Compile time check for usage of #pragma pack

Question

Most compiliers support changing the packing of a class by using a #pragma pack(N) directive, where N is the new minimum acceptable alignment for each member.

Is it possible to check at compile-time whether or not a #pragma pack(N) has been specified. Additionally, is there a way to determine N?

Answer 1

You cannot test the struct packing directly, instead you have to create a test structure and check its size:

struct Test_Pack_Struct {
    unsigned char   bVal;
    __int64         lVal;
};
#define GetStructPacking()  (sizeof(Test_Pack_Struct)-8)

At compile time you may check the apprriate size with a static assert (requires C++ 11 or higher), for example:

static_assert( GetStructPacking() == 4, "Error: 4 byte packing assumed" );

At runtime you can assign the value of GetStructPacking macro to a variable or use it in expressions:

int iPacking = GetStructPacking()

Keep in mind, that the size of the Test_Pack_Struct structure depends on the posistion where it is defined (headers, code files etc).

A drawback is that, if you want to make several packing checks in the same context, you have to defined different structures (and macros if you want to use it).