20 General utilities library [utilities]

20.15 Metaprogramming and type traits [meta]

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

This subclause contains templates that may be used to query properties of types at compile time.

Table 50: 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 names 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 $I^{th}$ dimension of T, where indexing of I is zero-based

Each of these templates shall be a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of integral_constant<size_t, Value>.

[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]

[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]