error LNK2001: unresolved external symbol with DLL - c++

Question

I created a DLL project and successfully built it. I then tried to use the DLL in another Project, TEST, and I am getting the following error.
Error 1 error LNK2001: unresolved external symbol "public: void __thiscall SnoMessage::setRawMessageName(class ATL::CStringT<wchar_t,class StrTraitMFC_DLL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >)" (?setRawMessageName#SnoMessage##QAEXV?$CStringT#_WV?$StrTraitMFC_DLL#_WV?$ChTraitsCRT#_W#ATL#####ATL###Z)
I added the required lib in the linker properties, and I also added the header files in the TEST include directory. So the function is being recognized, but it keeps giving those errors. The DLL is comprised of the following files
SnoMessage.h
#pragma once
#include "StdAfx.h"
class SnoMessage
{
public:
__declspec(dllexport) SnoMessage(void);
__declspec(dllexport) ~SnoMessage(void);
__declspec(dllexport) void setRawMessageName(CString messageName);
__declspec(dllexport) void setRawMessageType(CString messageType);
__declspec(dllexport) void setRawMessageAttributes(std::map<CString,CString> attributes);
__declspec(dllexport) CString getRawMessageName();
__declspec(dllexport) CString getRawMessageType();
__declspec(dllexport) std::map<CString,CString> getRawMessageAttributes();
private:
CString messageName;
CString messageType;
std::map<CString,CString> attributes;
};
SnoMessage.cpp
#include "stdafx.h"
#include "SnoMessage.h"
SnoMessage::SnoMessage(void)
{
}
SnoMessage::~SnoMessage(void)
{
}
void SnoMessage::setRawMessageName(CString messageName){
this->messageName = messageName;
}
void SnoMessage::setRawMessageType(CString messageType){
this->messageType = messageType;
}
void SnoMessage::setRawMessageAttributes(std::map<CString,CString> attributes){
this->attributes = attributes;
}
CString SnoMessage::getRawMessageName(){
return messageName;
}
CString SnoMessage::getRawMessageType(){
return messageType;
}
std::map<CString,CString> SnoMessage::getRawMessageAttributes(){
return attributes;
}
And in test I am doing the following:
test.cpp
// test.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "SnoMessage.h"
int _tmain(int argc, _TCHAR* argv[])
{
SnoMessage *msg = new SnoMessage();
msg->setRawMessageName("TEST");
return 0;
}
Let me know if you need more info, thanks.

Answer 1

In your dll define this in some header you want to use for your export defs...
MyExports.h
#ifdef SNOMESSAGE_EXPORTS
#define SNOMESSAGE_API __declspec(dllexport)
#else
#define SNOMESSAGE_API __declspec(dllimport)
#endif
Now in your dll you just define SNOMESSAGE_EXPORTS, then when your dll is compiled your class and methods will be visible to the exe. But when you include those same headers in the exe the Macro will import them instead of export.
//In the DLL this is == to export, in the executable this is import. Problem solved.
class SNOMESSAGE_API SnoMessage
{
public:
//...
};
You no longer need to export each member, just the class.

Answer 2

I would mark the whole class as exported, not just its member functions. Also, following the advice of this conversation, you need to specify __declspec(dllecport) or __declspec(dllimport) based on whether you are including the header in the DLL or the code that uses the DLL; and define the guarding macro in the DLL project.

Answer 3

When you compile the DLL you should have __declspec(dllexport), but when you compile exe you should have __declspec(dllimport). The easiest way to do this is to have a #define somewhere that has different value when "in DLL" and "out of DLL". Also do export the whole class instead of individual methods.

Answer 4

There is a case when dll compile use C call but exe use standard call, the link in x64 has no problem, but when using win32 will show this link error 2001. For that situation just use C call for both dll and exe for win32 platform (https://learn.microsoft.com/en-us/cpp/error-messages/tool-errors/name-decoration?view=msvc-160).