您可以在运行时将指向结构的void指针转换为特定的结构类型吗？

Question

I have a struct defined like this:

struct GameState {
    int score;
    int moves;
    bool won;

    void *metadata;
};
typedef struct GameState GameState;

The metadata pointer will point to another struct of a type decided at runtime. For example it might be:

struct KlondikeMetadata{
    bool draw3;
    int drawcount;
};
typedef struct KlondikeMetadata KlondikeMetadata;

Or maybe:

struct FreeCellMetadata{
    int reserveCells;
};
typedef struct FreeCellMetadata FreeCellMetadata;

The actual metadata struct used depends on the game the user is playing. 99% of the time this isn't a problem because I know what game the user is playing. However, there are cases where I don't (and can't) know this.

My question is, is there a way to determine or specify the correct metadata type at runtime?

For example if I could add a property to the GameState struct indicating that the metadata value is of type KlondikeMetadata and use that to cast the metadata to that type, I think I'd be golden. Is there a way to do this? Is there a way to specify a type and cast a variable at runtime in C?

Answer 1

You're going to have to encode it yourself.

The easiest solution is to declare an enumeration:

typedef enum {
 GameType_Klondike,
 GameType_FreeCell,
} GameType;

then add a field of that type before the pointer:

GameType game_type;
void     *metadata;

Of course, this means you're going to have to set the game_type field when you initialize metadata , to remember the type.

You can also go a bit object-oriented, and have the GameType be part of the metadata:

struct Metadata {
 GameType gametype;
};

struct FreeCellMetadata {
 struct Metadata meta;
 /* rest of fields here */
};

struct KlondikeMetadata {
 struct Metadata meta;
 /* rest of fields here */
};

Then you can cast your void * to struct Metadata * , and inspect the gametype field before casting the pointer to the proper type.

For bonus points, use a union:

struct Metadata {
 GameType type;
 union {
 struct KlondikeMetadata klondike;
 struct FreecellMetadata freecellL;
 } game;
};

Again, of course this requires you to maintain the data, ie when you initialize a struct KlondikeMetadata you must remember to set its gametype field, and so on.