Unresolved external symbol in a DLL

Naveen

George_George wrote:

CFoo foo;

This is not enough, you have to export the foo variable, like __declspec (dllexport) CFoo foo;

George_George wrote:

extern CFoo foo;

Also in the exe, you have to import the variable __declspec (dllimport) CFoo foo;

George_George

I am confused, nbugalia! In my design, I want to create object instance in DLL and access the object instance from the hosting executable from extern reference. Your code change impacts my design... :-) Any comments? regards, George

George_George

Thanks nave, I have tested your solution works. My question is, if I create a global variable inside a DLL, then in the hosting executable, could I use extern to refer to the variable and use it? regards, George

nbugalia

Here is the Dll file code - foo.h

class __declspec (dllexport) CFoo
{
public:
CFoo();
int foo1();
int foo2();
};

**__declspec (dllexport) CFoo foo;re>

Foo.cpp

#include "Foo.h"

CFoo::CFoo()
{
return;
}
int CFoo::foo1()
{
return 100;
}
int CFoo::foo2()
{
return 200;
}

and here is the console based application code -
Foo.h

class __declspec (dllimport) CFoo
{
public:
CFoo();
int foo1();
int foo2();
};

__declspec (dllimport) CFoo foo;

test.cpp
#include "foo.h"
int main()
{
int i = foo.foo1();
i = foo.foo2();
return 0;
}

Don't forget to set the dependency to Foo.lib file in the test project settings.
Hope this will help. :-D**

Naveen

George_George wrote:

could I use extern to refer to the variable and use it?

No. you should export it to use the variable in the hosting executable..

nave [OpenedFileFinder] [My Blog]

George_George

Cool, nbugalia! I want to confirm with you that we could expose, function/class/global variable from a DLL, correct? regards, George

George_George

Thanks nave, So, from learning from you, it is legal to expose both function/global variable/class from a DLL? (previously, I think only expose function is allowed.) regards, George

Naveen

George_George wrote:

So, from learning from you, it is legal to expose both function/global variable/class from a DLL?

Yes its legal to expose both functions and variables.

nave [OpenedFileFinder] [My Blog]

George_George

Thanks nave, And also legal for exposing class? regards, George

Naveen

Of course classes also :cool:

nave [OpenedFileFinder] [My Blog]

Stuart Dootson

You've got two issues:

In main.cpp, you want to declare CFoo as __declspec(dllimport). The way I do that is to put something like this in foo.h: #if !defined(FOO_DLL) #define FOO_EXP __declspec(dllimport) #else #define FOO_EXP __declspec(dllexport) #endif class FOO_EXP CFoo { ... }; You would define FOO_DLL when building the DLL using, for example, #define FOO_DLL in every source file in the DLL before #including foo.h. You need do nothing in main.cpp>/code>, as that's what the default case caters for.
As other replies have said, you would need to export CFoo foo; from the DLL.

George_George

Thanks Naveen! I have proved we could expose both class and global variable by DLL and using your mentioned elegant way. :-) But I am not 100% confident. Here is my code, could you have a code review please? DLL Part:

// foo.h

#ifdef _DLLPROJ_
#define DLLEXP __declspec (dllexport)
#else
#define DLLEXP __declspec (dllimport)
#endif

DLLEXP class CFoo
{
public:

DLLEXP int foo1();

DLLEXP int foo2();

};

DLLEXP class CFooImpl : CFoo
{
public:

DLLEXP CFooImpl();

DLLEXP int foo1();

DLLEXP int foo2();

};

// dllmain.cpp

#include "foo.h"

CFooImpl::CFooImpl()
{
return;
}

int CFooImpl::foo1()
{
return 100;
}

int CFooImpl::foo2()
{
return 200;
}

DLLEXP CFooImpl foo;

Hosting client part

// main.cpp

#include "foo.h"

DLLEXP CFooImpl foo;

int main()
{
CFooImpl foo1;

int i = foo.foo1();
i = foo.foo2();

return 0;

}

regards, George

George_George

Thanks Stuart, I have correct my code here, could you help to do a code review please? :-) http://www.codeproject.com/script/Forums/View.aspx?fid=1647&msg=2846175[^] regards, George

Naveen

I have noted the below problem

George_George wrote:

DLLEXP class CFoo

This should be modified as class DLLEXP CFoo everthing else seems ok.

nave [OpenedFileFinder] [My Blog]

Stuart Dootson

Aside from the issue Naveen showed you, IIRC you don't need to DLLEXP the class and the classes methods - one or the other, that's all that's needed really. I tend to DLLEXP the methods, as you have better control over what precisely is exported.

George_George

I think they should be the same? It compiles and runs well in VS. :-) regards, George

George_George

Hi Stuart, I have tried that if I only mark dllexport on class and not on methods, the methods could not be found during link process. So, I think we have to mark both. Could you have a try please? regards, George

Stuart Dootson

I have an awful lot of deployed code that uses this mechanism - here's a sample: #if!defined(COMMONLIB_API) #ifdef COMMONLIB_EXPORTS #define COMMONLIB_API __declspec(dllexport) #else #define COMMONLIB_API __declspec(dllimport) #endif #endif namespace MyAppLibrary { class MessageReporter { public: //ctor COMMONLIB_API MessageReporter(); //Add new reporting function COMMONLIB_API static void AddReportFunction(ReportFnPtr func); //Set MessageReporter Level on or off COMMONLIB_API static void SetReporterLevel(Level lev, bool on); //Set all levels to on COMMONLIB_API static void AllLevelsOn() {repLevels=~0;} //Set all levels to off COMMONLIB_API static void AllLevelsOff() {repLevels=0;} }; }; Is it possible you haven't exported necessary constructors?

George_George

But if you only expose class not expose methods, the methods could not be found when linking the client of the DLL (link error, unresolved symbols related to the methods). Could you have a try? I have tried. regards, George

Stuart Dootson

DLL interface defined in a.h

a.h

#if !defined(__A_H__) #define __A_H__ #if!defined(A_API) #ifdef A_EXPORTS #define A_API __declspec(dllexport) #else #define A_API __declspec(dllimport) #endif #endif class A_API A { public: A(); A(int a); void DoSomething(int b); int Result() const; private: int a_; }; #endif // !defined(__A_H__) DLL implemented in a.cpp and built with cl -EHsc -LD a.cpp

a.cpp

#include <windows.h> #define A_EXPORTS #include "a.h" BOOL APIENTRY DllMain( HANDLE hModule, DWORD ul_reason_for_call, LPVOID lpReserved ) { return TRUE; } A::A() : a_(0) {} A::A(int a) : a_(a) {} void A::DoSomething(int b) { a_ += b; } int A::Result() const { return a_; }
DLL used in b.cpp, built with cl -EHsc b.cpp a.lib:

b.cpp

#include <iostream> #include "a.h" int main(int argc, char** argv) { A a(argc); a.DoSomething(3); std::cout << a.Result() << std::endl; } Builds fine, runs OK and even produces the right result...which is nice.

modified on Saturday, December 20, 2008 6:19 AM