Re: static if - unexpected results
On Friday, 23 June 2023 at 18:43:06 UTC, Steven Schveighoffer wrote: It should be a spec change. Change POD to say "type" instead of "struct". The goal of `isPOD` is to determine how careful generic code needs to be to pass the type around, or copy it. Changing it to false implies that it is not "plain old data". I.e. it has a destructor, it has hidden members, or it cannot be copied via bit copying (all of these do not fit the type in question). The only other option is to error on calling `__traits(isPOD, char)`, but I think that's even worse. -Steve Yeah, I think that's also where I'm standing. The current behavior seems correct and useful, it's just not documented correctly.
Re: static if - unexpected results
On 6/23/23 10:31 AM, FeepingCreature wrote:
On Friday, 23 June 2023 at 14:22:24 UTC, DLearner wrote:
Hi
Was looking for compile-time detection of a struct variable.
However, the following test code gave the two 'FAILS' shown below.
Comments?
```
void main() {
import std.stdio : writeln;
import std.traits;
string mxnTst(string VarName) {
return
`static if (is(typeof(` ~ VarName ~ `) == char)) {` ~
`writeln("` ~ VarName ~ ` ", " is a char");` ~
`} else static if (__traits(isPOD, typeof(` ~ VarName ~ `))) {` ~
`writeln("` ~ VarName ~ ` ", " is a struct");` ~
`} else static if (is(typeof(` ~ VarName ~ `) == int)) {` ~
`writeln("` ~ VarName ~ ` ", " is an int");` ~
`} else {` ~
`static assert(false, "mxnTst Variable '` ~ VarName ~ `' is
of unknown type");` ~
`}`
;
}
char char1;
int int1;
byte byte1;
struct foo {
int fooint;
char foochar;
}
foo foovar1;
mixin(mxnTst("char1")); // Expected: char1 is a char. Actual:
char1 is a char. (ok)
mixin(mxnTst("int1")); // Expected: int1 is an int. Actual: int1
is a struct. (FAIL)
mixin(mxnTst("foovar1")); // Expected: foovar1 is a struct. Actual:
foovar1 is a struct. (ok)
mixin(mxnTst("byte1")); // Expected: Run to fail with the static
assert message. Actual: byte1 is a struct. (FAIL)
}
```
```
static assert(__traits(isPOD, int)); // ok.
static assert(__traits(isPOD, byte)); // ok.
```
It's a bug in either the spec or the compiler.
It should be a spec change. Change POD to say "type" instead of "struct".
The goal of `isPOD` is to determine how careful generic code needs to be
to pass the type around, or copy it. Changing it to false implies that
it is not "plain old data". I.e. it has a destructor, it has hidden
members, or it cannot be copied via bit copying (all of these do not fit
the type in question).
The only other option is to error on calling `__traits(isPOD, char)`,
but I think that's even worse.
-Steve
Re: static if - unexpected results
On Friday, 23 June 2023 at 16:51:16 UTC, Ali Çehreli wrote:
On 6/23/23 07:22, DLearner wrote:
>`} else static if (__traits(isPOD, typeof(` ~ VarName
~ `))) {` ~
Regardless, you can also use the 'is' expression with the
'struct' keyword. If T is a struct,
is (T == struct)
that will produce true at compile time.
Ali
Thanks for this - I can confirm it works.
Re: static if - unexpected results
On 6/23/23 07:22, DLearner wrote:
>`} else static if (__traits(isPOD, typeof(` ~ VarName ~ `))) {` ~
Regardless, you can also use the 'is' expression with the 'struct'
keyword. If T is a struct,
is (T == struct)
that will produce true at compile time.
Ali
Re: static if - unexpected results
On Friday, 23 June 2023 at 15:48:44 UTC, H. S. Teoh wrote: On Friday, 23 June 2023 at 15:22:36 UTC, DLearner wrote: On Friday, 23 June 2023 at 14:31:45 UTC, FeepingCreature wrote: On Friday, 23 June 2023 at 14:22:24 UTC, DLearner wrote: [...] ``` static assert(__traits(isPOD, int)); // ok. static assert(__traits(isPOD, byte)); // ok. ``` It's a bug in either the spec or the compiler. I am using ``` DMD64 D Compiler v2.103.0-dirty ``` under ``` Windows [Version 10.0.19045.3086] ``` Do I need to report this anywhere? Tested your original code on latest dmd git master, here's the output: d char1 is a char int1 is a struct foovar1 is a struct byte1 is a struct Looks like there isn't a problem? Or at least, it's now fixed in git master. Which exact version of dmd are you using? Did you download from dlang.org or did you build your own? --T Probably I misunderstand, but to me: ``` int1 is a struct ``` and ``` byte1 is a struct ``` are both errors. The 'struct' test is being triggered for things that are not structs.
Re: static if - unexpected results
On Friday, 23 June 2023 at 15:22:36 UTC, DLearner wrote: On Friday, 23 June 2023 at 14:31:45 UTC, FeepingCreature wrote: On Friday, 23 June 2023 at 14:22:24 UTC, DLearner wrote: [...] ``` static assert(__traits(isPOD, int)); // ok. static assert(__traits(isPOD, byte)); // ok. ``` It's a bug in either the spec or the compiler. I am using ``` DMD64 D Compiler v2.103.0-dirty ``` under ``` Windows [Version 10.0.19045.3086] ``` Do I need to report this anywhere? Tested your original code on latest dmd git master, here's the output: d char1 is a char int1 is a struct foovar1 is a struct byte1 is a struct Looks like there isn't a problem? Or at least, it's now fixed in git master. Which exact version of dmd are you using? Did you download from dlang.org or did you build your own? --T
Re: static if - unexpected results
On Friday, 23 June 2023 at 14:31:45 UTC, FeepingCreature wrote: On Friday, 23 June 2023 at 14:22:24 UTC, DLearner wrote: [...] ``` static assert(__traits(isPOD, int)); // ok. static assert(__traits(isPOD, byte)); // ok. ``` It's a bug in either the spec or the compiler. I am using ``` DMD64 D Compiler v2.103.0-dirty ``` under ``` Windows [Version 10.0.19045.3086] ``` Do I need to report this anywhere?
Re: static if - unexpected results
On Friday, 23 June 2023 at 14:22:24 UTC, DLearner wrote:
Hi
Was looking for compile-time detection of a struct variable.
However, the following test code gave the two 'FAILS' shown
below.
Comments?
```
void main() {
import std.stdio : writeln;
import std.traits;
string mxnTst(string VarName) {
return
`static if (is(typeof(` ~ VarName ~ `) == char)) {` ~
`writeln("` ~ VarName ~ ` ", " is a char");` ~
`} else static if (__traits(isPOD, typeof(` ~ VarName ~
`))) {` ~
`writeln("` ~ VarName ~ ` ", " is a struct");` ~
`} else static if (is(typeof(` ~ VarName ~ `) == int)) {`
~
`writeln("` ~ VarName ~ ` ", " is an int");` ~
`} else {` ~
`static assert(false, "mxnTst Variable '` ~ VarName ~
`' is of unknown type");` ~
`}`
;
}
char char1;
int int1;
byte byte1;
struct foo {
int fooint;
char foochar;
}
foo foovar1;
mixin(mxnTst("char1")); // Expected: char1 is a char.
Actual: char1 is a char. (ok)
mixin(mxnTst("int1"));// Expected: int1 is an int.
Actual: int1 is a struct. (FAIL)
mixin(mxnTst("foovar1")); // Expected: foovar1 is a struct.
Actual: foovar1 is a struct. (ok)
mixin(mxnTst("byte1")); // Expected: Run to fail with the
static assert message. Actual: byte1 is a struct. (FAIL)
}
```
```
static assert(__traits(isPOD, int)); // ok.
static assert(__traits(isPOD, byte)); // ok.
```
It's a bug in either the spec or the compiler.
static if - unexpected results
Hi
Was looking for compile-time detection of a struct variable.
However, the following test code gave the two 'FAILS' shown below.
Comments?
```
void main() {
import std.stdio : writeln;
import std.traits;
string mxnTst(string VarName) {
return
`static if (is(typeof(` ~ VarName ~ `) == char)) {` ~
`writeln("` ~ VarName ~ ` ", " is a char");` ~
`} else static if (__traits(isPOD, typeof(` ~ VarName ~
`))) {` ~
`writeln("` ~ VarName ~ ` ", " is a struct");` ~
`} else static if (is(typeof(` ~ VarName ~ `) == int)) {` ~
`writeln("` ~ VarName ~ ` ", " is an int");` ~
`} else {` ~
`static assert(false, "mxnTst Variable '` ~ VarName ~
`' is of unknown type");` ~
`}`
;
}
char char1;
int int1;
byte byte1;
struct foo {
int fooint;
char foochar;
}
foo foovar1;
mixin(mxnTst("char1")); // Expected: char1 is a char.
Actual: char1 is a char. (ok)
mixin(mxnTst("int1"));// Expected: int1 is an int.
Actual: int1 is a struct. (FAIL)
mixin(mxnTst("foovar1")); // Expected: foovar1 is a struct.
Actual: foovar1 is a struct. (ok)
mixin(mxnTst("byte1")); // Expected: Run to fail with the
static assert message. Actual: byte1 is a struct. (FAIL)
}
```
