How to check the size of a structure at compile time?

Question

I want to add code that during compilation checks the size of a structure to make sure that it is a predefined size. For example I want to make sure that the size of this structure is 1024 byte when I am porting this code or when I am adding/removing items from structure during compile time:

#pack(1)
struct mystruct
{
    int item1;
    int item2[100];
    char item3[4];
    char item5;
    char padding[615];
 }

I know how to do this during run time by using a code such as this:

 if(sizeof(mystruct) != 1024)
 { 
     throw exception("Size is not correct");
 }

But it is a waste of processing if I do this during run time. I need to do this during compile time.

How can I do this during compilation?

Answer 1

You can check the size during compilation:

static_assert (sizeof(mystruct) == 1024, "Size is not correct");

You need C++11 for that. Boost has a workaround for pre-c++11 compilers:

BOOST_STATIC_ASSERT_MSG(sizeof(mystruct) == 1024, "Size is not correct");

See the documentation.

Answer 2

If you don't have C++11 or Boost, you could try this:

typedef char assertion_on_mystruct[(   sizeof(mystruct)==1024   )*2-1 ];

If the statement is false, then this typedefs an array type with negative size, and your compiler should give an error message. If true, then the size will be one, a valid size. For example, g++ gives:

template.cpp:10:70: error: size of array ‘assertion_on_mystruct’ is negative

I admit it's not the most useful thing, because it only tells you the line number of the error. But it is the simplest, stand-alone, technique I can think of.

A more general macro is:

#define DUMB_STATIC_ASSERT(test) typedef char assertion_on_mystruct[( !!(test) )*2-1 ]

DUMB_STATIC_ASSERT( sizeof(mystruct)==1024 );
DUMB_STATIC_ASSERT( sizeof(my_other_struct)==23 );
DUMB_STATIC_ASSERT( sizeof(minimum_size_struct) >= 23 );

Answer 3

From C++11 you have static_assert which is handled on compilation:

static_assert(sizeof(mystruct) == 1024, "Size is not correct");

If the size is not 1024 bytes, you will get a compilation error.

Answer 4

If you want to check it at compile time you may use template metaprogramming phase.

In standard C++ you have boost's static assert, which is hidden by a macro BOOST_STATIC_ASSERT. You would use it in the following way:

#include <boost/static_assert.hpp>
...
BOOST_STATIC_ASSERT(sizeof(mystruct) == 1024);

The above code will fail to compile if the assertion is not met, with some semi-readable error message.

In C++11 you get simpler functionality with static assertions which introduces a new keyword static_assert.

static_assert(sizeof(mystruct) == 1024,"Size is not correct");

---

Doing the same thing at preprocessor phase cannot be done but it seems not to be really needed in your case.

Answer 5

Please note that padding is included in sizeof():

struct A { int i; bool b; };

typedef char assertion_on_A[( ((sizeof(A)))== 8 )*2-1 ];
static_assert(sizeof(A) == 8, "sizeof A");

Both the typedef and the static_assert expect size 8 here.