Purpose of Unions in C and C++

Question

I have used unions earlier comfortably; today I was alarmed when I read this post and came to know that this code

union ARGB
{
    uint32_t colour;

    str         


        

Answer 1

As you say, this is strictly undefined behaviour, though it will "work" on many platforms. The real reason for using unions is to create variant records.

union A {
   int i;
   double d;
};

A a[10];    // records in "a" can be either ints or doubles 
a[0].i = 42;
a[1].d = 1.23;

Of course, you also need some sort of discriminator to say what the variant actually contains. And note that in C++ unions are not much use because they can only contain POD types - effectively those without constructors and destructors.

Answer 2

In C++, Boost Variant implement a safe version of the union, designed to prevent undefined behavior as much as possible.

Its performances are identical to the enum + union construct (stack allocated too etc) but it uses a template list of types instead of the enum :)

Answer 3

In C it was a nice way to implement something like an variant.

enum possibleTypes{
  eInt,
  eDouble,
  eChar
}


struct Value{

    union Value {
      int iVal_;
      double dval;
      char cVal;
    } value_;
    possibleTypes discriminator_;
} 

switch(val.discriminator_)
{
  case eInt: val.value_.iVal_; break;

In times of litlle memory this structure is using less memory than a struct that has all the member.

By the way C provides

    typedef struct {
      unsigned int mantissa_low:32;      //mantissa
      unsigned int mantissa_high:20;
      unsigned int exponent:11;         //exponent
      unsigned int sign:1;
    } realVal;

to access bit values.

Answer 4

In the C language as it was documented in 1974, all structure members shared a common namespace, and the meaning of "ptr->member" was defined as adding the member's displacement to "ptr" and accessing the resulting address using the member's type. This design made it possible to use the same ptr with member names taken from different structure definitions but with the same offset; programmers used that ability for a variety of purposes.

When structure members were assigned their own namespaces, it became impossible to declare two structure members with the same displacement. Adding unions to the language made it possible to achieve the same semantics that had been available in earlier versions of the language (though the inability to have names exported to an enclosing context may have still necessitated using a find/replace to replace foo->member into foo->type1.member). What was important was not so much that the people who added unions have any particular target usage in mind, but rather that they provide a means by which programmers who had relied upon the earlier semantics, for whatever purpose, should still be able to achieve the same semantics even if they had to use a different syntax to do it.