title: How do you build an application in c++ modularly
body: <p><em>This is a question more about how to build a c++ application than it is about c++, per se</em> </p>
  
  <p>I'm building an application that was envisioned as a Graphical application, but the specifics of the implementation require a great deal of abstract functionality to facilitate the interface, such as reading lists of objects from files, web resources, etc. I have figured out how to implement this functionality without much difficulty, but I have no good way to test it.  Specifically, I've begun the implementation of the graphical environment, but it is not yet ready to work with the lower-level functionality.</p>
  
  <p>I have also built a good deal of what I want the lower layer to do, but it is as-yet untested.  all of this code resides in a single folder and is stored in a version control system with regular commits.  </p>
  
  <p>I'm relatively new to writing anything functional in C++ having only worked on class projects up to this point, but I have written a good number of programs, of various types, in PHP.  </p>
  
  <p>If this were a PHP project, it seems it would be simple to test any functionality: </p>
  
  <ol>
  <li>I would simply begin by implementing it interactively</li>
  <li>codify it into a small file</li>
  <li>write some code that used the functionality</li>
  <li>build it into a function</li>
  <li>import that function into my larger body of code.  </li>
  </ol>
  
  <p>This seems like a really awkward way to do the same with C++.  Have I got it all backwards, how do you solve small isolated problems, in your compiled programs and import them into your projects; is there a workflow that you find helpful.</p>