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++.
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
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.)
#ifdef __cplusplus on the right places to make that header file includeable. Ah that might get even more fuzzy.
I decided to choose the following path.
.member initialization.static const struct Foobar initialization of membersInstead declare the global variable:
extern "C" {
extern const struct Foobar foobar;
}
and initialize it in a global section:
struct Foobar foobar = { 0, 0, 0, 0 };
and instead of bugging the C++ compiler with modern ANSI C99 syntax I let the linker do the work be demangling C symbols.
This is my example how to make const union in C99 (also works in C++).
#include <stdio.h>
#include <stdint.h>
typedef union{
uint16_t u16;
uint8_t u8s[sizeof(uint16_t)];
}uint8_16_t;
int main()
{
const uint8_16_t val= {.u8s={0xAA, 0xBB}};
printf("0x%04X", val.u16);
return 0;
}
out: 0xBBAA
The .raw initializer (designated initializer) is added in C++20
// ok for c++20 or higher
static const CardInfo Y_CONSTANT = { .raw = {0, 0, 0, 0 } };
unions by the first element? I.e.static const Y_CONSTANT = {{0,0,0,0}};