联合与比特屏蔽和比特移位

Question

What are the disadvantages of using unions when storing some information like a series of bytes and being able to access them at once or one by one.

Example : A Color can be represented in RGBA. So a color type may be defined as,

typedef unsigned int RGBAColor;

Then we can use "shifting and masking" of bits to "retrieve or set" the red, green, blue, alpha values of a RGBAColor object ( just like it is done in Direct3D functions with the macro functions such as D3DCOLOR_ARGB() ).

But what if I used a union,

union RGBAColor
{
unsigned int Color;
struct RGBAColorComponents
{
    unsigned char Red;
    unsigned char Green;
    unsigned char Blue;
    unsigned char Alpha;
} Component;
};

Then I will not be needing to always do the shifting ( << ) or masking ( & ) for reading or writing the color components. But is there problem with this? ( I suspect that this has some problem because I haven't seen anyone using such a method. )

Can Endianness Be a broblem? If we always use Component for accessing color components and use Color for accessing the whole thing ( for copying, assigning, etc.. as a whole ) the endianness should not be a problem, right?

-- EDIT -- I found an old post which is the same problem. So i guess this question is kinda repost :P sorry for that. here is the link : Is it a good practice to use unions in C++?

According to the answers it seems that the use of unions for the given example is OK in C++. Because there is no change of data type in there, its just two ways to access the same data. Please correct me if i am wrong. Thanks. :)

Answer 1

This usage of unions is illegal in C++, where a union comprises overlapping, but mutually exclusive objects. You are not allowed to write one member of a union, then read out another member.

It is legal in C where this is a recommended way of type punning.

This relates to the issue of (strict) aliasing, which is a difficulty faced by the compiler when trying to determine whether two objects with different types are distinct. The language standards disagree because the experts are still figuring out what guarantees can safely be provided without sacrificing performance. Personally, I avoid all of this. What would the int actually be used for? The safe way to translate is to copy the bytes, as by memcpy .

There is also the endianness issue, but whether that matters depends on what you want to do with the int .

Answer 2

I believe using the union solves any problems related to endianness, as most likely the RGBA order is defined in network order. Also the fact that each component will be uint8_t or such, can help some compilers to use sign/zero extended loads, storing the low 8 bits directly to a nonaligned byte pointer and being even able to parallelize some byte operations (eg arm has some packed 4x8 bit instructions).