digitalmars.D.learn - Is there a way to see if .init has been set by the user?
- simendsjo (8/8) Sep 21 2013 I want to know if a variable has changed .init, but I don't know
- bearophile (7/15) Sep 21 2013 init is part of a type and you can't change it. a and by are both
- Andrej Mitrovic (13/16) Sep 21 2013 There is however a way to extract it if it's a part of an aggregate:
- simendsjo (4/21) Sep 21 2013 I'm though of using if for aggregates, yes. But this doesn't
- Maxim Fomin (32/35) Sep 21 2013 Well, D wouldn't be D, if it did not allow something like this
- Maxim Fomin (4/12) Sep 21 2013 Strictly speaking this is an ill-posed question. Are talking
- simendsjo (3/16) Sep 21 2013 No, at compile-time. It's trivial to see if init is changed, but
- monarch_dodra (10/29) Sep 21 2013 I think there is a misunderstanding in your "a.init" and
- Dicebot (6/25) Sep 21 2013 I see. No, I don't think this feature is suitable for declarative
I want to know if a variable has changed .init, but I don't know
if it's possible if the .init value is the same. Does anyone have
a solution for this?
int a;
int b = 0;
pragma(msg, a.init); // 0
pragma(msg, b.init); // 0
// how can I see that b has "= 0"?
Sep 21 2013
simendsjo:
I want to know if a variable has changed .init, but I don't
know if it's possible if the .init value is the same. Does
anyone have a solution for this?
int a;
int b = 0;
pragma(msg, a.init); // 0
pragma(msg, b.init); // 0
// how can I see that b has "= 0"?
init is part of a type and you can't change it. a and by are both
ints, and in both cases int.init is 0. I think there is no simple
way to know if a variable has being default-initialized or not.
Why do you want to know it?
Bye,
bearophile
Sep 21 2013
On 9/21/13, bearophile <bearophileHUGS lycos.com> wrote:I think there is no simple way to know if a variable has being default-initialized or not. Why do you want to know it?There is however a way to extract it if it's a part of an aggregate: ----- struct S { int x = 1; } void main() { static assert(S.x.init == 0); static assert(S.init.x == 1); } -----
Sep 21 2013
On Saturday, 21 September 2013 at 12:50:20 UTC, Andrej Mitrovic wrote:On 9/21/13, bearophile <bearophileHUGS lycos.com> wrote:I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I think there is no simple way to know if a variable has being default-initialized or not. Why do you want to know it?There is however a way to extract it if it's a part of an aggregate: ----- struct S { int x = 1; } void main() { static assert(S.x.init == 0); static assert(S.init.x == 1); } -----
Sep 21 2013
On Saturday, 21 September 2013 at 13:30:19 UTC, simendsjo wrote:I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I don't think those two case are any different from the type system point of view, manual or default.
Sep 21 2013
On Saturday, 21 September 2013 at 13:38:44 UTC, Dicebot wrote:On Saturday, 21 September 2013 at 13:30:19 UTC, simendsjo wrote:Need some __traits(hasUserSuppliedInit, T) then :)I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I don't think those two case are any different from the type system point of view, manual or default.
Sep 21 2013
On Saturday, 21 September 2013 at 13:48:00 UTC, simendsjo wrote:On Saturday, 21 September 2013 at 13:38:44 UTC, Dicebot wrote:I am not sure those differ even at compiler level (other than at syntax stage) - why would you need it?On Saturday, 21 September 2013 at 13:30:19 UTC, simendsjo wrote:Need some __traits(hasUserSuppliedInit, T) then :)I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I don't think those two case are any different from the type system point of view, manual or default.
Sep 21 2013
On Saturday, 21 September 2013 at 13:55:00 UTC, Dicebot wrote:On Saturday, 21 September 2013 at 13:48:00 UTC, simendsjo wrote:I was thinking of "query by example". If a field has been set, it should be used in a where clause. But if I cannot see the difference between int.init and 0 for instance, it is impossible to get it right. It's possible to solve this using UDAs of course.On Saturday, 21 September 2013 at 13:38:44 UTC, Dicebot wrote:I am not sure those differ even at compiler level (other than at syntax stage) - why would you need it?On Saturday, 21 September 2013 at 13:30:19 UTC, simendsjo wrote:Need some __traits(hasUserSuppliedInit, T) then :)I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I don't think those two case are any different from the type system point of view, manual or default.
Sep 21 2013
"Namespace" <rswhite4 googlemail.com> On Saturday, 21 September 2013 at 14:11:44 UTC, simendsjo wrote:On Saturday, 21 September 2013 at 13:55:00 UTC, Dicebot wrote:0 and int.init is the same. ;)On Saturday, 21 September 2013 at 13:48:00 UTC, simendsjo wrote:I was thinking of "query by example". If a field has been set, it should be used in a where clause. But if I cannot see the difference between int.init and 0 for instance, it is impossible to get it right. It's possible to solve this using UDAs of course.On Saturday, 21 September 2013 at 13:38:44 UTC, Dicebot wrote:I am not sure those differ even at compiler level (other than at syntax stage) - why would you need it?On Saturday, 21 September 2013 at 13:30:19 UTC, simendsjo wrote:Need some __traits(hasUserSuppliedInit, T) then :)I'm though of using if for aggregates, yes. But this doesn't handle the case where you set the same value is T.init.I don't think those two case are any different from the type system point of view, manual or default.
Sep 21 2013
On Saturday, 21 September 2013 at 11:35:11 UTC, bearophile wrote:init is part of a type and you can't change it. Bye, bearophileWell, D wouldn't be D, if it did not allow something like this for aggregate types: import core.stdc.string, std.stdio; pure hack(T)(T value) if (is(T == class)) { byte[] init_mem = new byte[T.classinfo.init.length]; memcpy(init_mem.ptr, cast(byte*)value, T.classinfo.init.length); T.classinfo.init = init_mem; } class A { int a; } pure foo(int val) { A a = new A; a.a++; hack(a); return a.a + val; } void main() { writeln(0.foo, 0.foo, 0.foo); // prints 1,2,3 } And taking into accout some additional runtime magic, assertion below may fail: class A { int i = 5; } struct S { A a;} S s; assert(s.a is null);
Sep 21 2013
On Saturday, 21 September 2013 at 11:13:57 UTC, simendsjo wrote:
I want to know if a variable has changed .init, but I don't
know if it's possible if the .init value is the same. Does
anyone have a solution for this?
int a;
int b = 0;
pragma(msg, a.init); // 0
pragma(msg, b.init); // 0
// how can I see that b has "= 0"?
Strictly speaking this is an ill-posed question. Are talking
about testing at runtime whether some variable was changed from
init to some non-default value?
Sep 21 2013
On Saturday, 21 September 2013 at 15:43:11 UTC, Maxim Fomin wrote:On Saturday, 21 September 2013 at 11:13:57 UTC, simendsjo wrote:No, at compile-time. It's trivial to see if init is changed, but I want to see if it's set even if it's to T.init.I want to know if a variable has changed .init, but I don't know if it's possible if the .init value is the same. Does anyone have a solution for this? int a; int b = 0; pragma(msg, a.init); // 0 pragma(msg, b.init); // 0 // how can I see that b has "= 0"?Strictly speaking this is an ill-posed question. Are talking about testing at runtime whether some variable was changed from init to some non-default value?
Sep 21 2013
On Saturday, 21 September 2013 at 16:09:35 UTC, simendsjo wrote:On Saturday, 21 September 2013 at 15:43:11 UTC, Maxim Fomin wrote:I think there is a misunderstanding in your "a.init" and "b.init": This is just calling a static property on an instance. It's no different from T.init. It *won't* tell you what a/b was initially initialized with. If I understand correctly, you want: int b = 3; static assert(__traits(initialValue, b) == 3); ? I don't think we have that. I'm not even sure we *can* have that.On Saturday, 21 September 2013 at 11:13:57 UTC, simendsjo wrote:No, at compile-time. It's trivial to see if init is changed, but I want to see if it's set even if it's to T.init.I want to know if a variable has changed .init, but I don't know if it's possible if the .init value is the same. Does anyone have a solution for this? int a; int b = 0; pragma(msg, a.init); // 0 pragma(msg, b.init); // 0 // how can I see that b has "= 0"?Strictly speaking this is an ill-posed question. Are talking about testing at runtime whether some variable was changed from init to some non-default value?
Sep 21 2013
On Saturday, 21 September 2013 at 16:09:35 UTC, simendsjo wrote:On Saturday, 21 September 2013 at 15:43:11 UTC, Maxim Fomin wrote:I see. No, I don't think this feature is suitable for declarative approach abuse ;) Even if you manage to hack something, it is unlikely to be reliable, I'd recommend to stick to UDA's until compiler exposes that info.On Saturday, 21 September 2013 at 11:13:57 UTC, simendsjo wrote:No, at compile-time. It's trivial to see if init is changed, but I want to see if it's set even if it's to T.init.I want to know if a variable has changed .init, but I don't know if it's possible if the .init value is the same. Does anyone have a solution for this? int a; int b = 0; pragma(msg, a.init); // 0 pragma(msg, b.init); // 0 // how can I see that b has "= 0"?Strictly speaking this is an ill-posed question. Are talking about testing at runtime whether some variable was changed from init to some non-default value?
Sep 21 2013