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 );