infer type argument in classe constructor?

[Puming via Digitalmars-d-learn]

On Tuesday, 29 March 2016 at 10:29:46 UTC, Simen Kjaeraas wrote:
> On Tuesday, 29 March 2016 at 10:13:28 UTC, Puming wrote:
>> Hi,
>>
>> I'm writing a generic class：
>>
>> ```d
>>
>> struct Message { ... }
>>
>> class Decoder(MsgSrc) {
>> }
>> ```
>>
>> When using it, I'd have to include the type of its argument:
>>
>> ```
>> void main() {
>>    Message[] src = ...;
>>
>>    auto decoder = new Decoder!(Message[])(src);
>>
>>    ...
>> }
>> ```
>>
>> Can it be inferred so that I only need to write?
>>
>> ```d
>> auto decoder = new Decoder(src); // you can infer the type 
>> from src.
>> ```
>
> Nope. To see why, consider a class like this:
>
> class A(T) {
>   T data;
>   this(int n) {
>   }
> }
>
> void main() {
>    auto a = new A(3); // What is T?
> }
>
Sorry I don't see it. In this case, I don't see an ambiguity?

`int n` and T are not connected, so invoking A(3) means you are 
only setting the argument n to 3, so T can not be infered, and 
the compiler could just complain 'generic type T is not provided'.

> The common solution is a simple 'create' function:
>
> Decoder!T decoder(T)(T msg) {
>     return new Decoder!T(msg);
> }
>
> --
>   Simen