string::length() returning garbage value

Question

I am trying to use string::length() inside a function I wrote, named match(), but I'm getting garbage values.

Why does the marked line is outputting garbage values?

#include<iostream>
#include<stdio.h>
#include<string>
using namespace std;

void match(string st1,string st2)
{
    for(int i=0;i<st1.length();i++)
    {
        cout<<"Why this value is garbage "<<(i-st1.length()-1)<<"\t";
        // this expression gives wrong values ^^^^^^^^^^^^^^^
    }
}

int main()
{
    string st1,st2;
    cout<<"Enter the required string\n";
    cin>>st1>>st2;
    match(st1,st2);
    return 0;
}

Answer 1

imagine a string "foo":

i-st1.length()-1 means:

when i is 0:

0 - 3 = -3
  - 1 = -4

but st1.length() is a size_t, which is unsigned, so all terms in the expression are promoted to unsigned values.

(unsigned)0 - (unsigned)3 = 0xfffffffffffffffd
                      - 1 = 0xfffffffffffffffc

                          = 18446744073709551612