Re: Template alias parameter: error: need 'this' for ...
On Friday, 24 February 2023 at 15:28:18 UTC, Steven Schveighoffer wrote: On 2/24/23 7:00 AM, Elfstone wrote: Seems like the same bug is still there after ten years. `static` should not affect module-level functions, but also, this code should work without `static`. Reported, not sure if there's a previous bug, it was hard to come up with a good description: https://issues.dlang.org/show_bug.cgi?id=23738 -Steve It was marked duplicate. I read the comments to the previous issue and felt like reading C++ discussions, except C++ is a lot better documented. Anyway surely `__traits(getAttributes, Bar.t)` compiles. The compiler should "need 'this'" when it actually need 'this' - unless there are attributes that can be bound to 'this'.
Re: Template alias parameter: error: need 'this' for ...
On Friday, 24 February 2023 at 14:22:17 UTC, user1234 wrote:
you can break using `goto`, restore `static` everywhere, and
using local introspection determine whether the result exists.
```d
struct Bar
{
@("hello") int t;
}
static bool hasAttribute(alias F, T)()
{
static foreach (a; __traits(getAttributes, F))
{
static if (is(typeof(a) : T))
{
enum result = true;
goto L0;
}
}
L0:
static if (is(typeof(result)))
return result;
else
return false;
}
void main()
{
import std.stdio;
writeln(hasAttribute!(Bar.t, string));
}
```
Unfortunately there is a serious bug in this code. Take a look at
what happens when you try it with this `struct Bar`:
```d
struct Bar
{
@("hello") @("goodbye") int t;
}
```
Re: Template alias parameter: error: need 'this' for ...
On 2/24/23 7:00 AM, Elfstone wrote: Seems like the same bug is still there after ten years. `static` should not affect module-level functions, but also, this code should work without `static`. Reported, not sure if there's a previous bug, it was hard to come up with a good description: https://issues.dlang.org/show_bug.cgi?id=23738 -Steve
Re: Template alias parameter: error: need 'this' for ...
On Friday, 24 February 2023 at 12:00:41 UTC, Elfstone wrote:
Seems like the same bug is still there after ten years.
```d
struct Bar
{
@("hello") int t;
}
static bool hasAttribute(alias F, T)()
{
bool result = false;
foreach (a; __traits(getAttributes, F))
{
static if (is(typeof(a) : T))
{
result = true; // couldn't simply return true, 'cause the
compiler complains about "unreachable code".
}
}
return result;
}
void main()
{
import std.stdio;
writeln(hasAttribute!(Bar.t, string));
}
```
you can break using `goto`, restore `static` everywhere, and
using local introspection determine whether the result exists.
```d
struct Bar
{
@("hello") int t;
}
static bool hasAttribute(alias F, T)()
{
static foreach (a; __traits(getAttributes, F))
{
static if (is(typeof(a) : T))
{
enum result = true;
goto L0;
}
}
L0:
static if (is(typeof(result)))
return result;
else
return false;
}
void main()
{
import std.stdio;
writeln(hasAttribute!(Bar.t, string));
}
```
Template alias parameter: error: need 'this' for ...
https://forum.dlang.org/post/imnannjdgtjnlzevh...@forum.dlang.org
On Saturday, 24 August 2013 at 11:47:43 UTC, Matej Nanut wrote:
On Friday, 23 August 2013 at 22:54:33 UTC, Jonathan M Davis
wrote:
Because without static it's a member variable, which means
that you have to
have a constructed object to access it (since it's part of the
object). When
you declare a variable in a class or struct static, then
there's only one for
the entire class or struct, so it can be accessed without an
object. And when
you do StructName.var or ClassName.var your accessing the
variable via the
struct or class rather than an object, so the variable must be
static.
- Jonathan M Davis
But I declared the template static, not the variable.
Is there a better way to pass a ‘member get’ expression to a
template?
I need this for calling ‘.offsetof’ on it, and for checking if
the member's parent is a certain struct type.
Seems like the same bug is still there after ten years.
struct Bar
{
@("hello") int t;
}
static bool hasAttribute(alias F, T)()
{
bool result = false;
foreach (a; __traits(getAttributes, F))
{
static if (is(typeof(a) : T))
{
result = true; // couldn't simply return true, 'cause the
compiler complains about "unreachable code".
}
}
return result;
}
void main()
{
import std.stdio;
writeln(hasAttribute!(Bar.t, string));
}
Re: Template alias parameter: error: need 'this' for ...
On Friday, 23 August 2013 at 22:54:33 UTC, Jonathan M Davis wrote: Because without static it's a member variable, which means that you have to have a constructed object to access it (since it's part of the object). When you declare a variable in a class or struct static, then there's only one for the entire class or struct, so it can be accessed without an object. And when you do StructName.var or ClassName.var your accessing the variable via the struct or class rather than an object, so the variable must be static. - Jonathan M Davis But I declared the template static, not the variable. Is there a better way to pass a ‘member get’ expression to a template? I need this for calling ‘.offsetof’ on it, and for checking if the member's parent is a certain struct type.
Template alias parameter: error: need 'this' for ...
Hello!
I've run into this issue that I don't understand, maybe someone
can enlighten me. :)
This code:
---
struct Thing
{
int i;
}
void main()
{
t!(Thing.i)();
}
void t(alias a)()
{
return;
}
---
fails to compile with: ‘Error: need 'this' for 't' of type 'pure
nothrow @safe void()'’.
If I declare ‘t’ as static, it works (and does what I want it to
do).
Re: Template alias parameter: error: need 'this' for ...
On Friday, August 23, 2013 23:28:46 Matej Nanut wrote:
Hello!
I've run into this issue that I don't understand, maybe someone
can enlighten me. :)
This code:
---
struct Thing
{
int i;
}
void main()
{
t!(Thing.i)();
}
void t(alias a)()
{
return;
}
---
fails to compile with: ‘Error: need 'this' for 't' of type 'pure
nothrow @safe void()'’.
If I declare ‘t’ as static, it works (and does what I want it to
do).
Because without static it's a member variable, which means that you have to
have a constructed object to access it (since it's part of the object). When
you declare a variable in a class or struct static, then there's only one for
the entire class or struct, so it can be accessed without an object. And when
you do StructName.var or ClassName.var your accessing the variable via the
struct or class rather than an object, so the variable must be static.
- Jonathan M Davis
