21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.6 Type property queries [meta.unary.prop.query]

1
#
The templates specified in Table 48 may be used to query properties of types at compile time.
Table 48: Type property queries [tab:meta.unary.prop.query]
Template
Value
template<class T>
struct alignment_of;
alignof(T).

Mandates: alignof(T) is a valid expression ([expr.alignof])
template<class T>
struct rank;
If T is an array type, an integer value representing the number of dimensions of T; otherwise, 0.
template<class T,
unsigned I = 0>
struct extent;
If T is not an array type, or if it has rank less than or equal to I, or if I is 0 and T has type “array of unknown bound of U”, then 0; otherwise, the bound ([dcl.array]) of the dimension of T, where indexing of I is zero-based
2
#
Each of these templates shall be a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of integral_constant<size_t, Value>.
3
#
[Example 1: // the following assertions hold: assert(rank_v<int> == 0); assert(rank_v<int[2]> == 1); assert(rank_v<int[][4]> == 2); — end example]
4
#
[Example 2: // the following assertions hold: assert(extent_v<int> == 0); assert(extent_v<int[2]> == 2); assert(extent_v<int[2][4]> == 2); assert(extent_v<int[][4]> == 0); assert((extent_v<int, 1>) == 0); assert((extent_v<int[2], 1>) == 0); assert((extent_v<int[2][4], 1>) == 4); assert((extent_v<int[][4], 1>) == 4); — end example]