20 General utilities library [utilities]

20.15 Metaprogramming and type traits [meta]

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

This subclause contains templates that may be used to query properties of types at compile time.
Table 49: Type property queries   [tab:meta.unary.prop.query]
Template
Value
template<class T>
struct alignment_­of;
alignof(T).

Requires: alignof(T) shall be 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 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
:
// the following assertions hold:
assert(rank_v<int> == 0);
assert(rank_v<int[2]> == 1);
assert(rank_v<int[][4]> == 2);
— end example
 ]
Example
:
// 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
 ]