CODEWITHSUNDEEP
c++
Jake Petroules
Jake Petroules

Reputation: 24150

Compile-time assertions in C++?

I recently came upon the need to have compile-time assertions in C++ to check that the sizes of two types were equal.

I found the following macro on the web (stated to have come from the Linux kernel):

#define X_ASSERT(condition) ((void)sizeof(char[1 - 2*!!(condition)]))

which I used like so:

X_ASSERT(sizeof(Botan::byte) != sizeof(char));

This gets me curious - although this works, is there a cleaner way to do so? (obviously there's more than one way, as it is) Are there advantages or disadvantages to certain methods?

Upvotes: 5

Views: 512

Answers (5)

TerryP
TerryP

Reputation: 1072

To do it right you need a C++0x friendly compiler, see James McNellis' and Jerry Coffins answers.

You can't do much with the 1998 or 2003 C++ standards. Take a look at these links for examples:

http://en.wikipedia.org/wiki/Assertion_(computing)#Static_assertions http://ksvanhorn.com/Articles/ctassert.html

Upvotes: 1

Ben Zotto
Ben Zotto

Reputation: 71008

Some other interesting options are here: http://www.jaggersoft.com/pubs/CVu11_3.html

Neat reading as the author walks the C (not C++) spec looking for syntax that can be leveraged as compile-time assertions.

Upvotes: 1

Alex Martelli
Alex Martelli

Reputation: 881595

There's an excellent #error preprocessor directive (see here for a good essay about it), but I believe it needs to be within a #if as opposed to being used in a "free-standing" as in your example use.

Upvotes: 0

Jerry Coffin
Jerry Coffin

Reputation: 490108

You might want to take a look at Boost StaticAssert. The internals aren't exactly clean (or weren't the last time I looked) but at least it's much more recognizable, so most people know what it means. It also goes to some pains to produce more meaningful error messages if memory serves.

Upvotes: 6

James McNellis
James McNellis

Reputation: 355049

In C++0x, there is a new language feature, static_assert, which provides a standard way to generate compile-time assertions. For example,

static_assert(sizeof(Botan::byte) != 1, "byte type has wrong size");

Visual C++ 2010 supports static_assert, as do g++ 4.3 (and greater) and Intel C++ 11.0.

Upvotes: 10

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