Re: How get struct value by member name string ?
On Tuesday, 30 May 2023 at 15:43:12 UTC, Steven Schveighoffer
wrote:
On 5/30/23 4:46 AM, 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);
}
compile time parameters come before runtime parameters:
```d
string getTMember(string memberName)(T t) {
return __traits(getMember, t, memberName);
}
// used like
auto v = getTMember!"name"(t);
```
-Steve
Yeah I did fix it in my message right after
Re: How get struct value by member name string ?
On 6/1/23 10:58 PM, John Xu wrote:
Ok, thanks for all you gentlemen's help. I tried following function,
now it works like C/C++/Python way:
string getTMember(T t, string columnName) {
foreach(member; __traits(allMembers, T)){
if (member == columnName) {
return __traits(getMember, mcu, member).to!string;
}
}
return "";
}
You can use a switch, and the search for the right member name should be
more optimized, though it's possible the compiler might recognize your
pattern and do this anyway:
```d
string getTMember(T t, string columnName) {
switch(columnName)
{
static foreach(member; __traits(allMembers, T)) {
case member:
return __traits(getMember, t, member).to!string;
}
default:
return "";
}
}
```
This is a very common pattern in e.g. serialization libs in D.
-Steve
Re: How get struct value by member name string ?
A correction:
string getTMember(T t, string columnName) {
foreach(member; __traits(allMembers, T)){
if (member == columnName) {
return __traits(getMember, t, member).to!string;
}
}
return "";
}
Re: How get struct value by member name string ?
On Thursday, 1 June 2023 at 15:38:08 UTC, Steven Schveighoffer
wrote:
On 5/31/23 12:08 AM, John Xu wrote:
When render vibe.d diet template,
string[] allMembers = __traits(allMembers, t);
enum allMembers = __traits(allMembers, t);
res.render!("index.dt", t, allMembers)
if I don't want write memberName one by one in diet template:
table
- foreach(memberName; allMembers)
tr
td #{memberName}
td #{getTMember!memberName(t)}
Problem: memberName is not known at compile time.
The problem is that you stored the member name list as a
runtime variable, and that is not known at compile time. Try
the enum.
Or really, just `foreach(memberName; __traits(allMembers, t))`
right in the diet template.
-Steve
Ok, thanks for all you gentlemen's help. I tried following
function,
now it works like C/C++/Python way:
string getTMember(T t, string columnName) {
foreach(member; __traits(allMembers, T)){
if (member == columnName) {
return __traits(getMember, mcu, member).to!string;
}
}
return "";
}
Re: How get struct value by member name string ?
On 5/31/23 12:08 AM, John Xu wrote:
When render vibe.d diet template,
string[] allMembers = __traits(allMembers, t);
enum allMembers = __traits(allMembers, t);
res.render!("index.dt", t, allMembers)
if I don't want write memberName one by one in diet template:
table
- foreach(memberName; allMembers)
tr
td #{memberName}
td #{getTMember!memberName(t)}
Problem: memberName is not known at compile time.
The problem is that you stored the member name list as a runtime
variable, and that is not known at compile time. Try the enum.
Or really, just `foreach(memberName; __traits(allMembers, t))` right in
the diet template.
-Steve
Re: How get struct value by member name string ?
On Tuesday, 30 May 2023 at 15:43:12 UTC, Steven Schveighoffer
wrote:
On 5/30/23 4:46 AM, 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);
}
compile time parameters come before runtime parameters:
```d
string getTMember(string memberName)(T t) {
return __traits(getMember, t, memberName);
}
// used like
auto v = getTMember!"name"(t);
```
-Steve
When render vibe.d diet template,
string[] allMembers = __traits(allMembers, t);
res.render!("index.dt", t, allMembers)
if I don't want write memberName one by one in diet template:
table
- foreach(memberName; allMembers)
tr
td #{memberName}
td #{getTMember!memberName(t)}
Problem: memberName is not known at compile time.
Re: How get struct value by member name string ?
On 5/30/23 4:46 AM, 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);
}
compile time parameters come before runtime parameters:
```d
string getTMember(string memberName)(T t) {
return __traits(getMember, t, memberName);
}
// used like
auto v = getTMember!"name"(t);
```
-Steve
Re: How get struct value by member name string ?
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));
```
Re: How get struct value by member name string ?
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, "..."));
```
Re: How get struct value by member name string ?
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));
}
```
Re: How get struct value by member name string ?
On Tuesday, 30 May 2023 at 01:33:54 UTC, H. S. Teoh wrote:
On Tue, May 30, 2023 at 01:24:46AM +, 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?
Re: How get struct value by member name string ?
On Tue, May 30, 2023 at 01:24:46AM +, 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
Re: How get struct value by member name string ?
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?
Re: How get struct value by member name string ?
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
