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

You might also try to check the output of expanded macros. If you are using gcc as a compiler, gcc -E <filename> >> full_src.txt' shall help. More details are here: Seeing expanded C macros

Answer 2

You might like the undef-free version of X_Macros,
it reduces the necessary care taken with defining and undefining around each use
and is better suited for definition in a header und useage in multiple code files:

#define LIST_OF_STRUCT_MEMBERS_foo(mode) \  
    X_##mode(a) \  
    X_##mode(b) \  
    X_##mode(c)  

#define X_struct(name) int name;  
#define X_list(name) -1,  
#define foo_DEFAULT_VALUE  { LIST_OF_STRUCT_MEMBERS_foo(list) }  

struct foo {  
     LIST_OF_STRUCT_MEMBERS_foo(struct)  
};  

static inline void foo_invalidate(struct foo* in) {  
     *in = (struct foo){  
     LIST_OF_STRUCT_MEMBERS_foo(list)  
     };  
}  

static struct foo test = foo_DEFAULT_VALUE;  

Output (gcc -E):

struct foo {
     int a; int b; int c;
};

static inline void foo_invalidate(struct foo* in) {
     *in = (struct foo){
     -1, -1, -1,
     };
}

static struct foo test = { -1, -1, -1, };

Answer 3

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.

---

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, };