Size of implementation specific std::mbstate_t

Question

The docs on this are rather lacking so I'm hoping the community can run a simple test and post results here so that I, and anybody else, has a reference.

#include <cwchar>
sizeof( std::mbstate_t );

If you could post the results here and also mention which compiler you are using, I would be very grateful.

On VS2010 it's declared as typedef int mbstate_t; and it's size is 4 bytes for both 32 and 64 bit builds.

I'm asking this because mbstate_t is a member of streampos. I need to use this member to store the conversion state of an encoding. The minimum space I can get away with is 3 bytes so I need to know if any implementation is going to break my code.

Thanks in advance.

Answer 1

gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 on x86_64

size = 8

gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3 on armv7l

size = 8

Answer 2

You just want know the results of the sizeof?

Qt 5.1 with GCC x86 32bit under Debian:

size = 8

Answer 3

From the C11 specification (7.29.1/2):

   mbstate_t

which is a complete object type other than an array type that can hold the conversion state information necessary to convert between sequences of multibyte characters and wide characters;

So while I was wrong in that is can be an array, it could be anything else (including a structure containing an array). The language in the specification doesn't say anything about how it should be implemented, just that it's "a complete object type other than an array type".

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From the C++11 specification (multiple places, for example 21.2.3.1/4):

The type mbstate_t is defined in <cwchar> and can represent any of the conversion states that can occur in an implementation-defined set of supported multibyte character encoding rules.

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In conclusion, you can not rely on mbstate_t being an integer type, or of a specific size, if you want to be portable. If you want to be portable, you have to let the standard library manage the state for you.