Hello,

i wanted to ask why this isnt working:

struct Text(T : char)
{
	size_t _len;
	T* _ptr;
}

Thanks :)

With working i mean that
Text X;
Doesnt compile!

On Sunday, November 19, 2017 19:22:51 Jiyan via Digitalmars-d-learn wrote:
> Hello,
>
> i wanted to ask why this isnt working:
>
> struct Text(T : char)
> {
>   size_t _len;
>   T* _ptr;
> }
>
> Thanks :)

What about it isn't working? I think that you need to explain what you're trying to do and what you think that this code should be doing but isn't.

- Jonathan M Davis

On Sunday, 19 November 2017 at 19:28:37 UTC, Jonathan M Davis wrote:
> On Sunday, November 19, 2017 19:22:51 Jiyan via Digitalmars-d-learn wrote:
>> Hello,
>>
>> i wanted to ask why this isnt working:
>>
>> struct Text(T : char)
>> {
>>   size_t _len;
>>   T* _ptr;
>> }
>>
>> Thanks :)
>
> What about it isn't working? I think that you need to explain what you're trying to do and what you think that this code should be doing but isn't.
>
> - Jonathan M Davis

Text X;

Here X would be an Object from type Text with T being char when i'm right.
But this doesn't compile!
Thanks :)

On Sunday, 19 November 2017 at 19:31:53 UTC, Jiyan wrote:
> Text X;

You still need to instantiate it, even with default args.

Text!() X;

will work

On Sunday, November 19, 2017 19:25:40 Jiyan via Digitalmars-d-learn wrote:
> With working i mean that
> Text X;
> Doesnt compile!

Okay. For starters,

struct Text(T : char)
{
    size_t _len;
    T* _ptr;
}

is a template specialization, which means that it's only going to compile if T is char or a type that implicitly converts to char. It's similar (but not quite the same as) doing

struct Text(T)
    if(is(T : char))
{
}

https://dlang.org/spec/template.html#TemplateTypeParameterSpecialization

I suspect that what you meant to do was

struct Text(T = char)
{
    size_t _len;
    T* _ptr;
}

which would make char the default value for T. However, not even that is going to allow you to do

Text t;

There is no implicit instantiation of templated types. Having the default value for T will allow you to do

Text!() x;

instead of

Text!char x;

but it won't allow you to do

Text x;

and there's no way to do that. Templated types must always be explicitly instantiated. What you could do that would give you a similar effect would be to do

struct TextImpl(T)
{
    size_t len;
    T* ptr;
}

alias Text = TextImpl!char;

and then when you use Text, it will be replaced with TextImpl!char, which would allow you to do

Text x;

The only time that templates are ever implicitly instantiated is with function templates where all of the template parameters can be inferred from the function arguments. It never occurs with any other kind of template, even if the template has no parameters.

- Jonathan M Davis

On Sunday, 19 November 2017 at 19:42:02 UTC, Jonathan M Davis wrote:
> On Sunday, November 19, 2017 19:25:40 Jiyan via Digitalmars-d-learn wrote:
>> [...]
>
> Okay. For starters,
>
> [...]

Ah ok thanks very much, this helped me a lot :)

On 11/19/17 2:41 PM, Adam D. Ruppe wrote:
> On Sunday, 19 November 2017 at 19:31:53 UTC, Jiyan wrote:
>> Text X;
> 
> You still need to instantiate it, even with default args.
> 
> Text!() X;
> 
> will work
> 

If that's the case, then he needs to use Text(T = char) (default type) vs. Text(T : char) (specialization)

-Steve