Using getSymbolsByUDA in a static foreach loop
I'm trying to use getSymbolsByUDA in order to loop over all of
the members in a struct with a certain UDA, and then call a
function on the member. The plan is to use this to avoid looping
over an array of function pointers.
However, the compiler is giving a strange error and the
documentation of getSymbolsByUDA is unhelpful, as there are no
practical use-case examples.
Here's a very simplified version of my code
```d
import std.traits;
enum Runnable;
struct SubSystem
{
void run();
}
struct Manager
{
@Runnable SubSystem subsystem;
void run()
{
static foreach(system; getSymbolsByUDA!(Manager,
Runnable))
{
system.run();
}
}
}
void main()
{
Manager m;
m.run();
}
```
Result:
```
onlineapp.d(16): Error: value of `this` is not known at compile
time
```
This seems to me to be the logical way to write this code. What
am I missing?
Re: Using getSymbolsByUDA in a static foreach loop
On Wednesday, 19 January 2022 at 20:46:17 UTC, Jack Stouffer
wrote:
static foreach(system; getSymbolsByUDA!(Manager,
Runnable))
{
system.run();
onlineapp.d(16): Error: value of `this` is not known at compile
time
The getSymbols returns aliases, meaning you hit what I wrote
about a few days ago:
http://dpldocs.info/this-week-in-d/Blog.Posted_2022_01_10.html#tip-of-the-week
The `this` is a runtime value and all the other `static` things
work on compile time info.
So you want to `__traits(child, system, this).run()` and it
should work - the traits child will re-attach a this value.
Re: Using getSymbolsByUDA in a static foreach loop
On Wednesday, 19 January 2022 at 20:53:29 UTC, Adam D Ruppe wrote: So you want to `__traits(child, system, this).run()` and it should work - the traits child will re-attach a this value. The error is actually coming from trying to use the result of getSymbolsByUDA in the right part of the `static foreach`, not the call to the `run` function. Which was odd to me because I thought it just returned a `AliasSeq`. Here's a link to the erroring code with your traits change: https://run.dlang.io/is/gO84ox
Re: Using getSymbolsByUDA in a static foreach loop
On Wednesday, 19 January 2022 at 21:44:57 UTC, Jack Stouffer wrote: The error is actually coming from trying to use the result of getSymbolsByUDA in the right part of the `static foreach` huh.. I never use most of std.traits, they just complicate things. Bleh idk, I wouldn't bother with it and loop through the __traits instead.
Re: Using getSymbolsByUDA in a static foreach loop
On Wednesday, 19 January 2022 at 21:49:12 UTC, Adam D Ruppe wrote:
I never use most of std.traits, they just complicate things.
Bleh idk, I wouldn't bother with it and loop through the
__traits instead.
Unless I'm missing something obvious this has to be a DMD bug,
because this prints nothing:
```d
import std.traits;
import std.stdio;
enum Runnable;
struct SubSystem
{
void run()
{
writeln("SubSystem ran");
}
}
struct Manager
{
@Runnable SubSystem subsystem;
void run()
{
static foreach(member; __traits(allMembers, Manager))
{
static foreach (attribute;
__traits(getAttributes, member))
{
static if (attribute == Runnable)
{
__traits(child, Manager, member).run();
}
}
}
}
}
void main()
{
Manager m;
m.run();
}
```
The `__traits(getAttributes, member)` call always returns an
empty tuple. Calling `__traits(getAttributes, Manager.subsystem)`
manually works as expected.
Re: Using getSymbolsByUDA in a static foreach loop
On Thursday, 20 January 2022 at 00:55:33 UTC, Jack Stouffer wrote: static foreach(member; __traits(allMembers, Manager)) member here is a string, not the member. I prefer to call it memberName. Then you __traits(getMember, Manager, memberName) to actually get the alias you can pass to getAttributes.
Re: Using getSymbolsByUDA in a static foreach loop
On Thursday, 20 January 2022 at 01:14:51 UTC, Adam Ruppe wrote:
On Thursday, 20 January 2022 at 00:55:33 UTC, Jack Stouffer
wrote:
static foreach(member; __traits(allMembers,
Manager))
member here is a string, not the member. I prefer to call it
memberName.
Then you __traits(getMember, Manager, memberName) to actually
get the alias you can pass to getAttributes.
Thanks, that fixed it. Final working version for anyone who finds
this thread:
```d
import std.traits;
import std.stdio;
enum Runnable;
struct SubSystem
{
void run()
{
writeln("SubSystem ran");
}
}
struct Manager
{
@Runnable SubSystem subsystem;
void run()
{
static foreach(memberName; __traits(allMembers, Manager))
{
static foreach (attribute; __traits(getAttributes,
__traits(getMember, Manager, memberName)))
{
static if (is(attribute == Runnable))
{
__traits(getMember, Manager,
memberName).run();
}
}
}
}
}
void main()
{
Manager m;
m.run();
}
```
