[conv.ptr] Fix unclear specification of derived-to-base conversions for null pointers #3823
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source/expressions.tex
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| object. The null pointer value is converted to the null pointer value of | ||
| the destination type. | ||
| that necessitates this conversion is ill-formed. | ||
| If the source value is a null pointer value, the result is a |
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What's the "source value"? Can we reformulate without using this novel term?
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@jensmaurer In [conv], there's a mixture of "source value", "source expression" and "source prvalue". I suppose that the best replacement here would be "source prvalue".
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Sounds better, because it aligns with the phrasing in the "If" at the start of the paragraph.
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@jensmaurer Applied
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I really don't like using "source prvalue" here. A prvalue is a kind of expression, so this sounds like it's talking about a (static) property of the expression, which it's not.
I think something like "A null pointer value of the source type is converted to a null pointer value of the destination type. A pointer to an object o of type D is converted to a pointer to the B base class subobject of o." would work, and fits much better into our pointer value taxonomy in [basic.compound]. Arguably this would need CWG review since it makes it much more clear that such a conversion applied to any other pointer value is undefined.
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@jensmaurer I applied the changes in [conv.ptr] to [conv.mem], [expr.static.cast], [expr.reinterpret.cast], and [expr.const.cast]. Another small error I fixed was that in [conv.mem] we say "The result refers to the member in |
zygoloid
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@sdkrystian, could you please rebase and implement @zygoloid's suggestion, which sounds like an improvement? |
[conv.ptr] p3 states:
While null pointer values are established as a singular value a pointer can have in [basic.compound], it is unique to the pointer type. Saying "The null pointer value" is rather unclear in what it refers to, as there is no single null pointer value.