[clang] 5c689e4 - Improve documentation for the [[clang::lifetimebound]] attribute.

[David Blaikie via cfe-commits]

On Thu, Mar 18, 2021 at 7:58 PM Richard Smith via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

>
> Author: Richard Smith
> Date: 2021-03-18T19:58:21-07:00
> New Revision: 5c689e4bb0473e08645547ddbf9874b5e2fa04d0
>
> URL:
> https://github.com/llvm/llvm-project/commit/5c689e4bb0473e08645547ddbf9874b5e2fa04d0
> DIFF:
> https://github.com/llvm/llvm-project/commit/5c689e4bb0473e08645547ddbf9874b5e2fa04d0.diff
>
> LOG: Improve documentation for the [[clang::lifetimebound]] attribute.
>
> Added:
>
>
> Modified:
>     clang/include/clang/Basic/AttrDocs.td
>
> Removed:
>
>
>
>
> ################################################################################
> diff  --git a/clang/include/clang/Basic/AttrDocs.td
> b/clang/include/clang/Basic/AttrDocs.td
> index f73fbd08e3bf..734cf026ae87 100644
> --- a/clang/include/clang/Basic/AttrDocs.td
> +++ b/clang/include/clang/Basic/AttrDocs.td
> @@ -3032,10 +3032,39 @@ is retained by the return value of the annotated
> function
>  (or, for a parameter of a constructor, in the value of the constructed
> object).
>  It is only supported in C++.
>
> -This attribute provides an experimental implementation of the facility
> -described in the C++ committee paper `P0936R0 <http://wg21.link/p0936r0
> >`_,
> -and is subject to change as the design of the corresponding functionality
> -changes.
> +This attribute causes warnings to be produced if a temporary object does
> not
> +live long enough. For example:
> +
> +.. code-block:: c++
> +
> +    template<typename T, typename U>
> +    const U &get_or_default(std::map<T, U> &m, const T &key,
> +                            const U &default_value
> [[clang::lifetimebound]]);
> +
> +    std::map<std::string, std::string> m;
> +    // warning: temporary "bar"s that might be bound to local reference
> 'val'
> +    // will be destroyed at the end of the full-expression
> +    const std::string &val = get_or_default(m, "foo"s, "bar"s);
> +
> +When applied to a reference parameter, the referenced object is assumed
> to be
> +retained by the return value of the function. When applied to a
> non-reference
> +parameter (for example, a pointer or a class type), all temporaries
> referenced
> +by the parameter are assumed to be retained by the return value of the
> +function.
>

"referenced by" seems a bit vague to me - I guess it means all reference
members of the parameter? (what about reference members of subobjects?)


> +
> +The attribute can be applied to the implicit ``this`` parameter of a
> member
> +function by writing the attribute after the function type:
> +
> +.. code-block:: c++
> +
> +    struct string_view {
> +      // ...
> +      const char *data() const [[clang::lifetimebound]];
> +    };
> +
> +This attribute is inspired by the C++ committee paper `P0936R0
> +<http://wg21.link/p0936r0>`_, but does not affect whether temporary
> objects
> +have their lifetimes extended.
>    }];
>  }
>
>
>
>
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