Strange C++/CLI linking issue - unresolved external symbols but header file is linked

Question

I'm trying to do something very simple in C++/CLI, however I'm running into issues with Visual Studio and I can't figure out why.

I have two projects, one which contains a header file named JNIUtils.h. Its content is very barebones -

#pragma once

class JNIUtils
{
public:
    JNIUtils( void );
    ~JNIUtils( void );
};

and its implementation is very simple as well -

#include "stdafx.h"

#include "JNIUtils.h"

JNIUtils::JNIUtils( void )
{

}

JNIUtils::~JNIUtils( void )
{

}

All I'm trying to do is to create an object of this newly defined type in another project. In the project where I'm including this header file, I've gone to

Project Properties > Configuration Properties > VC++ Directories > Include Directories

I added an entry with the path to where the header file I'm including resides (JNIUtils.h)

The CPP file I'm trying to create an instance of this object in looks as follows:

#include "stdafx.h"
#include "JNIUtils.h"

using namespace System;

int main(array<System::String ^> ^args)
{
    JNIUtils *util = new JNIUtils();
    Console::WriteLine(L"Hello World");
    return 0;
}

When I try to compile I get the following errors:

Error   3   error LNK1120: 2 unresolved externals   C:\zcarter\UDK\WebSvcClients\Debug\JNITester.exe    JNITester
Error   2   error LNK2019: unresolved external symbol "public: __thiscall JNIUtils::JNIUtils(void)" (??0JNIUtils@@$$FQAE@XZ) referenced in function "int __clrcall main(cli::array<class System::String ^ >^)" (?main@@$$HYMHP$01AP$AAVString@System@@@Z)   C:\zcarter\UDK\WebSvcClients\JNITester\JNITester.obj    JNITester
Error   1   error LNK2028: unresolved token (0A000009) "public: __thiscall JNIUtils::JNIUtils(void)" (??0JNIUtils@@$$FQAE@XZ) referenced in function "int __clrcall main(cli::array<class System::String ^ >^)" (?main@@$$HYMHP$01AP$AAVString@System@@@Z)  C:\zcarter\UDK\WebSvcClients\JNITester\JNITester.obj    JNITester

Can anyone tell me what I'm doing wrong here?

Typically you would need to use the object file or library that the other object file is included in. — Vaughn Cato, Jan 12 '12 at 03:38
Can you tell me where to add the obj file? There is no lib file... — Zachary Carter, Jan 12 '12 at 03:43
Could you specify what version of Visual Studio you are using? — Coincoin, Jan 12 '12 at 03:48
When you compile the JNIUtils.cpp, you get a JNIUtils.o, you need to provide that file to the linker. The header file provides only the prototype of the function, which is sufficient for compilation but not linking — Chethan Ravindranath, Jan 12 '12 at 03:50

Coincoin · Accepted Answer · 2012-01-12T04:07:17.140

When you want to use classes from a project into another one, there are two things that you need to do:

1) Link the library compiled code (.lib) to your main application code. If you have vs2010, simply add the project in the Project->Properties->Common->References section, all the hard work will be done for you.

If you have vs2008 or lower, you must add the project to the dependencies in Project->Dependencies.... Then, go in Project->Properties->Linker->Input and add the .lib file to the existing list of .dll. Use a relative path so that this will work on other computers. Note that there is also a Reference section in vs2008, but I believe it only works for CLR assemblies.

2) Add the headers to the include directories

You figured that part already, but I recommend you to add them to the Project->Properties->C++->Additional Include Directories instead, as the method you used will not work on another computer unless they make the same configuration as you.

Thank you very much Coincoin, this is exactly what I needed to do. — Zachary Carter, Jan 12 '12 at 11:06

score 1 · Answer 2 · answered Jan 12 '12 at 03:44

the key to your problem is that your trying to use this object in another project.

to do this, you must compile the class into a class library (i suggest a static one here), and then link this library in your compiler settings for the second program. this is somewhat tedious actually, but very good to try out!

note that there are two separate "linkings" here. one in your code, where you include the header file; and one in your linker (happens after compiler) where you link the object file (which is now inside a static library file).

The first "linking" isn't linking at all, really. It's just an include (that tells the compiler that `JNIUtils::JNIUtils` exists). — cHao, Jan 12 '12 at 03:52

score 0 · Answer 3 · answered Jul 10 '21 at 09:40

In my case the error message show LNK2019 unresolved external symbol __imp_someFunctionName referenced in function ..., and I find that the prefix __imp_ of function name, means that the program is using __declspec(dllimport), not __declspec(dllexport).

So I add the proper name to Project->Properties->Configuration Properties->C/C++->Preprocessor, let __declspec(dllexport) work, then can solve error and build success.

reference to: How can I get rid of the __imp__ prefix in the linker in VC++?

Strange C++/CLI linking issue - unresolved external symbols but header file is linked

3 Answers3