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Disgusting C-ism

I wrote some code that makes some assumptions about the length of int when converted to a string.  This is in code that is strictly internal and is only ever called by internal code in very controlled circumstances.  Still, when I wrote that assumption, it made me a little bit itchy.  I asked myself a question I frequently ask, "what can I do to make sure that the compiler will catch an error for me?"

Essentially, what I wanted was to see if sizeof(int) == 4 like this:

#if sizeof(int) == 4
#error Call Steve - his assumption on sizeof(int) was wrong and this code will fail

Actually - it's better than that - the code has enough information to make it clear how to fix the code, should this arise.

I could do a runtime check wherein I count the number of bits/bytes in an int and use that, but I'd rather make this a compiler check.

I could also use:

#error Call Steve...

but I found this little gem:

#define CCASSERT(predicate) _x_CCASSERT_LINE(predicate, __LINE__)
#define _x_CCASSERT_LINE(predicate, line) \
typedef char constraint_violated_on_line_##line[2*((predicate)!=0)-1];

This defines a new type which is an array of a length that depends on the value of a predicate and will generate an illegal type on a false predicate.  Nice.

the usage in my case is:

CCASSERT(sizeof(int) == 4)

which stops the compiler cold. 

Published Monday, October 29, 2007 4:27 PM by Steve Hawley


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