One of the features of C++ is the ability to declare a union, without specifying a name. Here’s an example:
1 2 3 4 | union { int a; char b;}; |
(yes, they can be useful, and I’ll give an example shortly). The standard [class.union.anon / 12.3.1] states the following limitations:
- Names of members of the anonymous union need to be distinct from any other in the scope of the declared union.
- Each variable declared in the union will have the same address (C++14)
- Anonymous unions can’t contain member functions
- all members are public
The main effect is that the members of a union are defined in scope after the anonymous union is defined. The members may be referenced as though they were any other variable in scope. For example:
1 2 3 4 5 6 7 8 | int c;union { int a; char b;};a = 1;c = 2; |
Where it is useful
Tagged unions are an interesting use-case:
1 2 3 4 5 6 7 8 | Class Tagged{ union { int a; char b; }; enum { INT_USED = 1, CHAR_USED=2 } tag;}; |