Question

I’ve created example C++ project in Visual Studio of dll type. It contains header file SqlLtDb.h:

 using namespace std;
    // This class is exported from the SqlLtDb.dll
    class CSqlLtDb {
    public:
        CSqlLtDb(char *fileName);
        ~CSqlLtDb();
        // TODO: add your methods here.
        bool SQLLTDB_API open(char* filename);
        vector<vector<string>> SQLLTDB_API query(char* query);
        bool SQLLTDB_API exec(const char* query);
        void SQLLTDB_API close();
        int SQLLTDB_API getNameOfClass();
    private:
        sqlite3 *database;
    };

extern "C" SQLLTDB_API CSqlLtDb* getInstanceCSblLtDb();
extern SQLLTDB_API int nSqlLtDb;
extern "C" SQLLTDB_API int fnSqlLtDb();

And in SqlLtDb.cpp methods are implemented as follows (I’m showing only two implementation):

...

int SQLLTDB_API CSqlLtDb::getNameOfClass()
{
    return 777;
}

extern "C" SQLLTDB_API CSqlLtDb* getInstanceCSblLtDb()
{
    CSqlLtDb* instance = new CSqlLtDb("");
    return instance;
}

SqlLtDb.def file look like this:

LIBRARY "SqlLtDb"
EXPORTS
getInstanceCSblLtDb
open
query
exec
close
getNameOfClass

SqlLtDb.lib file is generated by LIB command, using above .def file.
This is my SqlLtDb.dll file.

Now I want to include this file to my consoleApplication application.
ConsoleApplication is in VS 2008. I’ve set:

Properties->Configuration Properties->Linker->Input->Additional Dependencies : SqlLtDb.lib;

Properties->Configuration Properties->Linker->General->Additional Library
Directories: E:\PM\SqlLtDb\Release;

Runtime Library is set as it was: Multi-threaded Debug DLL (/MDd) (I didn’t change it).

I copied files: SqlLtDb.dll, SqlLtDb.lib, SqlLtDb.def, sqlite3.dll into Debug folder where consoleApplication.exe is generated. And I added SqlLtDb.h file into folder where consoleApplication's source files are stored.

Function main in consoleApplication look like this:

#include "stdafx.h"
#include "SqlLtDb.h";

int _tmain(int argc, _TCHAR* argv[])
{
    CSqlLtDb* mySqlClass = getInstanceCSblLtDb();  // here is ok, this method is 
                                                   // exported rigth
    mySqlClass->open("");  // here is error whit open method
    return 0;
}

When I compile this code I get error:

Error 1 error LNK2019: unresolved external symbol "__declspec(dllimport) public: 
bool __thiscall CSqlLtDb::open(char *)" (__imp_?open@CSqlLtDb@@QAE_NPAD@Z) 
referenced in function _wmain consoleApplication.obj consoleApplication

Method getInstanceCSblLtDb is exported successful, but the problem is with export methods from class. I won’t to export all class, better is export pointer to class.

Thanks

Answer 1

You need to export the class in the DLL with __declspec(dllexport), and import it in the linking code with __declspec(dllimport). Example:

class SQLLTDB_API CSqlLtDb {
    ...
};

You don’t need SQLLTDB_API for each member, only the class – the linker will generate exports for each method for you.