How do I cast int** to void**? - c++

Question

With the following snippet:
int n = 11;
int* c = &n;
void** v = &c;
I receive the following error in visual studio:
the value of type int** cannot be used to initialize an entity of type void **.
This works fine:
int n = 11;
int* c = &n;
void* v = c;
But this code snippet is for a larger problem in someone's library.
What am I doing wrong with casting a variable to void**?
Complete Example
Using the caen digitizer library the way they try to collect data from the peripheral device has this prototype:
/******************************************************************************
* X742_DecodeEvent(char *evtPtr, void **Evt)
* Decodes a specified event stored in the acquisition buffer writing data in Evt memory
* Once used the Evt memory MUST be deallocated by the caller!
*
* [IN] EventPtr : pointer to the requested event in the acquisition buffer (MUST BE NULL)
* [OUT] Evt : event structure with the requested event data
* : return 0 = Success;
******************************************************************************/
int32_t X742_DecodeEvent(char *evtPtr, void **Evt);
And this is the implementation:
int32_t X742_DecodeEvent(char *evtPtr, void **Evt) {
CAEN_DGTZ_X742_EVENT_t *Event;
uint32_t *buffer;
char chanMask;
uint32_t j,g,size;
uint32_t *pbuffer;
uint32_t eventSize;
int evtSize,h;
evtSize = *(long *)evtPtr & 0x0FFFFFFF;
chanMask = *(long *)(evtPtr+4) & 0x0000000F;
evtPtr += EVENT_HEADER_SIZE;
buffer = (uint32_t *) evtPtr;
pbuffer = (uint32_t *) evtPtr;
eventSize = (evtSize * 4) - EVENT_HEADER_SIZE;
if (eventSize == 0) return -1;
Event = (CAEN_DGTZ_X742_EVENT_t *) malloc(sizeof(CAEN_DGTZ_X742_EVENT_t));
if (Event == NULL) return -1;
memset( Event, 0, sizeof(CAEN_DGTZ_X742_EVENT_t));
for (g=0; g<X742_MAX_GROUPS; g++) {
if ((chanMask >> g) & 0x1) {
for (j=0; j<MAX_X742_CHANNEL_SIZE; j++) {
Event->DataGroup[g].DataChannel[j]= malloc(X742_FIXED_SIZE * sizeof (float));
if (Event->DataGroup[g].DataChannel[j] == NULL) {
for (h=j-1;h>-1;h++) free(Event->DataGroup[g].DataChannel[h]);
return -1;
}
}
size=V1742UnpackEventGroup(g,pbuffer,&(Event->DataGroup[g]));
pbuffer+=size;
Event->GrPresent[g] = 1;
}
else {
Event->GrPresent[g] = 0;
for (j=0; j<MAX_X742_CHANNEL_SIZE; j++) {
Event->DataGroup[g].DataChannel[j] = NULL;
}
}
}
*Evt = Event;
return 0;
}
I use this by:
CAEN_DGTZ_X742_EVENT_t* Evt = NULL; // Creating my event pointer
//Doing some config of the device
X742_DecodeEvent(evtptr, &Evt); //Decode the event data for me to read (Throws error)
Hope this gives some context.

Answer 1

void** means a pointer to a void* object. But there is no void* object in that code to point at! void** does NOT mean "a pointer to any kind of pointer", so please avoid using it as such. If you have a pointer to something which might be an int*, might be a double*, or etc., void* is a better type than void**. Even better would be a template or std::variant or std::any.
But if you have to use a library that is using void** to mean "a pointer to a pointer to a type unknown at compile time" or something like that, you might need to create a void* pointer to work with, or might need to add in casts to get around the fact that the compiler doesn't like this conversion (for good reason). The problem is, there are at least two reasonable ways to do this! (They will end up doing exactly the same thing on many common computer architectures, but this is not guaranteed.)
// LibraryFunc1 takes a void** argument that somehow means an int* pointer.
// But which call is correct?
int* data_in = generate_data();
LibraryFunc1(reinterpret_cast<void**>(&data_in)); // ?
void* p1 = data_in;
LibraryFunc1(&p1); // ?
// LibraryFunc2 returns a void** argument that somehow means an int* pointer.
void** p2 = LibraryFunc2();
int* data_out_1 = static_cast<int*>(*p2); // ?
int* data_out_2 = *reinterpret_cast<int**>(p2); // ?
Based on the function definition shown, the safe usage is unfortunately:
void* tmpEvt;
X742_DecodeEvent(evtptr, &tmpEvt);
auto* Evt = static_cast<CAEN_DGTZ_X742_EVENT_t*>(tmpEvt);
since the library function assumes at *Evt = Event; that *Evt is actually a void* object it can modify. It may usually work to do the simpler thing instead:
CAEN_DGTZ_X742_EVENT_t* Evt = NULL;
X742_DecodeEvent(evtptr, reinterpret_cast<void**>(&Evt));
but this is undefined behavior by the C++ Standard, and might do the wrong thing on some architectures.
You could make the correct way easier by wrapping it in a function:
inline CAEN_DGTZ_X742_EVENT_t* Get_X742_DecodeEvent(char* evtPtr)
{
void* tmpEvt;
X742_DecodeEvent(evtPtr, &tmpEvt);
return static_cast<CAEN_DGTZ_X742_EVENT_t*>(tmpEvt);
}

Answer 2

What am I doing wrong with casting a variable to void**?
There is no meaningful way to convert int** to void**, so what you're trying to do is wrong.
What you may do is
int n = 11;
void* c = &n;
void** v = &c;
But without a complete example, it is not possible to say whether applies to your problem.

Answer 3

That's simply how the language works.
void * pointers get special treatment: a pointer to an arbitrary type can be converted to a pointer to void (as long as doing so doesn't remove cv-qualifiers from the pointer).
void ** gets none of that special treatment. It's just a regular pointer type, like int **.
int32_t X742_DecodeEvent(char *evtPtr, void **Evt)
Since you want to pass CAEN_DGTZ_X742_EVENT_t ** to your function, you should change the parameter type accordingly: CAEN_DGTZ_X742_EVENT_t **Evt.
In comments you were suggested to use void ** v = (void**)&c;.
While you could probably make it work in practice, strictly speaking any access to *v would violate strict aliasing and cause undefined behavior. I wouldn't use that solution.