also
[static]
special value. The exact meaning depends on the context
(public static member constant of std::basic_string_view<CharT,Traits>)