Why is this nested macro replacement failing?

Question

I am trying to apply the X Macro concept, in order to have the possibility to initialize all struct members to a custom default (invalid) value. I write the following code:<

Answer 1

Let's pretend that we are the preprocessor and encountering the line:

static struct foo test = foo_DEFAULT_VALUE;

Pass 1:

static struct foo test = { LIST_OF_STRUCT_MEMBERS_foo };

Pass 2:

static struct foo test = { X(a) X(b) X(c) };

Pass 3: Nothing to expand as X is undefined on this line.

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One workaround could be defining a const variable (possibly but not necessarily static) to be used as default value:

#define X(name) -1,
static const struct foo foo_DEFAULT_VALUE = { LIST_OF_STRUCT_MEMBERS_foo };
#undef X

Which generates:

static const struct foo foo_DEFAULT_VALUE = { -1, -1, -1, };