CODEWITHSUNDEEP
c++c
michael_s
michael_s

Reputation: 2575

Check in C++ that a struct is well aligned or contains gaps

I have a problem that a struct shall be checked - at compile time - if it is well aligned or if it contains gaps. The checking may be done in additional test code, but I don't want "packed" data in the real implementation code.

This is an example header file (MyData.h) with the typical include guards:

#ifndef MYDATA_H_
#define MYDATA_H_

struct uneven
{
  int bla_u32;
  short bla_u16;
  char bla_u8;
  /* <-- this gap will be filled in the unpacked version */
};

#endif // MYDATA_H

I found one possible solution - see below.

Questions:

Upvotes: 13

Views: 1731

Answers (3)

Dutow
Dutow

Reputation: 5668

You could use a function instead of a namespace (on ideone):

This solution also works in C

Header File:

typedef struct
{
  int bla_u32;
  short bla_u16;
  char bla_u8;

  /* <-- this gap will be filled in the unpacked version */
}  uneven;

Source File:

#include "MyData.h"

#define StaticAssert(cond, msg) switch(0){case 0:case cond:;}

void checkSizes()
{
  uneven unpacked_uneven;
#pragma pack(push, 1)
  #undef MYDATA_H_ // force re-including "MyData.h"
  #include "MyData.h"
#pragma pack(pop)
  uneven packed_uneven;
  StaticAssert(sizeof(unpacked_uneven) == sizeof(packed_uneven), "uneven contains gaps");
}

You can place your StaticAssert into the function for a compile time error.

Upvotes: 4

eerorika
eerorika

Reputation: 238461

A compiler specific solution that works for both C and C++: GCC has a warning option -Wpadded, that produces a warning for every definition that change size due to alignment.

Upvotes: 5

michael_s
michael_s

Reputation: 2575

I found one (somehow nasty and very tricky) solution for the problem that only works with C++, not C.

#define StaticAssert(cond, msg) switch(0){case 0:case cond:;}

#pragma pack(push, 1)
namespace packed
{
#include "MyData.h"
}
#pragma pack(pop)

#undef MYDATA_H_ // force re-including "MyData.h"
#include "MyData.h"

void checkSizes()
{
  StaticAssert(sizeof(packed::uneven) == sizeof(uneven), "uneven contains gaps");
}

This StaticAssertmacro fails for the given uneven struct data - as the packed version's size is of 7 bytes and the unpacked (normal) version is 8 bytes. If an additional charis added at the end of the struct the test succeeds - both versions have 8 bytes then.

Upvotes: 2

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