string::npos为啥不是-1

#include <iostream>
#include<cmath>
#include<string>
using namespace std;




int main()
{
	string s("noon is 12 pm; midnight is not.");
	cout<<s.find("is")<<endl;
	cout<<s.rfind("is")<<endl;
	cout<<s.find_first_of("misop")<<endl;
	cout<<s.find_last_of("misop")<<endl;
	cout<<s.find_first_of(" ")<<endl;
	cout<<s.find_first_not_of("noon is 12 pm; midnight is not.")<<endl;
	cout<<string::npos<<endl;;
	return 0;
}