mikeash.com pyblog/friday-qa-2011-04-15-compile-time-tips-and-tricks.html comments

mikeash - 2011-04-18 15:12:25

Mon, 18 Apr 2011 15:12:25 GMT

Trevor: Yes, either that or #if !MYTYPE_DEFINED. I went with the latter, but in any case it's fixed now. Thanks!

Aidan Steele - 2011-04-17 22:21:54

Sun, 17 Apr 2011 22:21:54 GMT

I quite like the following macros:

#define likely(x)       __builtin_expect(!!(x), 1)

#define unlikely(x)     __builtin_expect(!!(x), 0)

Trevor - 2011-04-17 20:33:05

Sun, 17 Apr 2011 20:33:05 GMT

#if MYTYPE_DEFINED

Shouldn't that be

#ifndef MYTYPE_DEFINED

Jérôme Noël - 2011-04-16 23:52:34

Sat, 16 Apr 2011 23:52:34 GMT

We use a combination of Chris and Jesper macro for compile-time checks:



#define  lang_CHECK_CST(name, cond)    \

    typedef int lang_CheckCst_##name##_##__LINE__ [(cond) ? 1 : -1]

It has a meaningful name but uses a typedef which avoids the declaration of externs.

Dave Zarzycki - 2011-04-16 12:06:42

Sat, 16 Apr 2011 12:06:42 GMT

Here is a nice trick for making a generic assert that dynamically becomes a static assert when possible:

#define my_assert(e) do { \
                if (__builtin_constant_p(e)) { \
                        extern char __compile_time_assert__[(bool)(e) ? 1 : -1]; \
                } else { \
                        assert(e); \
                } \
        } while (0)

Chris Suter - 2011-04-15 22:01:32

Fri, 15 Apr 2011 22:01:32 GMT

Here’s the macro I use:

#define CHECK__(line, x) typedef char check_##line##_[(x) ? 1 : -1];

#define CHECK_(line, x) CHECK__(line, x)

#define CHECK(x) CHECK_(__LINE__, (x))

Jesper Särnesjö - 2011-04-15 16:53:55

Fri, 15 Apr 2011 16:53:55 GMT

That last tip about compile-time asserts is a good one. I like to wrap it up in macro, to save on typing and improve readability somewhat:

#define STATIC_ASSERT(cond, message_var_name) \

  extern char static_assert_##message_var_name[(cond) ? 1 : -1]



STATIC_ASSERT(sizeof(void *) == 8, 64_bit_pointers);

STATIC_ASSERT(sizeof(int) == 4, 32_bit_ints);