0013 - Named Casts

Introduction

This proposal introduces C++-like named casts which perform specific defined casts and have more strict semantics than the general C-style casting syntax.

Motivation

Long ago C++ adopted named casts to remove ambiguity around what data transformation a cast performs. These take the forms static_cast, reinterpret_cast, const_cast, dynamic_cast, and most recently bit_cast.

This proposal introduces named casts for HLSL to allow unambiguous ways to cast values from a source type to a destination type with tight validation.

Proposed solution

HLSL will introduce three new named cast templates, hlsl::static_cast<T>, hlsl::elementwise_cast<T> and hlsl::bit_cast<T>.

The hlsl::static_cast<T>(V) will behave similar to the C++ static_cast<T>(V) expression, except as changes are required due to other differences between C++ and HLSL. It will convert the expression V to the result type T. The main difference from C++ is that HLSL’s hlsl::static_cast<T> will always produce an rvalue since HLSL does not have spellable lvalue reference types, and may not be used for polymorphic casts for the same reason.

The hlsl::elementwise_cast<T>(V) will perform an element-wise conversion (\ref{Conv.Flat}). The type of the expression V and the result type T must be scalar layout compatible (\ref{Basic.Types.Scalarized}).

The hlsl::bit_cast<T>(V) cast function template will convert the expression V to type T without modifying the precise bit representation. The type of the expression V and the result type T must not be intangible types (\ref{Basic.Types.Intangible}), and must be of the same size.

The HLSL static_cast, bit_cast and elementwise_cast templates are defined in the hlsl namespace aligning with modern C++ conventions to avoid additions to the global namespace. Additionally the static_cast construct will be exposed via a using declaration into the global namespace allowing compatibility with the historical C++ definition which remains in modern C++.