• Jiyan (8/8) Nov 19 2017 Hello,
• Jiyan (3/3) Nov 19 2017 With working i mean that
• Jonathan M Davis (48/51) Nov 19 2017 Okay. For starters,
• Jiyan (3/8) Nov 19 2017 Ah ok thanks very much, this helped me a lot :)
• Jonathan M Davis (4/12) Nov 19 2017 What about it isn't working? I think that you need to explain what you'r...
• Jiyan (7/24) Nov 19 2017 Text X;
• Adam D. Ruppe (4/5) Nov 19 2017 You still need to instantiate it, even with default args.
• Steven Schveighoffer (4/13) Nov 20 2017 If that's the case, then he needs to use Text(T = char) (default type)

Jiyan <jiyan jiyan.info> writes:

Hello,

i wanted to ask why this isnt working:

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

Thanks :)

Jiyan <jiyan jiyan.info> writes:

With working i mean that
Text X;
Doesnt compile!

Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:

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

Jiyan <jiyan jiyan.info> writes:

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 :)

Jonathan M Davis <newsgroup.d jmdavisprog.com> writes:

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

Jiyan <jiyan jiyan.info> writes:

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 :)

Adam D. Ruppe <destructionator gmail.com> writes:

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

Steven Schveighoffer <schveiguy yahoo.com> writes:

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