Can you write a static assert to verify the offset of data members?

Question

Given the following struct:

struct ExampleStruct {
    char firstMember[8];
    uint64_t secondMember;
};

Is there a way to write a static assert to verify that the offset of secondMember is some multiple of 8 bytes?

Answer 1

Offsetof

You can use the offsetof marco brought by the cstddef library. Here I first get the offset, then I use the modulus operator to check if it is a multiple of 8. Then, if the remainder is 0, the offset is indeed a multiple of 8 bytes.

// Offset.cpp
#include <iostream>
#include <string>
#include <cstddef>
#include <stdarg.h>

struct ExampleStruct {
    char firstMember[8];
    uint64_t secondMember;
};


int main()
{
     size_t offset = offsetof(ExampleStruct, secondMember);
     if(offset%8==0)
        std::cout << "Offset is a multiple of 8 bytes";
}

Demo here

Offsetof with static_assert

Or by the context of this question, the goal is to have a static_assert. Well, that is pretty much the same thing:

// OffsetAssert.cpp
#include <iostream>
#include <string>
#include <cstddef>
#include <stdarg.h>

struct ExampleStruct {
    char firstMember[8];
    uint64_t secondMember;
};


int main()
{
     size_t offset = offsetof(ExampleStruct, secondMember); // Get Offset
     static_assert(offsetof(ExampleStruct, secondMember)%8==0,"Not Aligned 8 Bytes"); // Check if the offset modulus 8 is remainer 0 , if so it is a multiple of 8
     std::cout << "Aligned 8 Bytes" << std::endl; // If Assert Passes it is aligned 8 bytes
}

Demo here

Type Uses

I use std::size_t type because that's the type you normally use to store sizes of variables, objects, and so on. And also because it expands to the std::size_t expression according to cppreference.com:

The macro offsetof expands to an integral constant expression of type std::size_t, the value of which is the offset, in bytes, from the beginning of an object of specified type to its specified member, including padding if any.

References

cpprefrence

cplusplus

Answer 2

If your type has standard layout, you can use the offsetof macro:

#include <cstddef>

static_assert(offsetof(ExampleStruct, secondMember) % 8 == 0, "Bad alignment");

This the offsetof macro results in a constant expression, you can use a static assertion to produce a translation error if the condition is not met.