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.

You could change the order, so the "most defaulted" argument appears
last (reduces the number of time "default" would be useful).

Nowadays you could also pass a struct and name the arguments, like

f({.x = 1, .z = 3});

So how many use cases are there left to motivate a language change?