static method and fatal error LNK1120: 1 unresolved externals

dbag (24)
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: 

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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:

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#include <string>

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


TestClass.cpp:

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

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

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


Entry Point:

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

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

Repeater (3046)
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.

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#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.
Last edited on
dbag (24)
@Repeater, So you cannot create static variables inside a class period?
dbag (24)
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 (11237)
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 
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#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 
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#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 (24)
@Peter87, Ahh. The light bulb finally clicked. Thank you Peter that was certainly the crucial point I was missing.
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