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

[Richard Smith via cfe-commits]

On Mon, 22 Mar 2021 at 13:31, David Blaikie <dblaikie at gmail.com> wrote:

> 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?)
>

I'll send you a further improvement to review :)


> +
>> +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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>
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