Real-world use of X-Macros

Question

I just learned of X-Macros. What real-world uses of X-Macros have you seen? When are they the right tool for the job?

Answer 1

X-Macros are essentially parameterized templates. So they are the right tool for the job if you need several similar things in several guises. They allow you to create an abstract form and instantiate it according to different rules.

I use X-macros to output enum values as strings. And since encountering it, I strongly prefer this form which takes a "user" macro to apply to each element. Multiple file inclusion is just far more painful to work with.

/* x-macro constructors for error and type
   enums and string tables */
#define AS_BARE(a) a ,
#define AS_STR(a) #a ,

#define ERRORS(_) \
    _(noerror) \
    _(dictfull) _(dictstackoverflow) _(dictstackunderflow) \
    _(execstackoverflow) _(execstackunderflow) _(limitcheck) \
    _(VMerror)
enum err { ERRORS(AS_BARE) };
char *errorname[] = { ERRORS(AS_STR) };
/* puts(errorname[(enum err)limitcheck]); */

I'm also using them for function dispatch based on object type. Again by hijacking the same macro I used to create the enum values.

#define TYPES(_) \
    _(invalid) \
    _(null) \
    _(mark) \
    _(integer) \
    _(real) \
    _(array) \
    _(dict) \
    _(save) \
    _(name) \
    _(string) \
/*enddef TYPES */

#define AS_TYPE(_) _ ## type ,
enum { TYPES(AS_TYPE) };

Using the macro guarantees that all my array indices will match the associated enum values, because they construct their various forms using the bare tokens from the macro definition (the TYPES macro).

typedef void evalfunc(context *ctx);

void evalquit(context *ctx) { ++ctx->quit; }

void evalpop(context *ctx) { (void)pop(ctx->lo, adrent(ctx->lo, OS)); }

void evalpush(context *ctx) {
    push(ctx->lo, adrent(ctx->lo, OS),
            pop(ctx->lo, adrent(ctx->lo, ES)));
}

evalfunc *evalinvalid = evalquit;
evalfunc *evalmark = evalpop;
evalfunc *evalnull = evalpop;
evalfunc *evalinteger = evalpush;
evalfunc *evalreal = evalpush;
evalfunc *evalsave = evalpush;
evalfunc *evaldict = evalpush;
evalfunc *evalstring = evalpush;
evalfunc *evalname = evalpush;

evalfunc *evaltype[stringtype/*last type in enum*/+1];
#define AS_EVALINIT(_) evaltype[_ ## type] = eval ## _ ;
void initevaltype(void) {
    TYPES(AS_EVALINIT)
}

void eval(context *ctx) {
    unsigned ades = adrent(ctx->lo, ES);
    object t = top(ctx->lo, ades, 0);
    if ( isx(t) ) /* if executable */
        evaltype[type(t)](ctx);  /* <--- the payoff is this line here! */
    else
        evalpush(ctx);
}

Using X-macros this way actually helps the compiler to give helpful error messages. I omitted the evalarray function from the above because it would distract from my point. But if you attempt to compile the above code (commenting-out the other function calls, and providing a dummy typedef for context, of course), the compiler would complain about a missing function. For each new type I add, I am reminded to add a handler when I recompile this module. So the X-macro helps to guarantee that parallel structures remain intact even as the project grows.

Edit:

This answer has raised my reputation 50%. So here's a little more. The following is a negative example, answering the question: when not to use X-Macros?

This example shows the packing of arbitrary code fragments into the X-"record". I eventually abandoned this branch of the project and did not use this strategy in later designs (and not for want of trying). It became unweildy, somehow. Indeed the macro is named X6 because at one point there were 6 arguments, but I got tired of changing the macro name.

/* Object types */
/* "'X'" macros for Object type definitions, declarations and initializers */
// a                      b            c              d
// enum,                  string,      union member,  printf d
#define OBJECT_TYPES \
X6(    nulltype,        "null",     int dummy      ,            ("")) \
X6(    marktype,        "mark",     int dummy2      ,           ("")) \
X6( integertype,     "integer",     int  i,     ("%d",o.i)) \
X6( booleantype,     "boolean",     bool b,     (o.b?"true":"false")) \
X6(    realtype,        "real",     float f,        ("%f",o.f)) \
X6(    nametype,        "name",     int  n,     ("%s%s", \
        (o.flags & Fxflag)?"":"/", names[o.n])) \
X6(  stringtype,      "string",     char *s,        ("%s",o.s)) \
X6(    filetype,        "file",     FILE *file,     ("",(void *)o.file)) \
X6(   arraytype,       "array",     Object *a,      ("",o.length)) \
X6(    dicttype,        "dict",     struct s_pair *d, ("",o.length)) \
X6(operatortype,    "operator",     void (*o)(),    ("")) \

#define X6(a, b, c, d) #a,
char *typestring[] = { OBJECT_TYPES };
#undef X6

// the Object type
//forward reference so s_object can contain s_objects
typedef struct s_object Object;

// the s_object structure:
// a bit convoluted, but it boils down to four members:
// type, flags, length, and payload (union of type-specific data)
// the first named union member is integer, so a simple literal object
// can be created on the fly:
// Object o = {integertype,0,0,4028}; //create an int object, value: 4028
// Object nl = {nulltype,0,0,0};
struct s_object {
#define X6(a, b, c, d) a,
    enum e_type { OBJECT_TYPES } type;
#undef X6
unsigned int flags;
#define Fread  1
#define Fwrite 2
#define Fexec  4
#define Fxflag 8
size_t length; //for lint, was: unsigned int
#define X6(a, b, c, d) c;
    union { OBJECT_TYPES };
#undef X6
};

One big problem was the printf format strings. While it looks cool, it's just hocus pocus. Since it's only used in one function, overuse of the macro actually separated information that should be together; and it makes the function unreadable by itself. The obfuscation is doubly unfortunate in a debugging function like this one.

//print the object using the type's format specifier from the macro
//used by O_equal (ps: =) and O_equalequal (ps: ==)
void printobject(Object o) {
    switch (o.type) {
#define X6(a, b, c, d) \
        case a: printf d; break;
OBJECT_TYPES
#undef X6
    }
}

So don't get carried away. Like I did.