title: convert string to integer in c++
body: <p>Hello
  I know it was asked many times but I hadn't found answer to my specific question.</p>
  
  <p>I want to convert only string that contains only decimal numbers:</p>
  
  <p>For example 256 is OK but 256a is not.</p>
  
  <p>Could it be done without checking the string?</p>
  
  <p>Thanks</p>