Purpose of the code Member in SDL_UserEvent

You're right, the code member (Sint32 code) in the SDL_UserEvent structure often seems less utilized than data1 and data2, especially when complex data is involved. Its primary purpose is to provide a simple, application-defined integer code associated with a specific user event instance.

Think of it as a secondary way to classify or add simple integer data to an event, supplementing the main event type. Here are its common uses:

1. Sub-Typing or Disambiguation

You might register a single custom event type (e.g., ENTITY_UPDATE) but want to specify different kinds of updates without registering many distinct event types. The code member is perfect for this.

1// Assume ENTITY_UPDATE is a registered event type
2namespace EntityUpdateCode {
3  const Sint32 POSITION_CHANGED = 0;
4  const Sint32 HEALTH_CHANGED = 1;
5  const Sint32 STATE_CHANGED = 2;
6}
7
8// Pushing an event
9void NotifyPositionChanged(Entity* entity) {
10  SDL_Event event;
11  event.type = UserEvents::ENTITY_UPDATE;
12  // Use 'code' to specify the kind of update
13  event.user.code = EntityUpdateCode::POSITION_CHANGED; 
14  // Use 'data1' for the entity pointer
15  event.user.data1 = entity;
16  event.user.data2 = nullptr;
17  SDL_PushEvent(&event);
18}
19
20// Handling the event
21void HandleEvent(SDL_Event& event) {
22  if (event.type == UserEvents::ENTITY_UPDATE) {
23    Entity* entity = static_cast<Entity*>(event.user.data1);
24    // Switch on the 'code' to handle specific update types
25    switch (event.user.code) { 
26      case EntityUpdateCode::POSITION_CHANGED:
27        std::cout << "Entity " << entity->GetId()
28                  << " position changed.\\n";
29        // ... handle position update
30        break;
31      case EntityUpdateCode::HEALTH_CHANGED:
32        std::cout << "Entity " << entity->GetId()
33                  << " health changed.\\n";
34        // ... handle health update
35        break;
36      // ... other cases
37    }
38  }
39}

2. Passing Simple Integer Data

If the only data you need to associate with an event is a single integer (like an entity ID, an item index, a score adjustment, etc.), using the code member can be simpler and potentially more efficient than allocating memory just to store that integer and passing its pointer via data1 or data2.

1// Assume SCORE_CHANGED is a registered event type
2
3// Pushing an event
4void AddScore(int points) {
5  SDL_Event event;
6  event.type = UserEvents::SCORE_CHANGED;
7  event.user.code = points; // Store score directly in code 
8  event.user.data1 = nullptr;
9  event.user.data2 = nullptr;
10  SDL_PushEvent(&event);
11}
12
13// Handling the event
14void HandleEvent(SDL_Event& event) {
15  if (event.type == UserEvents::SCORE_CHANGED) {
16    int scoreChange = event.user.code; 
17    UpdatePlayerScore(scoreChange);
18  }
19}

3. Index or Identifier

It can serve as a simple index into an array or map, or act as a basic identifier related to the event's context.

Comparison to data1/data2

• code: An Sint32 value directly embedded in the event structure. Good for simple integer values, flags, or sub-type identifiers.
• data1/data2: void* pointers. Necessary for pointing to complex objects, strings, heap-allocated memory, or any non-integer data.

Whether you use code depends on your event design. If data1 points to a struct that already contains all necessary information (including any sub-type identifiers or integer values), then code might be redundant. However, it provides a convenient, readily available integer slot directly within the event structure for simpler cases or disambiguation.