Re: [boost] [config] What Feature is this??

Subject: Re: [boost] [config] What Feature is this??
From: Edward Diener (eldiener_at_[hidden])
Date: 2017-07-24 18:26:52

On 7/24/2017 12:40 PM, John Maddock via Boost wrote:
> 
> 
> On 24/07/2017 16:18, Edward Diener via Boost wrote:
>> On 7/24/2017 8:29 AM, Andrey Semashev via Boost wrote:
>>> On 07/24/17 14:37, Edward Diener via Boost wrote:
>>>> On 7/23/2017 2:50 PM, John Maddock via Boost wrote:
>>>>> Folks, I think I'm looking to add a new C++11 defect macro, but 
>>>>> don't know what the feature is called, nor can I see a way of 
>>>>> detecting it with clang's __has_feature function.
>>>>>
>>>>> The code I want to enable involves function overloading based on a 
>>>>> decltype return type as in:
>>>>
>>>> I never realized that function overloading could occur based on any 
>>>> return type. I always thought it had to be based on the parameter 
>>>> types. Is this part of the C+11 standard ? Where in the standard is 
>>>> this specified ?
>>>
>>> I think it's not about overloading but about disabling function 
>>> template instantiations via SFINAE.
>>
>> I am glad to hear that. I misinterpreted the original message and 
>> probably should have realized it. Thanks !
> 
> It's sort of about both actually - we're using SFINAE to select between 
> two overloads based on the properties of the argument types (sort of 
> poor man's Concepts), so we can write:
> 
> template <class Arg>
> auto foo(const Arg&) -> decltype(expression-involving-type-Arg)
> 
> and that function overload will be considered only if the expression 
> inside the decltype compiles.  You may have to go through heroics to get 
> the return type correct (use of the comma operator for example), but 
> it's a very easy way to select between function overloads based on which 
> concepts the argument types adhere to.

Thanks, John, I just fell asleep by not realizing you were talking about 
SFINAE ala enable_if etc. I am surprised that any compiler would operate 
differently in regard to SFINAE between using the C++03 'return-type 
some_function(etc.)' form and the C++11 'auto some_function(etc.) -> 
return-type' form, whether decltype was used or not.

> 
> John.
> 
> 
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