Named Requirements

[named_requirements]

This section describes named requirements used in the oneAPI Threading Building Blocks Specification.

A named requirement is a set of requirements on a type. The requirements may be syntactic or semantic. The named_requirement term is similar to “Requirements on types and expressions” term which is defined by the ISO C++ Standard (chapter “Library Introduction”) or “Named Requirements” section on the cppreference.com site.

For example, the named requirement of sortable could be defined as a set of requirements that enable an array to be sorted. A type T would be sortable if:

  • x < y returns a boolean value, and represents a total order on items of type T.

  • swap(x,y) swaps items x and y

You can write a sorting template function in C++ that sorts an array of any type that is sortable.

Two approaches for defining named requirements are valid expressions and pseudo-signatures. The ISO C++ standard follows the valid expressions approach, which shows what the usage pattern looks like for a requirement. It has the drawback of relegating important details to notational conventions. This document uses pseudo-signatures, because they are concise, and can be cut-and-pasted for an initial implementation.

For example, the table below shows pseudo-signatures for a sortable type T:

Sortable Requirements : Pseudo-Signature, Semantics

bool operator<(const T &x, const T &y)

Compare x and y.

void swap(T &x, T &y)

Swap x and y.

A real signature may differ from the pseudo-signature that it implements in ways where implicit conversions would deal with the difference. For an example type U, the real signature that implements operator``<`` in the table above can be expressed as int operator<( U x, U y ), because C++ permits implicit conversion from int to bool, and implicit conversion from U to (const U&). Similarly, the real signature bool operator<( U& x, U& y ) is acceptable because C++ permits implicit addition of a const qualifier to a reference type.

Task scheduler