XDispatch C++ Unresolved Externals

Question

I'm work with GCD in C++ with xdisptach, libdispatch in Visual Studio 2012 on Windows 7.

I am declaring a class with a global variable that is a dispatch queue. Other functions in the class call the queue's function. Everything compiles fine, except when i instantiate the queue in the constructor.

xdispatch::queue* dispatch_queue;
AsyncNode()
{
    dispatch_queue = new xdispatch::queue(Name);
}

When dispatch_queue = new xdispatch::queue(Name); is commented out, it all compiles fine. Otherwise i get the following errors.

Error   50  error LNK2019: unresolved external symbol "__declspec(dllimport) public: virtual __thiscall xdispatch::queue::~queue(void)" (__imp_??1queue@xdispatch@@UAE@XZ) referenced in function "public: virtual void * __thiscall xdispatch::queue::`scalar deleting destructor'(unsigned int)" (??_Gqueue@xdispatch@@UAEPAXI@Z)

Error   49  error LNK2019: unresolved external symbol "__declspec(dllimport) public: __thiscall xdispatch::queue::queue(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (__imp_??0queue@xdispatch@@QAE@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: __thiscall AsyncNode::AsyncNode(void)" (??0AsyncNode@@QAE@XZ)  

Error   53  error LNK2001: unresolved external symbol "public: virtual void __thiscall xdispatch::object::resume(void)" (?resume@object@xdispatch@@UAEXXZ)  

Error   51  error LNK2001: unresolved external symbol "public: virtual void * __thiscall xdispatch::queue::native(void)const " (?native@queue@xdispatch@@UBEPAXXZ)  

Error   52  error LNK2001: unresolved external symbol "public: virtual struct dispatch_queue_s * __thiscall xdispatch::queue::native_queue(void)const " (?native_queue@queue@xdispatch@@UBEPAUdispatch_queue_s@@XZ) 

This is the main site for xdispatch, but i can't find anything in terms of a forum or help with xdispatch in particular. There are many for objective-c.... :/

http://opensource.mlba-team.de/xdispatch/docs/current/index.html

Answer 1

First of all, the behavior regarding deleting the "problematic" line is normal - when you're defining a pointer you're not invoking any function, and that is the reason you are not getting the "unresolved external errors".

When you're initializing the variable by calling the constructor, you then run into problems because you're trying to call a function which is not available at link time.

When working with external libraries using DLLs, you must link to the appropriate import libraries, usually supplied as "lib" files. In your case, there is a folder named "lib" in the zip package. Also, the DLLs must availble at run-time - by putting them in the folder of the executable, or adding them to the PATH environment variable.

In order to link to a library, follow these steps (taken from MSDN ):

1. Open the project's Property Pages dialog box. For details, see Setting Visual C++ Project Properties.
2. Click the Linker folder.
3. Click the Input property page.
4. Modify the Additional Dependencies property.