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Creation of global class in a DLL

Posted on 2011-03-11
Medium Priority
Last Modified: 2012-08-13

I am building is a series of .lib files that come together to form a single DLL. I also have a class called TestObject that manages some data.

1. A single TestObject instance must be globally accessible by all functions in the DLL libs.
2. DllMain should do some init + cleanup of the global TestObject.

The immediate problem is that I can't get past the linker:

error LNK2019: unresolved external symbol "public: __thiscall TestObject::~TestObject(void)" (??1TestObject@@QAE@XZ) referenced in function "void __cdecl `dynamic atexit destructor for 'testObj''(void)" (??__FtestObj@@YAXXZ)      mylib.lib
error LNK2019: unresolved external symbol "public: __thiscall TestObject::TestObject(void)" (??0TestObject@@QAE@XZ) referenced in function "void __cdecl `dynamic initializer for 'testObj''(void)" (??__EtestObj@@YAXXZ)      mylib.lib
error LNK2019: unresolved external symbol "public: void __thiscall TestObject::PublicFunc(void)" (?PublicFunc@TestObject@@QAEXXZ) referenced in function _DllMain@12      mylib.lib

See code below. Should the TestObject reside elsewhere to be global? How can I get past the linker and have TestObject be fully global?

- Vashren
#include "TestObject.h"

TestObject g_testObj;

// DllMain()
BOOL APIENTRY DllMain(HINSTANCE hModule, DWORD dwReason, VOID* pReserved)

    switch (dwReason)
        case DLL_PROCESS_ATTACH:
            // some init of testObj
        case DLL_PROCESS_DETACH:
            // some cleanup of testObj

    return TRUE;

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Question by:Vashren
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LVL 86

Expert Comment

ID: 35110214
Well, it just seems that you have not provided both a constructor and a destructor for your class - are you linking with 'TestObject.cpp'?

Author Comment

ID: 35110315
TestObject.cpp contains an implementation of the constructor & destructor.

However, I'm not well versed with the linker. Do you mean to say I need to do something with the CPP file?
LVL 86

Accepted Solution

jkr earned 2000 total points
ID: 35110358
You need to add that file to your project, otherwise it won't be compiled or linked wirth and cause the errors you mentioned in your question. Right-click pn your projext, choose 'Add|Existing Item' and add 'TestObject.cpp'. "'Project|Add existing Item' (CTRL+SHIFT+A) will do the same.
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Author Comment

ID: 35110455
Awesome - it builds correctly now.

With the linker out of the way, I'll now see of the "globalness" works.
LVL 86

Assisted Solution

jkr earned 2000 total points
ID: 35110923
Should work fine ;o)

BTW, you might be interested in the http://en.wikipedia.org/wiki/Singleton_pattern - this takes the whole concept of 'global objects' to a new pattern. See also http://www.codeproject.com/KB/cpp/singleton_template.aspx ("A Singleton Template Class") for an example.

Author Comment

ID: 35112284
Thanks. Time to gobble up some points!

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