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How to Instantiate struct defined in template
Sep 23, 2012
Craig Dillabaugh
Sep 23, 2012
Jonathan M Davis
Sep 23, 2012
Craig Dillabaugh
Sep 23, 2012
Jonathan M Davis
Sep 23, 2012
Craig Dillabaugh
```Hello,
I am trying to figure out how templates work and tried to define
the
following template to define vertices of any dimension and
operations
on them.

import std.stdio;
import std.random;
import std.range;
import std.conv;
import std.math;

/*
* T must be one of the floating point types
*   float, double, real _ I should be enforcing this.
*/
template  vt(T)
{
struct vertex {
this(T[] crds...) {
if( crds.length < 2 ) {
throw new.Exception("To small!");
} else {
coords.length = crds.length;
foreach(ref v_coord, in_coord; lockstep(coords, crds))
{
v_coord = in_coord;
}
}
}
@property T x() { return coords[0];}
@property T y() { return coords[1];}
@property T z() { return ( coords.length < 3 ? T.nan :
coords[2] ); }
}

T euclid_dist(T a, T b) {
T sum = 0;
foreach( ref a_crd, ref b_crd; lockstep( a.coords, b.coords )
) {
sum += (a_crd - b_crd)*(a_crd - b_crd );
}
return sqrt(sum);
}
} //End of template

int main(string argv[] ) {
vt!(float).vertex v1(0.0,0.0);
vt!(float).vertex v2(3.0,4.0);
writefln("The distance between vertex 1 and vertex 2 is ",
vt!(float).euclid_dist(v1, v2) );
}

When I try to compile this I get the message:

vertex.d(57): Error: found 'v1' when expecting ';' following
statement
vertex.d(58): Error: found 'v2' when expecting ';' following
statement

Lines 57 and 58 are the first two lines in my main() function. I
can't figure out why, from the examples I've looked at I am using
the syntax properly, but am clearly missing something.

Also, since I don't really know what I am doing any criticisms on
how I am defining the vertex template are welcome.

Cheers,

Craig
```
```On Sunday, September 23, 2012 05:49:06 Craig Dillabaugh wrote:
> Hello,
> I am trying to figure out how templates work and tried to define
> the
> following template to define vertices of any dimension and
> operations
> on them.
>
> import std.stdio;
> import std.random;
> import std.range;
> import std.conv;
> import std.math;
>
> /*
>    * T must be one of the floating point types
>    *   float, double, real _ I should be enforcing this.
>    */
> template  vt(T)
> {
>     struct vertex {
>       this(T[] crds...) {
>         if( crds.length < 2 ) {
>           throw new.Exception("To small!");
>         } else {
> 	coords.length = crds.length;
> 	foreach(ref v_coord, in_coord; lockstep(coords, crds))
> 	{
> 	  v_coord = in_coord;
> 	}
>         }
>       }
>       @property T x() { return coords[0];}
>       @property T y() { return coords[1];}
>       @property T z() { return ( coords.length < 3 ? T.nan :
> coords[2] ); }
>     }
>
>     T euclid_dist(T a, T b) {
>       T sum = 0;
>       foreach( ref a_crd, ref b_crd; lockstep( a.coords, b.coords )
> ) {
>         sum += (a_crd - b_crd)*(a_crd - b_crd );
>       }
>       return sqrt(sum);
>     }
> } //End of template
>
> int main(string argv[] ) {
> 	vt!(float).vertex v1(0.0,0.0);
> 	vt!(float).vertex v2(3.0,4.0);
> 	writefln("The distance between vertex 1 and vertex 2 is ",
> 		vt!(float).euclid_dist(v1, v2) );
> }
>
> When I try to compile this I get the message:
>
> vertex.d(57): Error: found 'v1' when expecting ';' following
> statement
> vertex.d(58): Error: found 'v2' when expecting ';' following
> statement
>
> Lines 57 and 58 are the first two lines in my main() function. I can't figure out why, from the examples I've looked at I am using the syntax properly, but am clearly missing something.
>
> Also, since I don't really know what I am doing any criticisms on how I am defining the vertex template are welcome.

Before anything, I'd question why you declared vt at all. If all you're putting in it is a single struct, then just templatize the struct directly:

struct Vertex(T)
{
...
}

Now, it looks like you have a free function in there as well - euclid_dist - but there's no reason to put that in the same template. Just templatize it directly. Then you get

T euclid_dist(T)(T a, T b) {...}

And main ends up looking something like

void main()
{
auto v1 = Vertex!float(0.0, 0.0);
auto v2 = Vertex!float(2.0, 4.0);
writefln("The distance between vertex 1 and vertex 2 is %s",
euclid_dist(v1, v2));
}

It's actually fairly to explicitly declare a template in D outside of eponymous templates. If you're dealing with a user-defined type or function, it almost always makes more sense to templatize them directly.

Now, as for the exact error message, it's because the syntax that you're using to define v1 and v2 is illegal. You'd need to do either

vt!float.vertex v1 = vt!float(0.0, 0.0);

or preferrably.

auto v1 = vt!float(0.0, 0.0);

You can't construct the type on the left-hand side of the assignment operator like that.

- Jonathan M Davis
```
```On Sunday, 23 September 2012 at 04:03:28 UTC, Jonathan M Davis
wrote:
> On Sunday, September 23, 2012 05:49:06 Craig Dillabaugh wrote:
>> Hello,

clip

>
> Before anything, I'd question why you declared vt at all. If all you're
> putting in it is a single struct, then just templatize the struct directly:
>
> struct Vertex(T)
> {
>     ...
> }
>
> Now, it looks like you have a free function in there as well - euclid_dist -
> but there's no reason to put that in the same template. Just templatize it
> directly. Then you get
>
> T euclid_dist(T)(T a, T b) {...}
>
> And main ends up looking something like
>
> void main()
> {
>     auto v1 = Vertex!float(0.0, 0.0);
>     auto v2 = Vertex!float(2.0, 4.0);
>     writefln("The distance between vertex 1 and vertex 2 is %s",
>              euclid_dist(v1, v2));
> }
there was a typo in my euclid_dist function, the code for it, and
main now look like:

T euclid_dist(T)(vertex!T a, vertex!T b) {
T sum = 0;
foreach( ref a_crd, ref b_crd; lockstep( a.coords, b.coords ) ) {
sum += (a_crd - b_crd)*(a_crd - b_crd );
}
return sqrt(sum);
}

int main(string argv[] ) {
auto v1 = vertex!float(0.0,0.0);
auto v2 = vertex!float(3.0,4.0);
writeln("The distance between vertex 1 and vertex 2 is ",
euclid_dist!float(v1, v2) );
return 0;
}

One question. Is there any way to get the function template to
deduce the type of T from the vertices I pass, so that I can
call:

euclid_dist(v1, v2) )

euclid_dist!float(v1, v2) );

>
> It's actually fairly to explicitly declare a template in D outside of
> eponymous templates. If you're dealing with a user-defined type or function, it
> almost always makes more sense to templatize them directly.
>
> Now, as for the exact error message, it's because the syntax that you're using
> to define v1 and v2 is illegal. You'd need to do either
>
> vt!float.vertex v1 = vt!float(0.0, 0.0);
>
> or preferrably.
>
> auto v1 = vt!float(0.0, 0.0);
>
> You can't construct the type on the left-hand side of the assignment operator
> like that.
>
> - Jonathan M Davis

,

```
```On Sunday, September 23, 2012 06:37:30 Craig Dillabaugh wrote:
> One question. Is there any way to get the function template to deduce the type of T from the vertices I pass, so that I can call:
>
> euclid_dist(v1, v2) )
>
>
> euclid_dist!float(v1, v2) );

It should infer the types just fine as-is. Templated functions can pretty much always infer the template arguments from their function arguments. It's with types that that doesn't work (so a constructor requires template arguments, but a function does not).

- Jonathan M Davis
```
```On Sunday, 23 September 2012 at 04:51:31 UTC, Jonathan M Davis
wrote:
> On Sunday, September 23, 2012 06:37:30 Craig Dillabaugh wrote:
>> One question. Is there any way to get the function template to
>> deduce the type of T from the vertices I pass, so that I can
>> call:
>>
>> euclid_dist(v1, v2) )
>>
>>
>> euclid_dist!float(v1, v2) );
>
> It should infer the types just fine as-is. Templated functions can pretty much
> always infer the template arguments from their function arguments. It's with
> types that that doesn't work (so a constructor requires template arguments,
> but a function does not).
>
> - Jonathan M Davis

Yes, you are right. I thought I had tried it without the !float
and it hadn't worked, but I guess not.

Thanks again for all your help.
Craig
```