Added the pad_int_with_zeros() function.

dlib/string/string.h

@@ -4,9 +4,11 @@
 #define DLIB_STRINg_ 
 
 #include "string_abstract.h"
+#include <sstream>
 #include "../algs.h"
 #include <string>
 #include <iostream>
+#include <iomanip>
 #include "../error.h"
 #include "../assert.h"
 #include "../uintn.h"
@@ -257,6 +259,18 @@ namespace dlib
     }
 #endif
 
+// ----------------------------------------------------------------------------------------
+
+    inline std::string pad_int_with_zeros (
+        int i,
+        unsigned long width = 6
+    )
+    {
+        std::ostringstream sout;
+        sout << std::setw(width) << std::setfill('0') << i;
+        return sout.str();
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class string_cast_error : public error

dlib/string/string_abstract.h

@@ -113,6 +113,19 @@ namespace dlib
                 item into a std::string.  
     !*/
 
+// ----------------------------------------------------------------------------------------
+
+    std::string pad_int_with_zeros (
+        int i,
+        unsigned long width = 6
+    );
+    /*!
+        ensures
+            - converts i into a string of at least width characters in length.  If
+              necessary, the string will be padded with leading zeros to get
+              to width characters.
+    !*/
+
 // ----------------------------------------------------------------------------------------
 
     template <