C++ 这个 function 会导致未定义的行为吗？

Question

Hello i think this is causing an undefined behavior but not sure

PlayerMoving *unpackPlayerMoving(BYTE *data)
{
    PlayerMoving *dataStruct = new PlayerMoving;
    dataStruct->packetType = *(int *)(data);
    dataStruct->netID = *(int *)(data + 4);
    dataStruct->characterState = *(int *)(data + 12);
    dataStruct->plantingTree = *(int *)(data + 20);
    dataStruct->x = *(float *)(data + 24);
    dataStruct->y = *(float *)(data + 28);
    dataStruct->XSpeed = *(float *)(data + 32);
    dataStruct->YSpeed = *(float *)(data + 36);
    dataStruct->punchX = *(int *)(data + 44);
    dataStruct->punchY = *(int *)(data + 48);
    return dataStruct;
}

does it cause it and if it does how could i fix it?

this is playermoving

struct PlayerMoving
{
    int packetType;
    int netID;
    float x;
    float y;
    int characterState;
    int plantingTree;
    float XSpeed;
    float YSpeed;
    int punchX;
    int punchY;
    int secondnetID;
};

and byte is

typedef unsigned char BYTE;

Answer 1

The behaviour of your code is undefined.

Even if BYTE is an unsigned char type ( unsigned char is an exception to the strict aliasing rule), there is no guarantee that

*(int *)(data + 4);

and so on meets the alignment requirements for an int .

Your best bet is to memcpy the data from the BYTE array to each structure member, and trust the compiler to optimise (check the generated assembly).

Answer 2

The code causes undefined behavior if any of the following assumptions are violated:

• BYTE is char or unsigned char
• none of your array access go out of bounds
• your implementation allows unaligned accesses, OR the required alignments of int and float do not exceed 4 bytes AND your array starts at an appropriately aligned address
• no bit patterns are invalid for int and float on your platform, OR these patterns do not appear in your array

You shouldn't return an allocated raw pointer though. Use unique_ptr .