Question

When I was compiling a C++ program using icc 11, it gave this warning:

warning #21: type qualifiers are meaningless in this declaration
typedef const direction_vector_ref_t direction_vector_cref_t;

It is saying const just meaningless. I am curious about this since if this typedef expands it will turn into const array<double,3>& and the const is definitely meaningful. Why it gave this warning?

Answer 1

Are you sure? Try:

array<double, 3> a;
direction_vector_cref_t b = a;
b[0] = 1.0;

The issue here, is that when you use a typedef, it conceptually adds parentheses around the type, so you are conceptually using const (array<double, 3>&) as opposed to (const array<double, 3>)&, so you are not actually making the referent object constant. So your declaration is more like:

typedef array<double, 3>& const direction_vector_cref_t;

And in the above, the const for the variable (rather than the referent type) needs to be deferred till later.