Compile-time check to make sure that there is no padding anywhere in a struct

Question

Is there a way to write a compile-time assertion that checks if some type has any padding in it?

For example:

struct This_Should_Succeed
{
    int a;         


        

Answer 1

To get the total field size without retyping each struct member you can use an X Macro

First define all the fields

#define LIST_OF_FIELDS_OF_This_Should_Fail    \
    X(int, a)          \
    X(char, b)         \
    X(int, c)

#define LIST_OF_FIELDS_OF_This_Should_Succeed \
    X(long long, a)    \
    X(long long, b)    \
    X(int, c)          \
    X(int, d)          \
    X(int, e)          \
    X(int, f)

then declare the structs

struct This_Should_Fail {
#define X(type, name) type name;
    LIST_OF_FIELDS_OF_This_Should_Fail
#undef X
};

struct This_Should_Succeed {
#define X(type, name) type name;
    LIST_OF_FIELDS_OF_This_Should_Succeed
#undef X
};

and check

#define X(type, name) sizeof(This_Should_Fail::name) +
static_assert(sizeof(This_Should_Fail) == LIST_OF_FIELDS_OF_This_Should_Fail 0);
#undef X

#define X(type, name) sizeof(This_Should_Succeed::name) +
static_assert(sizeof(This_Should_Succeed) == LIST_OF_FIELDS_OF_This_Should_Succeed 0);
#undef X

or you can just reuse the same X macro to check

#define X(type, name) sizeof(a.name) +
{
    This_Should_Fail a;
    static_assert(sizeof(This_Should_Fail) == LIST_OF_FIELDS_OF_This_Should_Fail 0);
}
{
    This_Should_Succeed a;
    static_assert(sizeof(This_Should_Succeed) == LIST_OF_FIELDS_OF_This_Should_Succeed 0);
}        
#undef X

See demo on compiler explorer

For more information about this you can read Real-world use of X-Macros