#ifdef NDEBUG
#define assert_or_show(a,b) ((void)0)
#else
#define assert_or_show(a,b) ((a) ? (void)0 : \
wsprintf(g_sz, "file %s, line %d: (%s), %d", __FILE__, __LINE__, #a, b), \
MessageBox(NULL, g_sz, NULL, 0))
#endif

char g_sz[1024];
assert_or_show(x >= 0 && x < WIDTH, x)