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Template with default parameter
Mar 11, 2022
Andrey Zherikov
Mar 11, 2022
bauss
Mar 11, 2022
Andrey Zherikov
Mar 11, 2022
bauss
Mar 11, 2022
Andrey Zherikov
Mar 11, 2022
Ali Çehreli
Mar 11, 2022
Adam D Ruppe
Mar 11, 2022
Andrey Zherikov
March 11, 2022
Gravatar of Andrey ZherikovPosted by Andrey ZherikovReply
Andrey Zherikov
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I have simple template:

template T(int i=3)
{
    mixin template M(int m)
    {
	    enum t = i;
    }
}

{
	mixin T!1.M!1;
	pragma(msg, t);   // 1
}
{
	mixin T!().M!1;
	pragma(msg, t);   // 3
}
{
	mixin T.M!1;      // Error: identifier `M` of `T.M` is not defined
					  // Error: mixin `M!1` is not defined
	pragma(msg, t);   // Error: undefined identifier `t`
					  //        while evaluating `pragma(msg, t)`
}

What should I do to be able to write T.M!...? I want to omit verbose !() for T. Note that mixins are essential here.

March 11, 2022
Gravatar of baussPosted by bauss
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bauss
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On Friday, 11 March 2022 at 04:41:40 UTC, Andrey Zherikov wrote:

>

I have simple template:

template T(int i=3)
{
    mixin template M(int m)
    {
	    enum t = i;
    }
}

{
	mixin T!1.M!1;
	pragma(msg, t);   // 1
}
{
	mixin T!().M!1;
	pragma(msg, t);   // 3
}
{
	mixin T.M!1;      // Error: identifier `M` of `T.M` is not defined
					  // Error: mixin `M!1` is not defined
	pragma(msg, t);   // Error: undefined identifier `t`
					  //        while evaluating `pragma(msg, t)`
}

What should I do to be able to write T.M!...? I want to omit verbose !() for T. Note that mixins are essential here.

Create an alias for T!() is the best you can do.

Ex.

alias t = T!();

There isn't really any better method as far as I know.

March 11, 2022
Gravatar of Andrey ZherikovPosted by Andrey Zherikov
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Andrey Zherikov
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On Friday, 11 March 2022 at 07:06:15 UTC, bauss wrote:

>

Create an alias for T!() is the best you can do.

Ex.

alias t = T!();

There isn't really any better method as far as I know.

I'd like to preserve the name (t != T) but alias T = T!() gives me Error: declaration T is already defined

March 11, 2022
Gravatar of baussPosted by bauss
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bauss
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On Friday, 11 March 2022 at 11:55:24 UTC, Andrey Zherikov wrote:

>

On Friday, 11 March 2022 at 07:06:15 UTC, bauss wrote:

>

Create an alias for T!() is the best you can do.

Ex.

alias t = T!();

There isn't really any better method as far as I know.

I'd like to preserve the name (t != T) but alias T = T!() gives me Error: declaration T is already defined

Yeah you can't really do much about that.

Then T would have to be something like TImpl and then you do alias T = TImpl!();

March 11, 2022
Gravatar of Andrey ZherikovPosted by Andrey Zherikov
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Andrey Zherikov
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There is some inconsistency between templates and template functions as this works as I want to:

import std.stdio;

void f(int i=3)()
{
    writeln(i);
}

void main()
{
    f!1;  // 1
    f!(); // 3
    f;    // 3
}
March 11, 2022
Gravatar of Ali ÇehreliPosted by Ali Çehreli
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Ali Çehreli
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On 3/11/22 04:01, Andrey Zherikov wrote:
> There is some inconsistency between templates and template functions as
> this works as I want to:
> ```d
> import std.stdio;
>
> void f(int i=3)()
> {
>      writeln(i);
> }
>
> void main()
> {
>      f!1;  // 1
>      f!(); // 3
>      f;    // 3
> }
> ```

Reduced:

template T(int i = 3) {
  enum value = i;
}

void main() {
  pragma(msg, T!().value);  // Requires T!()
}

The same inconsistency exists for user-defined type templates as well:

struct S(int i = 3) {
}

void main() {
  S!() s;  // Requires S!()
}

There has been requests to remove this inconsistency. Might be in bugzilla.

Ali
March 11, 2022
Gravatar of Adam D RuppePosted by Adam D Ruppe
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Adam D Ruppe
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On Friday, 11 March 2022 at 12:01:27 UTC, Andrey Zherikov wrote:
> There is some inconsistency between templates and template functions as this works as I want to:

Yeah, functions have the special feature of implicit instantiation from the function arguments. No other template do, if you want an instance there, you need to specify it as a !(). And since functions can't return aliases to types you can't use that to get around it. There's no way to do what you want to do, you either need to write the !() or use a different alias name.
March 11, 2022
Gravatar of Andrey ZherikovPosted by Andrey Zherikov
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Andrey Zherikov
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On Friday, 11 March 2022 at 12:53:56 UTC, Adam D Ruppe wrote:
> On Friday, 11 March 2022 at 12:01:27 UTC, Andrey Zherikov wrote:
>> There is some inconsistency between templates and template functions as this works as I want to:
>
> Yeah, functions have the special feature of implicit instantiation from the function arguments. No other template do, if you want an instance there, you need to specify it as a !(). And since functions can't return aliases to types you can't use that to get around it. There's no way to do what you want to do, you either need to write the !() or use a different alias name.

Thanks for explanation!