Union initialization in C++ and C

Question

I have built a working C library, that uses constants, in header files defined as

typedef struct Y {   union {     struct bit_field bits;     uint8_t raw[4];   } X; } CardInfo;  static const CardInfo Y_CONSTANT = { .raw = {0, 0, 0, 0 } }; 

I know that the .raw initializer is C only syntax.

How do I define constants with unions in them in a way such that I can use them in C and C++.

Answer 1

I had the same problem. For C89 the following is true:

With C89-style initializers, structure members must be initialized in the order declared, and only the first member of a union can be initialized

I found this explanation at: Initialization of structures and unions

Answer 2

I believe that C++11 allows you to write your own constructor like so:

union Foo {     X x;     uint8_t raw[sizeof(X)];      Foo() : raw{} { } }; 

This default-initializes a union of type Foo with active member raw, which has all elements zero-initialized. (Before C++11, there was no way to initialize arrays which are not complete objects.)