What Is wrong with this code?

Add inline (inline string WCHAR_TO_STRING(WCHAR* wc....) and continue investigations.

That caused the error to go away, but why?

Also, I have about 16 lines of code in the function, so would it be ideal to make it inline?

Not that I am complaining. I just want to understand what's happening.

Thanks.

It seems that the function was defined inside a header (.h) file, and there are several #include directives that simply paste the same code to multiple source files, which cause the errors. To solve the problem, you can add static — static string WCHAR_TO_STRING(.... — however this could result in code duplication (multiple local WCHAR_TO_STRING functions, larger program). I think that inline is more appropriate for short functions.

The traditional approach is to split the declarations and definitions:

// in "MyFunctions.h" file:

string WCHAR_TO_STRING(WCHAR* wc, int& chars);

// in one .cpp file, for example: "MyFunctions.cpp":

#include "MyFunctions.h"

string WCHAR_TO_STRING(WCHAR* wc, int& chars) 
{
	string ws = "";
	return ws;
}

// in other CPP files:

#include "MyFunctions.h"

void test1()
{
   // use the function:
   string s = WCHAR_TO_STRING(.....);
}

See also another modern approach:

// In "MyFunctions.h" (the "MyFunctions.cpp" is not needed):

namespace // unnamed namespace
{
   string WCHAR_TO_STRING(WCHAR* wc, int& chars) 
   {
	 string ws = "";
	 return ws;
   }
}

Haven't heard from you for a while. Have you solved your issue?