How to modify content of the original variable which is passed by value?

Question

There is an existing API function which does only allow the plugin(DLL) to receive three parameters and perform some action:

int ProcessMe(int nCommand, unsi         


        

Answer 1

Compiling for 32 bits platform you could cast to pointer the parameter:

#include <stdio.h>
typedef int BOOL;
#define FALSE 0
#define TRUE 1

int ProcessMe(int nCommand, unsigned int wParam, long lParam)
{
  // cast to match what you passed in
  *((BOOL*)wParam) = FALSE;
  return 0;
}

int cast_arguments_main(int, char **)
{
  BOOL bSave = TRUE;
  int nOption = 0;
  ProcessMe(0, (unsigned int)(&bSave), (long)(&nOption));
  if(FALSE==bSave)
    printf("bSave is modified!");
  return 1;
}

On a 64 bits platform the compiler complains about the impossibility to represent the pointer:

cast_arguments.cpp:17:37: error: cast from ‘BOOL*’ to ‘unsigned int’ loses precision

I had to change the unsigned int wParam parameter declaration to unsigned long wParam, and similar change at call point: ProcessMe(0, (unsigned long)(&bSave), (long)(&nOption)); but then you could simply declare the right type for your parameter, to be BOOL *. Thus feasibility depends on your target machine architecture...

Answer 2

Place the variables to modify in a struct and pass the pointer to the sturuct to the plug in:

struct MyStruct
{
    BOOL bSave;
    int nOption;
};

int ProcessMe(int nCommand, unsigned int wParam, long lParam)
{
    ((MyStruct*)lParam)->nOption = ...;
    return 0;
}

Use it like this:

int main()
{
  MyStruct struct;
  struct.bSave = TRUE;
  struct.nOption = 0;
  ProcessMe(0, 0, (long)(&struct));
  if(FALSE==struct.bSave)
    printf("bSave is modified!");
  return 1;
}

Strictly speaking this is undefined behavior. You need to check, whether it works on your platform.

Note: I used a struct here, because this way you can also pass more variables or larger variables such as double to the function.

Answer 3

No, there's no way to do that.

Passing by value means copies of the original ones are created. In the method scope, you will have no information about the original variables.

You need to either pass by reference or by value.

Answer 4

Try this... it is POSSIBLE!!!

include

int ProcessMe(int nCommand, unsigned int wParam, long lParam)

{

int *nCommand_ptr = (int*)nCommand;
unsigned int *wParam_ptr = (unsigned int*)wParam;
long *lParam_ptr = (long*)lParam;
*nCommand_ptr = 10;
*wParam_ptr = 10;
*lParam_ptr = 10;
return 0;

}

int main(){

int nCommand = 5;
unsigned int wParam = 5;
long lParam = 5;
printf("\n %d %u %lu",nCommand,wParam,lParam);
ProcessMe((int)&nCommand,(unsigned int)&wParam,(long)&lParam);
printf("\n %d %u %lu",nCommand,wParam,lParam);
return 0;

}

Output:

$>g++ passbyref.cc

$>./a.out

5 5 5

10 10 10$>