Motivation

The dimensions of multidimensional array reference class array_ref (P0009r2) are declared through either the data type or array properties template arguments.

template< typename DataType , typename Properties... >
struct array_ref ;

DataType may be an array type (8.3.4.p3). For example, array_ref<double[ ][3][8]> declares a rank-three array with a dynamic extent in dimension coordinate zero. This compact and intutive syntax is currently limited to a declaring a single dynamic extent in the array's leading coordinate. Removing this limitation allows declaration of an arbitrary number of dynamic extents in any coordinate.

Proposal

The current array type declarator constraints are defined in in n4567 8.3.4.p3 as follows.

When several “array of” specifications are adjacent, a multidimensional array type is created; only the first of the constant expressions that specify the bounds of the arrays may be omitted. In addition to declarations in which an incomplete object type is allowed, an array bound may be omitted in some cases in the declaration of a function parameter (8.3.5). An array bound may also be omitted when the declarator is followed by an initializer (8.5). In this case the bound is calculated from the number of initial elements (say, N) supplied (8.5.1), and the type of the identifier of D is “array of N T”. Furthermore, if there is a preceding declaration of the entity in the same scope in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class.

The preferred syntax requires a relaxation of array type declarator constraints defined in n4567 8.3.4.p3 exclusively for an incomplete object type. The following wording change is proposed.

When several “array of” specifications are adjacent, a multidimensional array type is created. The first of the constant expressions that specify the bounds of the array may be omitted in some cases in the declaration of a function parameter (8.3.5). The first array bound may also be omitted when the declarator is followed by an initializer (8.5). In this case the first bound is calculated from the number of initial elements (say, N) supplied (8.5.1), and the type of the identifier of D is “array of N T”. Furthermore, if there is a preceding declaration of the entity in the same scope in which the first bound was specified, an omitted array first bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class. In declarations in which an incomplete object type is allowed any of the constant expressions that specify the bounds of the array may be ommitted.

This minor language specification change has been implemented with a trivial (one line) patch to Clang and was permissible in gcc prior to version 5.