Question

how do I cast void *something to an object in standard C++?
Specifically I want want to cast void *userdata
to std::map<String, void*>

Is this possible? I am trying:

//void *user_data is a parameter of this function (callback)
std::map <String, void*> user_data_n; //this line is ok
user_data_n = static_cast<std::map<String, void *>>(*user_data); //I get the errors here.

ERRORs:

Spurious '>>' user '>' to terminate a template argument list
Expected '>' before '(' token
'void *' is not a pointer-to-object type

or is there a better way to carry information about the caller object and some other parameters I can pass to void *user_data?

UPDATE:

Ass suggested by @aaa carp I changed >> to > > and the first two errors were solved. The last is strange, Why do I get that kind of message when casting it here and not when putting that object when setting the callback?

std::map<String, void*> user_data_h;
user_data_h["Object"] = this; //this is a MainController object
user_data_h["h"] = h; //h was defined as int *h
createTrackbar("trackbar_H", winName, h, 255, trackbar_handler, &user_data_h);

where createTrackbar is defined as:

int createTrackbar( const string& trackbarname, const string& winname,
int* value, int count, TrackbarCallback onChange, void* userdata);

UPDATE2:

doing this solved my problem but following the same approach, why I still get error when trying to cast objects contained in my map object?

void trackbar_handler(int value, void *user_data){
std::map <String, void*> *user_data_map;
user_data_map = reinterpret_cast<std::map<String, void *> *>(user_data); //WORKED!! ;)

MainController *controller; //the same class type I put using "this" above
controller = reinterpret_cast<MainController *>( user_data_map["Object"]); //ERROR here

int *var = reinterpret_cast<int*> (user_data_map["h"]); //ERROR also here

Answer 1

When you’re casting from a void *, your result will be a pointer too. So the map declaration should be:

std::map <String, void*> *user_data_n;

Second, you should use reinterpret_cast for such (potentially dangerous) casts:

user_data_n = reinterpret_cast<std::map<String, void *> *>(user_data);

Update:

As others suggested, you could simply use a static_cast as well.

Why do I get that kind of message when casting it here and not when putting that object when setting the callback?

Any pointer can be implicitly converted to void *, but when converting it back to a pointer of some specific type, you need an explicit cast.

why I still get error when trying to cast objects contained in my map object?

As already mentioned in the comments, you need to dereference the pointer before using the map object. You might want to define a reference instead to make things easier:

std::map <String, void*> &user_data_map =
    *(static_cast<std::map<String, void *> *>(user_data));