On 2/1/23 22:18, Bo Persson via Std-Proposals wrote:
> On 2023-02-01 at 16:39, Andrey Semashev via Std-Proposals wrote:
>> On 2/1/23 17:25, Bo Persson via Std-Proposals wrote:
>>> On 2023-02-01 at 14:22, Andrey Semashev via Std-Proposals wrote:
>>>> On 2/1/23 16:19, Bo Persson via Std-Proposals wrote:
>>>>> On 2023-02-01 at 13:56, Andrey Semashev via Std-Proposals wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Would it be possible to be able to explicitly specify default
>>>>>> arguments
>>>>>> for template specializations and function calls?
>>>>>>
>>>>>> void foo(int x, int y = 10, int z = 20);
>>>>>>
>>>>>> foo(1, default, 3); // == foo(1, 10, 3);
>>>>>>
>>>>>>
>>>>>> // Use default ordering function
>>>>>> typedef std::set< int, default, my_alloc > my_set;
>>>>>>
>>>>>> // Use default compare and hash functions
>>>>>> typedef std::unordered_set< int, default, default, my_alloc >
>>>>>> my_uset;
>>>>>>
>>>>>>
>>>>>> template<
>>>>>> typename T,
>>>>>> typename = whatever,
>>>>>> bool = is_special< T >::value
>>>>>> >
>>>>>> struct trait {};
>>>>>>
>>>>>> // Specialization: trait< T, whatever, true >
>>>>>> template< typename T >
>>>>>> struct trait< T, default, true > {};
>>>>>>
>>>>>> This syntax is currently invalid, so the extension would be pretty
>>>>>> harmless. This would allow to avoid code duplication, where the
>>>>>> user of
>>>>>> the function or template, or the one who specializes the template,
>>>>>> has
>>>>>> to duplicate the default argument when he only needs to specify the
>>>>>> arguments after that.
>>>>>
>>>>> This seems to have been proposed 20 years ago.
>>>>>
>>>>> https://wg21.link/N1466
>>>>>
>>>>> Didn't go anywhere.
>>>>
>>>> Any particular reason why it got rejected or it was simply abandoned?
>>>>
>>>
>>> It's too long ago, I don't remember.
>>>
>>> One argument could be that there are several ways around the limitation.
>>>
>>> Like if the arguments are not all of the same type, you can have
>>> overloads with fewer arguments that forwards to the full function.
>>
>> Obviously, this would complicate maintenance as you'd have to provide
>> all permutations of the specified/not specified optional arguments.
>
> In the example we only have three parameters, and occationally want to
> skip the middle one. That't only one permutation.
>
>
>> Remember when we didn't have perfect forwarding in the core language?
>>
>
> But we do now. So let's use it.

My point was that trying to work around this problem through adding
overloads will lead to overload explosion similar to what we had when we
had to implement perfect forwarding in C++03.