How to call a C++ dll function at runtime ?

[Mike Pliam]

I think I get it now. My mistake was thinking that I could index the function pointer as one usually can do with a pointer, but that is not the case. Simply capturing the pointer returned by the function allows complete pointer indexing, and a single call to cleanup the pointer suffices. This little example works to return a vector from a dll using runtime dynamic linking without a memory leak.

///////////////////////////////////////////////////////////////////////////////
// mydll.h

#ifdef MY_EXPORTS
#define MY_API __declspec(dllexport)
#else
#define MY_API __declspec(dllimport)
#endif


extern MY_API double * pVector;

// Get rid of name mangeling
extern "C"
{

	MY_API double * fvec(void);

	MY_API void clean(double * dp);

}

////////////////////////////////////////////////////////////////////////////////////
// mydll.cpp

#include "stdafx.h"
#include "my.h"

MY_API double * pVector;

double * CreateVector(int nSize)
{
	// Allocate memory including leading size containing byte
	pVector = new double [nSize + 1];

	memset(pVector, 0x00, nSize+1);

	return pVector;

}// CreateVector(int nSize)

void FreeVector(double * pVector)
{
    delete [] pVector; 
	pVector = NULL;

}// FreeVector(double * pVector)

MY_API double * fvec(void)
{
	pVector = CreateVector(8);
	pVector[0] = 8.0;
	pVector[1] = 6.4342;
	pVector[2] = 1.5453;
	pVector[3] = 2.6546;
	pVector[4] = 3.7657;
	pVector[5] = 4.23423;
	pVector[6] = 5.1232;
	pVector[7] = 6.87654;
	pVector[8] = 7.7755;
	return pVector;
}

MY_API void clean(double * dp)
{
	if(dp != NULL)
	{
		delete [] dp;
		dp = NULL;
	}
}

/////////////////////////////////////////////////////////////////////////////////////////
// test.cpp

#include "stdafx.h"

#include <windows.h> 
#include <stdio.h> 

// Function pointers that will be used for the DLL functions.
typedef double *(*FVEC)();
typedef void (*CLEAN)(double *);

int _tmain(int argc, _TCHAR* argv[])
{
	int iOrg = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);	
	_CrtSetDbgFlag(iOrg | _CRTDBG_LEAK_CHECK_DF);

	vector<double> vf;
    HINSTANCE hinstLib; 
    BOOL fFreeResult, fRunTimeLinkSuccess = FALSE; 

	FVEC _MYVEC = NULL;
	CLEAN _MYCLEAN = NULL;

    // Get a handle to the DLL module.
 
    hinstLib = LoadLibrary(TEXT("my.dll")); 
 
 		if (NULL != _MYVEC) 
		{
			printf("\n_MYVEC is not NULL\n\n");

            fRunTimeLinkSuccess = TRUE;
 
			double * pVec = NULL;

			// get vector
			pVec =  _MYVEC();
			int N = (int) pVec[0];
			vf.resize(N);

			for(int i=1; i <= N; i++)
			{
				vf[i-1] = pVec[i];
	
			}
			printf("\n");
				
			_MYCLEAN(pVec);

			for(size_t i = 0; i < vf.size(); i++) { printf("&#37;0.4f\n", vf[i]); }

		}		}

        // Free the DLL module.
        fFreeResult = FreeLibrary(hinstLib); 
    } 

    // If unable to call the DLL function, use an alternative.
    if (! fRunTimeLinkSuccess) 
 		printf("Unable to call the DLL function\n");

	for(size_t i = 0; i < vf.size(); i++) { printf("%0.4f\n", vf[i]); }

	return 0;
}

Output:
Handle is valid

_MYVEC is not NULL

6.4342
1.5453
2.6546
3.7657
4.2342
5.1232
6.8765
7.7755

Thank you all for your kind help and patience with a slow learner.