On Monday, 29 May 2023 at 09:35:11 UTC, John Xu wrote:
>     Error: variable `column` cannot be read at compile time

you should generally getMember on a variable

T t;
__traits(getMember, t, "name")

like that, that's as if you wrote t.name

On Monday, 29 May 2023 at 11:21:11 UTC, Adam D Ruppe wrote:
> On Monday, 29 May 2023 at 09:35:11 UTC, John Xu wrote:
>>     Error: variable `column` cannot be read at compile time
>
> you should generally getMember on a variable
>
> T t;
> __traits(getMember, t, "name")
>
> like that, that's as if you wrote t.name

It seems I can't use variable as member name:

    struct T {int a; string name;}
    T t;
    string s = "name";
    writeln(__traits(getMember, t, s));

Above code fails to compile. Any help?

On Tue, May 30, 2023 at 01:24:46AM +0000, John Xu via Digitalmars-d-learn wrote:
> On Monday, 29 May 2023 at 11:21:11 UTC, Adam D Ruppe wrote:
> > On Monday, 29 May 2023 at 09:35:11 UTC, John Xu wrote:
> > >     Error: variable `column` cannot be read at compile time
> > 
> > you should generally getMember on a variable
> > 
> > T t;
> > __traits(getMember, t, "name")
> > 
> > like that, that's as if you wrote t.name
> 
> It seems I can't use variable as member name:
> 
>     struct T {int a; string name;}
>     T t;
>     string s = "name";
>     writeln(__traits(getMember, t, s));
> 
> Above code fails to compile. Any help?

Short answer:

`s` must be known at compile-time.  Or more precisely, known at the time of template expansion. In this case, use `enum`:

	enum s = "name";


Long answer:
	https://wiki.dlang.org/Compile-time_vs._compile-time


T

-- 
Which is worse: ignorance or apathy? Who knows? Who cares? -- Erich Schubert

On Tuesday, 30 May 2023 at 01:33:54 UTC, H. S. Teoh wrote:
> On Tue, May 30, 2023 at 01:24:46AM +0000, John Xu via Digitalmars-d-learn wrote:
>> On Monday, 29 May 2023 at 11:21:11 UTC, Adam D Ruppe wrote:
>> > On Monday, 29 May 2023 at 09:35:11 UTC, John Xu wrote:
>> > >     Error: variable `column` cannot be read at compile time
>> > 
>> > you should generally getMember on a variable
>> > 
>> > T t;
>> > __traits(getMember, t, "name")
>> > 
>> > like that, that's as if you wrote t.name
>> 
>> It seems I can't use variable as member name:
>> 
>>     struct T {int a; string name;}
>>     T t;
>>     string s = "name";
>>     writeln(__traits(getMember, t, s));
>> 
>> Above code fails to compile. Any help?
>
> Short answer:
>
> `s` must be known at compile-time.  Or more precisely, known at the time of template expansion. In this case, use `enum`:
>
> 	enum s = "name";
>
>
> Long answer:
> 	https://wiki.dlang.org/Compile-time_vs._compile-time
>
>
> T


How to put above enum as a function parameter? Following code wouldn't work:

    string getTMember(T t, enum string memberName) {
        return __traits(getMember, t, memberName);
    }

My database table is very wide, with many columns. Above ddbc allows a struct
to map db returned data. Then if I want a member's value to show in vibe.d template,
how do I use a function to get it?

30.05.2023 11:46, John Xu пишет:
> 
> 
> How to put above enum as a function parameter? Following code wouldn't work:
> 
>      string getTMember(T t, enum string memberName) {
>          return __traits(getMember, t, memberName);
>      }
> 
> My database table is very wide, with many columns. Above ddbc allows a struct
> to map db returned data. Then if I want a member's value to show in vibe.d template,
> how do I use a function to get it?

This works for me:
```D
import std;

void main() {
    struct T {int a; string name;}
     auto t = T(12, "got by member name");
     enum s = "name";
     writeln(__traits(getMember, t, s));
}
```

On Tuesday, 30 May 2023 at 08:46:43 UTC, John Xu wrote:
>
> How to put above enum as a function parameter? Following code wouldn't work:
>
>     string getTMember(T t, enum string memberName) {
>         return __traits(getMember, t, memberName);
>     }
>
> ...

As simple as this:
```
string getTMember(T t, string memberName)() {
    return __traits(getMember, t, memberName);
}

...

writeln(getTMember!(t, "..."));
```

On Tuesday, 30 May 2023 at 15:24:21 UTC, bauss wrote:
> On Tuesday, 30 May 2023 at 08:46:43 UTC, John Xu wrote:
>>
>> How to put above enum as a function parameter? Following code wouldn't work:
>>
>>     string getTMember(T t, enum string memberName) {
>>         return __traits(getMember, t, memberName);
>>     }
>>
>> ...
>
> As simple as this:
> ```
> string getTMember(T t, string memberName)() {
>     return __traits(getMember, t, memberName);
> }
>
> ...
>
> writeln(getTMember!(t, "..."));
> ```

Noticed a mistake in my code ...

```
getTMember(T t, string memberName)()
```

Should be:

```
getTMember(T, string memberName)(T t)
```
And calling it should be like:

```
writeln(getTMember!("...")(t));
```