Question

okay, i’m a bit befuddled with this error. What i’m trying to do here is make a basic_string that will be either char or wchar_t when UNICODE and _UNICODE are defined (this is in WINAPI). This does work, but for some reason, i’m unable to define a function that receives a std::basic_string outside the class where it’s declared. Here’s an example:

test.h

#ifndef TEST_H
#define TEST_H

#include <Windows.h>
#include <string>

class Test
{
public:
    void func(std::basic_string<TCHAR> stringInput);
};

#endif

test.cpp

#include "test.h"

void Test::func(std::basic_string<TCHAR> stringInput)
{
    MessageBox(NULL, stringInput.c_str(), TEXT("It works!"), MB_OK);
}

This yields a link error, claiming the test::func was never defined. However, if i just define inside the class like this:

test.h

#ifndef TEST_H
#define TEST_H

#include <Windows.h>
#include <string>

class Test
{
public:
    void func(std::basic_string<TCHAR> stringInput)
    {
        MessageBox(NULL, stringInput.c_str(), TEXT("It works!"), MB_OK);
    }
}

#endif

it works fine. However, i really like to keep my declarations and definitions in separate files to avoid redefinition errors and for organization. Here’s the kicker though. when i have func defined in test.cpp like before and don’t define UNICODE and _UNICODE in my main.cpp, i don’t get the link errors. So really, the only time i get a link error is when TCHAR becomes a wchar_t. So here’s my main and the error real quick…

main.cpp

#define UNICODE       // this won't compile when these are defined
#define _UNICODE

#include <Windows.h>
#include <string>

#include "test.h"

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,LPSTR lpCmdLine, int nCmdShow)
{
    Test test;
    test.func(TEXT("wakka wakka"));
    return 0;
}

error:

error LNK2019: unresolved external symbol "public: void __thiscall Test::func(class std::basic_string<wchar_t,struct std::char_traits<wchar_t>,class std::allocator<wchar_t> >)" (?func@Test@@QAEXV?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@@Z) referenced in function _WinMain@16

Anyone have a clue what’s going on and how i might go about fixing this?

Answer 1

I think because you are putting #define UNICODE in main.cpp, the other part doesn’t know about this. When test.cpp is compiled, UNICODE is not defined. You can try putting the UNICODE definition as project processor macro. Or in test.h, write #define UNICODE and #define _UNICODE before including Windows.h .

On another note, because you’ve included Windows.h in Test.h, you should not include it again in main.cpp .

Consider create a default project in visual studio, and use Precompiled Headers. This way, put such include in stdafx.h will address all your problems:

#define UNICODE
#include <windows.h>
#include <string>