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.