title: How to fix C++ warning of implicit conversion?
body: <p>I am just getting started in C++.</p>
  
  <p>I am trying to get the first three characters of the string 'str', and compare it to a known string, say, 'knownString'.</p>
  
  <p>To do that, I wrote this line of code:</p>
  
  <blockquote>
    <p>if (str.substr(start, 3) == knownString)</p>
  </blockquote>
  
  <p>where 'start' is an integer I declared before.
  But I keep getting this warning message: </p>
  
  <blockquote>
    <p><strong>warning: implicit conversion changes signedness: 'int' to</strong>
    <strong>'std::__cxx11::basic_string,**
    **std::allocator >::size_type' (aka 'unsigned int')</strong></p>
  </blockquote>
  
  <p>Does anyone knows what I can add or I missed in this statement to fix this?</p>