On Sunday, June 16, 2024 10:32:50 PM MDT Andy Valencia via Digitalmars-d-learn wrote: > In the alias: > > alias Unshared(T) = T; > alias Unshared(T: shared U, U) = U; > > as used in: > > cast(Unshared!mytype)value > > turns a mytype with shared attribute into one without shared. > > I deduce the alias is using some sort of type matching and > decomposition? > > I've read through the language spec, and didn't spot this > mechanism. Can somebody tell me what section of the spec covers > this very interesting mechanism? > > Thanks in advance, > Andy
Unshared is an eponymous template. https://dlang.org/spec/template.html#implicit_template_properties And it's using a shortcut syntax. alias Unshared(T) = T; alias Unshared(T: shared U, U) = U; is equivalent to template Unshared(T) { alias Unshared = T; } template Unshared(T : shared U, U) { alias Unshared = U; } The alias is simply a normal alias that's the result of evaluating the template. https://dlang.org/spec/declaration.html#alias The second template uses a template specialization https://dlang.org/spec/template.html#parameters_specialization that matches based on the fact that the given type implicitly converts to shared, and the shared U part separates out shared from the type so that you can get the type without shared. That's then used for the alias to get the type without shared. The way that the shared U part works comes from is expressions: https://dlang.org/spec/expression.html#is_expression Eponymous templates get used in a variety of circumstances, not just with aliases. For instance, they can be used with enums, e.g. enum sizeOf(T) = T.sizeof; which would be equivalent to template sizeOf(T) { enum sizeOf = T.sizeof; } and they're used with function templates, e.g. T identity(T)(T value) { return value; } is equivalant to template identity(T) { T identity(T value) { return value; } } - Jonathan M Davis
