Question

Can you define a macro that accesses a normal variable, but in a read-only fashion (other than defining it as a call to a function)? For example, can the VALUE macro in the following code be defined in such a way that the dostuff() function causes a compile error?

struct myobj {   int value; }  /* This macro does not satisfy the read-only requirement */ #define VALUE(o) (o)->value  /* This macro uses a function, unfortunately */ int getvalue(struct myobj *o) { return o->value; } #define VALUE(o) getvalue(o)  void dostuff(struct myobj *foo) {    printf('The value of foo is %d.\n', VALUE(foo)); /* OK */    VALUE(foo) = 1; /* We want a compile error here */    foo->value = 1; /* This is ok. */ } 

Answer 1

If the variable is always numeric, this works:

#define VALUE(x) (x+0) 

or in the context of your example,

#define VALUE(x) (x->value+0)