static method and fatal error LNK1120: 1 unresolved externals

[dbag]

I have created the following test to try and implement a static method to expose a static variable that I am usually able to do in C# in 2 seconds. The concepts seem almost identical and I can create the static variable and methods with no computation error but when I attempt to access my static methods I receive:

TestClass.obj : error LNK2001: unresolved external symbol "private: static class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > Test::TestClass::connectionString" (?connectionString@TestClass@Test@@0V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@A)

MyCppTestApp.exe : fatal error LNK1120: 1 unresolved externals

Here is my code:

TestClass.h:

#include <string>

namespace Test
{
	class TestClass
	{
	private:
		static std::string connectionString;
	public:
		static void setConnectionString(std::string str);
		static std::string getConnectionString();
	};
}

TestClass.cpp:

#include "stdafx.h"
#include "TestClass.h"

namespace Test
{
	void TestClass::setConnectionString(std::string str)
	{
		connectionString = str;
	}

	std::string TestClass::getConnectionString()
	{
		return connectionString;
	}
}

Entry Point:

#include "stdafx.h"
#include "TestClass.h"

int _tmain(int argc, _TCHAR* argv[])
{
   Test::TestClass::setConnectionString("Some Text");
}

[Repeater]

You need to create the string Test::TestClass::connectionString somewhere.

static member variables don't get created when an instance of a class is. There's only one instance of the static var, and you have to create it somewhere.

#include "stdafx.h"
#include "TestClass.h"
#include <string>

std::string Test::TestClass::connectionString;

int _tmain(int argc, _TCHAR* argv[])
{
   Test::TestClass::setConnectionString("Some Text");
}

or in TestClass.cpp , or anywhere, so long as it is actually created.

[dbag]

@Repeater, So you cannot create static variables inside a class period?

[dbag]

If I take it outside the class I also end up exposing it. If I cannot use it within a class and set it to private. How could I go about hiding the actual variable and exposing access only via static method?

[Peter87]

Declare it inside the class (in the header file) and define it outside the class (in the source file), just like you do with the functions.

TestClass.h

#include <string>

namespace Test
{
	class TestClass
	{
	private:
		static std::string connectionString; // This is only a declaration.
	public:
		static void setConnectionString(std::string str);
		static std::string getConnectionString();
	};
}

TestClass.cpp

#include "TestClass.h"

namespace Test
{
	// Defines the static variable. This is where you initialize the variable.
	std::string TestClass::connectionString; 
	
	void TestClass::setConnectionString(std::string str)
	{
		connectionString = str;
	}

	std::string TestClass::getConnectionString()
	{
		return connectionString;
	}
}

[dbag]

@Peter87, Ahh. The light bulb finally clicked. Thank you Peter that was certainly the crucial point I was missing.