Link DLL and class method explicitly - c++

Question

I'd like make a dynamic library by creating a DLL and import it into my main program.
But I can't run my program correctly since I switch from LIB to DLL.
This is my DLL .h file :
class Connector
{
public:
Connector(std::string _apiKey,
std::string _masterCode,
std::string _masterSystem,
std::string _masterVersion,
int INTERNAL_PARAMETER = -1);
virtual ~Connector();
std::string query(std::string method,
std::map<std::string,
std::string> params);
[...]
}
And this is the link code in my mainApp :
typedef std::string (CALLBACK* kcDLLFUNC_QUERY)(
std::string, std::map<std::string, std::string>, std::string);
HINSTANCE kcDLL = LoadLibrary(_T("Connect"));
kcDLLFUNC_QUERY kcDLLFUNC_query = (kcDLLFUNC_QUERY)GetProcAddress(kcDLL, "query");
std::map<std::string, std::string> params;
params["amount"] = "50";
std::string RES = kcDLLFUNC_query("de", params, "");
std::cout << RES << std::endl;
FreeLibrary(kcDLL);
Have I forgotten anything?

Answer 1

The main issue is that GetProcAddress() only works with extern "C" functions. The function you want to call is a member of a class, and you haven't exported either the function or the entire class.
I typically implement this by adding a define to the DLL project, and then create a header in the DLL project that defines a macro that indicates if the function/class is exported or imported. Something like this:
// Assumes IS_DLL is defined somewhere in the project for your DLL
// (such as in the project's Properties: C/C++ -> Preprocessor)
#ifdef IS_DLL
#define DLL_API __declspec(dllexport)
#else
#define DLL_API __declspec(dllimport)
#endif
And then modify your class like this:
#include "DllExport.h" // name of the header file defined above
class DLL_API Connector
{
public:
Connector(std::string _apiKey, std::string _masterCode, std::string _masterSystem, std::string _masterVersion, int INTERNAL_PARAMETER = -1);
virtual ~Connector();
std::string query(std::string method, std::map<std::string, std::string> params);
[...]
}
In your .exe, include the header for your class, and use it as usual. You also need to link to the DLL. In recent versions of Visual Studio, this is done as follows:
In the Solution Explorer, expand the project for the .exe.
Right click References, and select Add Reference....
In the dialog, select Solution in the list on the left.
Select the checkbox next to the DLL's project, and press OK.
If you end up creating multiple DLLs for your program, you'll need to change the name of the defines so they don't clash (I typically include the name of the DLL in the name of each define).