I was reading source code this morning and came across this curious line in a header file:
JS_STATIC_ASSERT(sizeof(JSGCThing) >= sizeof(JSString));
The effect of this macro is to check at compile time that sizeof(JSGCThing) >= sizeof(JSString). If the condition is not met, compilation fails with an error.
The puzzle is: how does this work? A hint is in the comments.
1 comment:
Hint: The error is not generated with #error; macros can't do that, as far as I know.
Hint: How else might a macro trigger a compile-time error?
Hint: Is there any other conceivable place in a C header file where an expression is allowed, other than preprocessor directives like #if?
Answer: See for yourself -- it's implemented on line 70 of this header file:
http://lxr.mozilla.org/seamonkey/source/js/src/jsutil.h#70
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