[cfe-dev] C++ name scoping question

[David Fontaine via cfe-dev]

?Thanks for the help Hubert.  Stating that the pFn declaration contains a function declarator certainly makes life easier.


-David


________________________________
From: Hubert Tong <hubert.reinterpretcast at gmail.com>
Sent: Friday, February 3, 2017 3:03 PM
To: David Fontaine
Cc: cfe-dev at lists.llvm.org
Subject: Re: [cfe-dev] C++ name scoping question

On Fri, Feb 3, 2017 at 5:39 PM, David Fontaine via cfe-dev <cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
(Also trying cfe-dev. Originally sent this to cfe-users.)

Hi,

I'm hoping a C++ language expert can help me with scoping rules for named function parameters.

As background, I'd like to allow named function parameters in non-function declarations (such as function-pointer-typed variables) to be referenced in attribute expressions.  Currently, attempting to do so results in "use of undeclared identifier".  However, I'm not even sure the standard allows me to do this, let alone what assumptions any patch to clang might run afoul of.

Consider the declaration:

    void (*pFn)(int x);

Reading the standard (specifically, I'm looking at C++11 draft N4527), I gather that:
?1. pFn is not a "function declarator". I didn't see a clear definition of this, but I assume function and variable are mutually exclusive descriptions of a declarator, and that this would be a variable declarator.
Within a declaration, certain portions of the syntax are considered declarators. The interpretation of this is that (*pFn)(int x) is the function declarator here.

2. According to [basic.scope.proto] (3.3.4), named parameters inside function declarators which are not also definitions have function prototype scope, but this only applies to function declarators.  I do not find a scoping rule that would apply to x in my pFn example, assuming point 1 is correct.
The scoping rule you found applies.


This seems like x has no applicable scoping rule and you shouldn't be able to reference it.  However, I note the following declaration is accepted as valid C++11:

    auto (*pSum)(int x, double y) -> decltype(x + y);
(*pSum)(int x, double y) -> decltype(x + y) is the function declarator here.


I'd appreciate any help understanding what portions of the standard apply to my examples and how to interpret them, and whether the standard in fact allows me to place an attribute on pFn which references x.
An attribute appertaining to the function type (placed after the optional noexcept-specifier) is part of the function declarator.



Thanks,
David Fontaine


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