C++ Equivalent to Designated Initializers?

Question

Recently I've been working on some embedded devices, where we have some structs and unions that need to be initialized at compile time so that we can keep certain things in flash or ROM that don't need to be modified, and save a little flash or SRAM at a bit of a performance cost. Currently the code compiles as valid C99, but without this adjustment it used to compile as C++ code as well, and it would be great to support things being compiled that way as well. One of the key things that prevents this is that we're using C99 designated initializers which do not work within the C subset of C++. I'm not much of a C++ buff, so I'm wondering what simple ways there might be to make this happen in either C++ compatible C, or in C++ that still allow initialization at compile time so that the structs and unions not need be initialized after program startup in SRAM.

One additional point of note: a key reason for designated initializer usage is initalizing as NOT the first member of a union. Also, sticking with standard C++ or ANSI C is a plus in order to maintain compatibility with other compilers (I know about the GNU extensions that provide something like designated initializers without C99).

Answer 1

I'm not sure you can do it in C++. For the stuff that you need to initialize using designated initializers, you can put those separately in a .c file compiled as C99, e.g.:

// In common header file
typedef union my_union
{
    int i;
    float f;
} my_union;

extern const my_union g_var;

// In file compiled as C99
const my_union g_var = { .f = 3.14159f };

// Now any file that #include's the header can access g_var, and it will be
// properly initialized at load time