The use of string::npos in C++

The role of string::npos

string::npos 的意思:The constant is the largest representable value of type size_type. It is assuredly larger than max_size(); hence it serves as either a very large value or as a special code.

It roughly means a constant, which is of type size_type and is a very large value

Its role: to indicate a location that does not exist,

Generally, when using find, it is used for judgments that cannot be found.

1 means a very large value, we can look at the following example

#include <iostream>
#include <string>
using namespace std;
int main()
{
    size_t aa = -1;
    printf("%zu\n",aa);
    if (aa == string::npos)
    {
      printf("相同");
    }
    
    
};

 You can see that aa and string:npos are the same

 2 use when find

(find search, return type size_type, which is an unsigned integer (according to the printed one). If the search is successful, return the position of the first character or substring found according to the search rules; if the search fails, return npos)

That is, find returns the coordinates when it finds a match, and returns string::npos when it does not match

In this way, you can use string::npos to judge whether it matches

#include <iostream>
#include <string>
using namespace std;
#include <algorithm> 
int main()
{
   string str = "hello world";
    char ch = 'e';
    //查找单个字符
    if(str.find(ch)!=string::npos){
         cout<<"YES"<<endl; 
        cout<<str.find(ch)<<endl;
    }
    else
        cout<<"NO"<<endl; 
};