Compile-time assert on datatype sizes

Question

I would like to perform a compile-time check on datatype sizes in a C/C++ project, and error on unexpected mismatches. Simple

#if sizeof foo_t != sizeof bar_t

does not compile - claims that sizeof is not a proper compile-time constant.

The desired scope of platforms - at the very least Visual C++ with Win32/64, and GCC on x86/amd64.

EDIT: compile-time, not necessarily preprocessor. Just not a run-time error.

EDIT2: the code assumes that wchar_t is 2 bytes. I want a compilation error if it's accidentally compiled with 4-byte wchar's.

How is the compiler supposed to know the size of a complex structure at the level of a preprocessor? There are probably some that could manage it if they want, but it seems definitely outside the scope of a #if. — Mark Ransom
– Mark Ransom, Commented Dec 5, 2011 at 16:27
It is better if you tell us why you want this. Because in this form you wouldn't get answer which could solve this. Preprocessor cannot evaluate sizeof. It is just text-replacement thing. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Dec 5, 2011 at 16:29
If an error is enough, see stackoverflow.com/questions/1980012/… for a solution valid for C and C++, if you want to do more complex handling, I agree with Mark, the pp can't have the needed information excepted in special cases. — AProgrammer
– AProgrammer, Commented Dec 5, 2011 at 16:35
@AProgrammer: the negative-size array trick sounds the neatest. Make an answer so that I can a +1. — Seva Alekseyev
– Seva Alekseyev, Commented Dec 5, 2011 at 16:55

111111 · Accepted Answer · 2011-12-05 16:38:49Z

10

in C++11 you can use static assert

static_assert(sizeof(foo_t) == sizeof(bar_t), "sizes do not match");

If it is pre C++11 then you can use boost static assert macro

http://www.boost.org/doc/libs/1_48_0/doc/html/boost_staticassert.html

BOOST_STATIC_ASSERT(sizeof(int)==sizeof(unsigned)); BOOST_STATIC_ASSERT_MSG(sizeof(int)==sizeof(unsigned), "sizes do not match");

edited Dec 5, 2011 at 16:38

answered Dec 5, 2011 at 16:27

111111

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3 Comments

111111 Over a year ago

He asked for compile time, besides does the preprocessor even have that information?

interjay Over a year ago

@Nawaz So what? It will give an error if the size doesn't match, which is exactly what the question asks.

111111 Over a year ago

Nawaz: To be honest it doesn't really matter how it is done, however the less that is done with the preprocessor and the more that is done with standardized language features the better.

ConfusedSushi · Accepted Answer · 2011-12-05 20:42:46Z

You have two options:

a) static_assert of C++11

b) BOOST_STATIC_ASSERT of boost

I would prefer the first one.

Edit:

The preprocessor is not really part of the language, as the name says it pre-processes a file, it has no knowledge of the language, so it does not know sizeof.

You could use some template to do some compile time code generation, for example:

template <typename T, bool x = sizeof(T) == 4> class X; template <typename T> class X<T, true> { T v; const char* msg() const { return "My size is 4"; } } template <typename T> class X<T, false> { T v; const char* msg() const { return "My size is NOT 4"; } } X<int> a; X<short> b;

Aaron McDaid · Accepted Answer · 2011-12-05 21:42:10Z

If you can't use C++11 or Boost, then you might find this useful:

template <typename A, typename B> struct MustBeSameSize { int c[sizeof(A)-sizeof(B)]; int d[sizeof(B)-sizeof(A)]; }; template struct MustBeSameSize<int, int>;

That will only compile if and only if the sizeof the two types is identical. If they are different like this:

template struct MustBeSameSize<char, int>;

then you'll get a compile-type error, but it won't be a very readable error; maybe something like (g++ 4.4.3):

error: overflow in array dimension

~~This works because any modern compiler should allow zero-length arrays, but not negative-length arrays.~~

This works for me, and I think G++ has allowed zero-length arrays for some time. But I'm not sure how portable this is. C99 allows flexible array members (i.e. unspecified size), but I don't think that's directly relevant. In short, if you need something portable, use C++11 or use Boost.

You sure about the latter bit? Um, Visual C++ 2010 does not. At least not on function level (i. e. NOT inside a struct).
@SevaAlekseyev, I'm not sure I understand. Are you confirming that VC++2010 does allow zero-length arrays in structs, and also saying that it does not allow them as function parameters?
I'm saying that void f(){char x[0];} yields compile error C2466: "cannot allocate an array of constant size 0".
You could add a "+1" to each array size to make it work with everything larger than a char. (assuming no odd-sized fundamental types larger than a char)

TheBeardyMan · Accepted Answer · 2012-01-19 14:48:29Z

You could define a compile time assert macro like this:

#define COMPILE_TIME_ASSERT( x ) \ switch ( x ) \ { \ case false: \ break; \ case ( x ): \ break; \ }

If the expression is false, you will get a duplicate case label error.

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