Unexpected output from to_string(int)

I am trying to count the length of an int that may or may not have leading 0s. For instance 0100. I tried using the to_string() method and it turned 0100 into the string "64" for some reason & I don't understand why. I am relatively new to C++ and I think I may have fundamentally misunderstood how to_string() works.

I am using C++11 for my compiler.