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I have been trying to implement a method similar to static_assert which is defined in the C++11 standard. The main problem is how does the C++ compiler write the text message being passed to static_assert as a const char*? I can get the compiler to write a message like A_is_not_POD. This is what I have:

#define MY_STATIC_ASSERT(condition, name) \ typedef char name[(condition) ? 1 : -1];

But it would be quite nice to get the compiler to write something like "Error: A is not POD." Any suggestions?

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    No that's not possible. The only thing you can get with older standards, is to spit out variable names. Commented Dec 6, 2013 at 14:12
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    “I can get the compiler to write a message like A_is_not_POD” How about showing what you have already done to achieve that? It would make your question clearer. The FAQ used to say “the best questions have a bit of source code in them” before it was re-revamped. Commented Dec 6, 2013 at 14:14
  • That's a shame. But where can we find the source code of static_assert function? The interesting thing is how does the compiler write the string to the compiler message. I have been trying to use templates somehow but have been unsuccessful so far. Commented Dec 6, 2013 at 14:14
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    @Shubham C++11's static_assert is not a function - it's a keyword, just like for. There's no source code for for either. That's why the compiler can do anything it really wants with the arguments. Commented Dec 6, 2013 at 14:17
  • I would guess it's impossible, since Boost's static assert can't print messages in C++03. Presumably, they tried pretty hard to find a way; and if it's impossible in C++03, it's almost certainly impossible in C99. Commented Dec 6, 2013 at 14:51

2 Answers 2

Reset to default
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Not sure i understand question, but C11 have _Static_assert(condition, errmessage). In C99 this functionality was missing but, depending on compiler, it could be possible to emulate. E.g. for gcc (unfortulately clang doesn't support attribute(error))

#define MY_STATIC_ASSERT(cnd, descr) ({ \ extern int __attribute__ ((error("static assert failed: (" #cnd ") (" #descr ")"))) \ compile_time_check(void); \ ((cnd) ? 0 : compile_time_check()), 0; \ })
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That's really nice. I didn't know we could do something like this!!! Its a shame clang doesn't provide this feature. Will however, will have a look around. Thanks a lot!!!
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In the c99 standard there is no "official" way to perform a static assertion in your C++ compiler.

"The main problem is how does the C++ compiler write the text message being passed to static_assert as a const char*?"

The C++ compiler detects an error in the code and prints out an appropriate error message based on a standard list of messages it has for each error that is known it can encounter. In c99, the compiler doesn't know what a "static assert error" is, so you need to cause some other error.

However, because creating static asserts with a c99 compiler is kind of a hack, it is not capable of printing out a nice error message exactly how you want.

For your example,

#define MY_STATIC_ASSERT(condition, name) \ typedef char name[(condition) ? 1 : -1]; MY_STATIC_ASSERT(false, my_error_msg)

will trigger the "error: size of array ‘my_error_msg’ is negative" message in the gcc compiler (and should a similar message in other compilers, you hope!). Giving the array an error message for the name was a way to print out your own info. There are various other techniques/hacks you can do on purpose such as bad templates, enums link

Note: you can provide custom compiler messages prior to C++11 using pre-processor macros such as #error or #pragma. However pre-process time is not the same as compile-time! The the pre-processor has a limited ability to evaluate many expressions and keywords such as "if", "sizeof", "return" and so on have no meaning to the pre-processor, only the compiler. link with some overview

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