How to acess member variable from a static function - c++

Question

i need to register to a callback function provided by a third party dll, this function does not accept any arguments, and hence i cannot pass in the "this" to it. here the pData is the just the data.
//This is declared as static function in my class
void CMyClass::ThirdPartyCallBack(void *pData)
{
//How to access the member variables of the class
}
how to do this without using a global variable, appreciate you attention and time

Answer 1

A void* is a pointer to whatever you like. You can pass anything you like as the data pointer to the dll, for example "this". And in your callback, you convert it back to an object pointer.

Answer 2

You can create a table of context pointers that is associated with a table of 'thunks' that are used as the callback function(s) you give to the DLL:
class foo {
public:
void callback(void* data);
};
enum {
kMAX_CALLBACKS = 3,
};
foo* context_table [kMAX_CALLBACKS] = {
0,
0,
0
// however many you might need...
};
extern "C" void callback_0( void* data)
{
foo* ctx = context_table[0];
ctx->callback(data);
}
extern "C" void callback_1( void* data)
{
foo* ctx = context_table[1];
ctx->callback(data);
}
extern "C" void callback_2( void* data)
{
foo* ctx = context_table[2];
ctx->callback(data);
}
// ....
extern "C" {
typedef void (*dll_callback)(void*);
}
dll_callback callback_table[kMAX_CALLBACKS] = {
callback_0,
callback_1,
callback_2
// however many you might need...
};
dll_callback set_callback_context(foo* ctx)
{
for (int i =0; i < kMAX_CALLBACKS; ++i) {
if (context_table[i] == 0) {
context_table[i] = ctx;
return callback_table[i];
}
}
return 0;
}
void clear_callback_context(foo* ctx)
{
for (int i =0; i < kMAX_CALLBACKS; ++i) {
if (context_table[i] == ctx) {
context_table[i] = 0;
return;
}
}
}
Now when you need to set up a callback to foo::callback() pass the DLLs function that sets up the callback with something like:
third_party_set_callback( set_callback_context(foo_instance_ptr));
and the callback will end up at foo:callback() with the appropriate this pointer.