Purpose of Unions in C and C

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I had previously used unions with ease, but when I read this post and learned that this code

union ARGB
{
    uint32_t colour;

    struct componentsTag
    {
        uint8_t b;
        uint8_t g;
        uint8_t r;
        uint8_t a;
    } components;

} pixel;

pixel.colour = 0xff040201;  // ARGB::colour is the active member from now on

// somewhere down the line, without any edit to pixel

if(pixel.components.a)      // accessing the non-active member ARGB::components

is in fact undefinable behaviour 

For example, reading from a union member other than the one who has lately been written to results in undefinable behaviour. 

What is the intended use of unions if not this? 

Could someone kindly provide a detailed explanation?

Jul 13 in C++ by Nicholas
• 5,020 points
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