What's the best way to handle endianness when working with structs that contain multiple different-sized members?

Question

Ryan McCombe · Accepted Answer

When working with structs containing multiple members of different sizes, we need to handle each member's endianness individually. Here's a structured approach to handling this common scenario:

Basic Approach

First, let's look at a simple struct and how to properly serialize it:

1#include <iostream>
2#include "SDL.h"
3
4struct PlayerData {
5  Uint16 Level;
6  Uint32 Experience;
7  float Health;
8};
9
10void WritePlayerData(SDL_RWops* Handle,
11                     const PlayerData& Data) {
12  SDL_WriteLE16(Handle, Data.Level); 
13  SDL_WriteLE32(Handle, Data.Experience); 
14  SDL_WriteLE32(Handle, Data.Health);      
15}
16
17PlayerData ReadPlayerData(SDL_RWops* Handle) {
18  PlayerData Data;
19  Data.Level = SDL_ReadLE16(Handle); 
20  Data.Experience = SDL_ReadLE32(Handle); 
21  Data.Health = SDL_ReadLE32(Handle);      
22  return Data;
23}
24
25int main() {
26  // Create test data
27  PlayerData TestData{
28    .Level = 42,
29    .Experience = 1000000,
30    .Health = 98.5f
31  };
32
33  // Write to file
34  SDL_RWops* WriteHandle{
35    SDL_RWFromFile("player.dat", "wb")};
36  if (WriteHandle) {
37    WritePlayerData(WriteHandle, TestData);
38    SDL_RWclose(WriteHandle);
39  }
40
41  // Read and verify
42  SDL_RWops* ReadHandle{
43    SDL_RWFromFile("player.dat", "rb")};
44  if (ReadHandle) {
45    PlayerData LoadedData{
46      ReadPlayerData(ReadHandle)};
47    SDL_RWclose(ReadHandle);
48
49    std::cout << "Level: " << LoadedData.Level
50      << "\nExp: " << LoadedData.Experience
51      << "\nHealth: " << LoadedData.Health
52      << '\n';
53  }
54}

1Level: 42
2Exp: 1000000
3Health: 98.5

Advanced Techniques

For more complex structs, you might want to create a serialization helper class:

1#include <iostream>
2#include "SDL.h"
3
4class BinaryWriter {
5  SDL_RWops* Handle;
6
7public:
8  explicit
9    BinaryWriter(SDL_RWops* Handle)
10    : Handle{Handle} {}
11
12  void Write(Uint16 Value) {
13    SDL_WriteLE16(Handle, Value);
14  }
15
16  void Write(Uint32 Value) {
17    SDL_WriteLE32(Handle, Value);
18  }
19
20  void Write(float Value) {
21    SDL_WriteLE32(Handle, Value);
22  }
23};
24
25struct PlayerData {
26  Uint16 Level;
27  Uint32 Experience;
28  float Health;
29
30  void Serialize(BinaryWriter& Writer) {
31    Writer.Write(Level);
32    Writer.Write(Experience);
33    Writer.Write(Health);
34  }
35};
36
37int main() {
38  PlayerData Data{
39    .Level = 42,
40    .Experience = 1000000,
41    .Health = 98.5f
42  };
43
44  SDL_RWops* Handle{
45    SDL_RWFromFile("player.dat", "wb")};
46  if (Handle) {
47    BinaryWriter Writer{Handle};
48    Data.Serialize(Writer); 
49    SDL_RWclose(Handle);
50  }
51}

Key points to remember:

Always handle each member individually with the appropriate endianness function
Keep track of the order of serialization to match when deserializing
Consider padding and alignment in your structs
Use helper classes to make the code more maintainable
Document your serialization format for future maintenance

Byte Order and Endianness

Handling Structs and Endianness

Basic Approach

Advanced Techniques

Byte Order and Endianness

Game Development with SDL2

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