+ void function (int n) { verify (n < 0); }
+
+ * For the verify macro, the struct verify_type__ will need to
+ somehow be embedded into a declaration. To be portable, this
+ declaration must declare an object, a constant, a function, or a
+ typedef name. If the declared entity uses the type directly,
+ such as in
+
+ struct dummy {...};
+ typedef struct {...} dummy;
+ extern struct {...} *dummy;
+ extern void dummy (struct {...} *);
+ extern struct {...} *dummy (void);
+
+ two uses of the verify macro would yield colliding declarations
+ if the entity names are not disambiguated. A workaround is to
+ attach the current line number to the entity name:
+
+ #define GL_CONCAT0(x, y) x##y
+ #define GL_CONCAT(x, y) GL_CONCAT0 (x, y)
+ extern struct {...} * GL_CONCAT(dummy,__LINE__);
+
+ But this has the problem that two invocations of verify from
+ within the same macro would collide, since the __LINE__ value
+ would be the same for both invocations.
+
+ A solution is to use the sizeof operator. It yields a number,
+ getting rid of the identity of the type. Declarations like
+
+ extern int dummy [sizeof (struct {...})];
+ extern void dummy (int [sizeof (struct {...})]);
+ extern int (*dummy (void)) [sizeof (struct {...})];
+
+ can be repeated.
+
+ * Should the implementation use a named struct or an unnamed struct?
+ Which of the following alternatives can be used?
+
+ extern int dummy [sizeof (struct {...})];
+ extern int dummy [sizeof (struct verify_type__ {...})];
+ extern void dummy (int [sizeof (struct {...})]);
+ extern void dummy (int [sizeof (struct verify_type__ {...})]);
+ extern int (*dummy (void)) [sizeof (struct {...})];
+ extern int (*dummy (void)) [sizeof (struct verify_type__ {...})];
+
+ In the second and sixth case, the struct type is exported to the
+ outer scope; two such declarations therefore collide. GCC warns
+ about the first, third, and fourth cases. So the only remaining
+ possibility is the fifth case:
+
+ extern int (*dummy (void)) [sizeof (struct {...})];
+
+ * This implementation exploits the fact that GCC does not warn about
+ the last declaration mentioned above. If a future version of GCC
+ introduces a warning for this, the problem could be worked around
+ by using code specialized to GCC, e.g.,:
+
+ #if 4 <= __GNUC__
+ # define verify(R) \
+ extern int (* verify_function__ (void)) \
+ [__builtin_constant_p (R) && (R) ? 1 : -1]
+ #endif
+
+ * In C++, any struct definition inside sizeof is invalid.
+ Use a template type to work around the problem. */
+
+
+/* Verify requirement R at compile-time, as an integer constant expression.
+ Return 1. */
+
+# ifdef __cplusplus
+template <int w>
+ struct verify_type__ { unsigned int verify_error_if_negative_size__: w; };
+# define verify_true(R) \
+ (!!sizeof (verify_type__<(R) ? 1 : -1>))
+# else
+# define verify_true(R) \
+ (!!sizeof \
+ (struct { unsigned int verify_error_if_negative_size__: (R) ? 1 : -1; }))
+# endif
+
+/* Verify requirement R at compile-time, as a declaration without a
+ trailing ';'. */
+
+# define verify(R) extern int (* verify_function__ (void)) [verify_true (R)]