Simple program gets LNK2019: unresolved external symbol Visual Studio 2013 - c++

Question

I've been struggling with this error on a more complex solution for days, so I simplified a test project to see if I can't figure it out and it still gets the same error:
1>------ Build started: Project: test, Configuration: Debug Win32 ------
1> Test.cpp
1> MainThing.cpp
1> main.cpp
1> Generating Code...
1>MainThing.obj : error LNK2019: unresolved external symbol "public: static int __cdecl Test::add(int)" (?add#Test##SAHH#Z) referenced in function "public: void __thiscall MainThing::run(void)" (?run#MainThing##QAEXXZ)
1>P:\Leif's Documents\Visual Studio 2013\Projects\test\Debug\test.exe : fatal error LNK1120: 1 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
MainThing.h
#pragma once
class MainThing
{
public:
MainThing();
~MainThing();
void run();
private:
int result;
};
MainThing.cpp
#include "MainThing.h"
#include "Test.h"
#include <cstdlib>
#include <iostream>
MainThing::MainThing()
{
}
MainThing::~MainThing()
{
}
void MainThing::run() {
int result = Test::add(5);
std::cout << result;
}
Test.h
#pragma once
class Test
{
public:
static int add(int a);
};
Test.cpp
#include "Test.h"
int add(int a){
int b = a;
int c = 5;
int d = b + c;
return d;
}
main.cpp
#include <iostream>
#include "MainThing.h"
int main(int argc, char** argv){
MainThing mainGame;
mainGame.run();
return 0;
}
This has been super frustrating and the fact that it even effects a fairly simple recreation test without any external dependencies makes it that much more so.

Answer 1

I think you just have a typo. Test.cpp should be
#include "Test.h"
int Test::add(int a){
int b = a;
int c = 5;
int d = b + c;
return d;
}
The way you have, you've written a free function that other things in Test.cpp could use, but no one else knows about. The compiler is looking for Test's method called add and isn't finding one.