31 Oct 2019 • C++ tricks: member array count

I recently wanted to make an array with the same size as some other array. Normally I would use ARRAY_COUNT, But this time the reference array was a struct member, and I was in a context where I had no struct instance I could use.

So this:

struct A {
	int a[ 4 ];

struct B {
	int b[ ??? ];

The solution is to use C++'s "pointer to data member" functionality:

template< typename T, typename M, size_t N >
constexpr size_t ARRAY_COUNT( M ( T::* )[ N ] ) {
       return N;

struct B {
	int b[ ARRAY_COUNT( &A::a ) ];

Weird but it works.