At a loss ... Can't figure out why XCode 6.1 won't compile my C++ program

Question

Although XCode is not flagging any errors before compile time, it brings up 4 when I actually compile it. They are

Undefined symbols for architecture i386:
  "HtmlProcessor::HtmlProcessor()", referenced from:
      _main in main.o
  "HtmlProcessor::~HtmlProcessor()", referenced from:
      _main in main.o
  "DocTree::_hp", referenced from:
      DocTree::setTree(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) in HtmlProcessor.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I have searched the web high and low for answers. Most of them mention changing the Architectures settings. Right now I have

Architectures: Universal (32/64-bit Intel) (x86_64, i386)
Base SDK: Latest OFX (OS X 10.9)
Build Active Architecture Only: No
Supported Platforms: OSX 
Valid Architectures: i386

and I've fiddled around with everything to try and get my damn program to compile. I don't even care what the target architecture is ..... I'm making this program for my own amusement and want it to run on my machine, a MacBook Pro running OS X 10.9.4. I just want this damn console program to work. You would think that XCode would have default configurations for your program to run on your machine.

Here are the source files: https://www.dropbox.com/sh/yu7tblwj14eoq1l/AAC8PfDi6la3CjE167Iz1C0da?dl=0

Nobel Prize to the Stack Overflow guru who bails me out of this one.

Answer 1

You declared a static class member, but you did not define it in any module:

 class DocTree {
 //...
 static HtmlProcessor _hp;
 //...
};

This needs to have this in one and only one module:

HtmlProcessor DocTree::_hp;

Answer 2

You are declaring the functions in the header file but do not define (implement) them in the .cpp file. I couldn't find the definition for the constructor and destructor in the .cpp file, although you have the declaration in the header. The linker then complains as it is not able to find the needed object code to create the instance of HtmlProcessor.

So make sure that you either declare the ctor as empty, like

HtmlProcessor(){}

or remove the declaration altogether, or use =default (if you use C++11).

Same for the static declaration of DocTree::_hp;, you need to define it somewhere.