how to find length of string in c++

// CPP program to illustrate 
// Different methods to find length 
// of a string 
#include <iostream> 
#include <string.h> 
using namespace std; 
int main() 
{ 
    // String obj 
    string str = "GeeksforGeeks"; 
  
    // 1. size of string object using size() method 
    cout << str.size() << endl; 
  
    // 2. size of string object using length method 
    cout << str.length() << endl; 
  
    // 3. size using old style 
    // size of string object using strlen function 
    cout << strlen(str.c_str()) << endl; 
  
    // The constructor of string will set it to the 
    // C-style string, 
    // which ends at the '\0' 
  
    // 4. size of string object Using while loop 
    // while 'NOT NULL' 
    int i = 0; 
    while (str[i]) 
        i++; 
    cout << i << endl; 
  
    // 5. size of string object using for loop 
    // for(; NOT NULL  
    for (i = 0; str[i]; i++) 
        ; 
    cout << i << endl; 
  
    return 0; 
} 
  string txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
cout << "The length of the txt 
  string is: " << txt.size(); str.length();