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digitalmars.D.learn - std.typecons Typedef initializers?

reply Chris Katko <ckatko gmail.com> writes:
````D
struct pair
{
float x,y;
}

alias sPair = Typedef!pair; // pair of xy in screen space 
coordinates
alias vPair = Typedef!pair; // pair of xy in viewport space 
coordinates
//etc

void test()
{
pair v0 = pair(1f, 2f); // works fine, but what about the 
typedefs?

vPair v1 = vPair(1f, 2f); //nope

vPair v2 = Typedef!vPair(1f, 2f); //nope
}

````

How do you initialize a typedef'd struct?
Apr 25 2022
next sibling parent reply Mike Parker <aldacron gmail.com> writes:
On Monday, 25 April 2022 at 08:54:52 UTC, Chris Katko wrote:

 ````D
 struct pair
 {
 float x,y;
 }

 alias sPair = Typedef!pair; // pair of xy in screen space 
 coordinates
 alias vPair = Typedef!pair; // pair of xy in viewport space 
 coordinates
 //etc



 How do you initialize a typedef'd struct?


``d
vPair v1 = vPair(pair(1f, 2f));
```
Apr 25 2022
parent reply Chris Katko <ckatko gmail.com> writes:
On Monday, 25 April 2022 at 12:53:14 UTC, Mike Parker wrote:

 On Monday, 25 April 2022 at 08:54:52 UTC, Chris Katko wrote:

 ````D
 struct pair
 {
 float x,y;
 }

 alias sPair = Typedef!pair; // pair of xy in screen space 
 coordinates
 alias vPair = Typedef!pair; // pair of xy in viewport space 
 coordinates
 //etc



 How do you initialize a typedef'd struct?


 ``d
 vPair v1 = vPair(pair(1f, 2f));
 ```


So to use a typedef'd struct... I have to basically add the 
original type on top of the typedef'd type every time? Surely 
it's not this clunky?

I mean, why even use a typedef then. Why not use just pair, 
sPair, vPair, etc as  separate types with identical members and 
cast as necessary? I'm not sure what the benefit typedef is 
adding here.

Thanks
Apr 25 2022
parent Stanislav Blinov <stanislav.blinov gmail.com> writes:
On Monday, 25 April 2022 at 23:41:47 UTC, Chris Katko wrote:


 So to use a typedef'd struct... I have to basically add the 
 original type on top of the typedef'd type every time? Surely 
 it's not this clunky?

 I mean, why even use a typedef then. Why not use just pair, 
 sPair, vPair, etc as  separate types with identical members and 
 cast as necessary? I'm not sure what the benefit typedef is 
 adding here.

 Thanks


It could just be an oversight in implementation and worth 
submitting an enhancement request on bugzilla.

Current implementation only defines a constructor that takes 
rvalue of original type, while what it ought to be doing is 
defining a variadic template constructor that would forward the 
arguments to underlying type's constructor.

To be fair, as far as your example code goes, it'd almost be 
easier to indeed simply duplicate the implementations, but have 
the compiler do it for you, e.g. like this:

```d
enum Space
{
     unspecified,
     screen,
     viewport,
}

struct TPair(Space space) { float x, y; }

alias Pair = Pair!(Space.unspecified);
alias sPair = Pair!(Space.screen);
alias vPair = Pair!(Space.viewport);
```
Apr 25 2022
prev sibling parent Paul Backus <snarwin gmail.com> writes:
On Monday, 25 April 2022 at 08:54:52 UTC, Chris Katko wrote:

 ````D
 alias sPair = Typedef!pair; // pair of xy in screen space 
 coordinates
 alias vPair = Typedef!pair; // pair of xy in viewport space 
 coordinates
 //etc
 ````


This doesn't do what you think it does. Both `sPair` and `vPair` 
are the same type. If you want to create two distinct `Typedef`s 
from the same base type, you must use the optional `cookie` 
argument, as shown in [the examples in the documentation:][1]

```d
alias MoneyEuros = Typedef!(float, float.init, "euros");
alias MoneyDollars = Typedef!(float, float.init, "dollars");

// The two Typedefs are _not_ the same type.
static assert(!is(MoneyEuros == MoneyDollars));
```

[1]: 
https://phobos.dpldocs.info/std.typecons.Typedef.html#examples
Apr 25 2022