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digitalmars.D.learn - Confusion about static arrays

reply Rick Mann <rmann-d-lang latencyzero.com> writes:
I had a little hiccup earlier when I tried to do this:

	static EventTypeSpec[1] events =
	[
		{ EventClass.kEventClassWindow, kEventWindowBoundsChanged }
	];
	writefln("events: 0x%x", events);

(EventTypeSpec is a struct with two uint members).

This resulted in a bus error inside writefln(). Someone pointed out that I
should pass events.ptr, which works. However, from the examples in
(http://www.digitalmars.com/d/arrays.html). For example, it says that static
arrays are analogous to C arrays. It also suggests this:

int* p;
int[3] s;
int[] a;

p = s;		// p points to the first element of the array s.
p = a;		// p points to the first element of the array a.

In both cases, assigning from s or a results in something pointing to the first
element of s and a. 

I guess I have two questions:

a) why would passing "events" to writefln() (with that specific format
specifier) cause it all to crash

b) Is the spec web page incorrect (or is it reasonable to be mislead by its
wording) when it seems to imply that one can simply pass the name of the array
around, rather than .ptr?

Thanks!
Jan 27 2007
next sibling parent reply Kirk McDonald <kirklin.mcdonald gmail.com> writes:
Rick Mann wrote:

 I had a little hiccup earlier when I tried to do this:
 
 	static EventTypeSpec[1] events =
 	[
 		{ EventClass.kEventClassWindow, kEventWindowBoundsChanged }
 	];
 	writefln("events: 0x%x", events);
 
 (EventTypeSpec is a struct with two uint members).
 
 This resulted in a bus error inside writefln(). Someone pointed out that I
should pass events.ptr, which works. However, from the examples in
(http://www.digitalmars.com/d/arrays.html). For example, it says that static
arrays are analogous to C arrays. It also suggests this:
 
 int* p;
 int[3] s;
 int[] a;
 
 p = s;		// p points to the first element of the array s.
 p = a;		// p points to the first element of the array a.
 
 In both cases, assigning from s or a results in something pointing to the
first element of s and a. 
 
 I guess I have two questions:
 
 a) why would passing "events" to writefln() (with that specific format
specifier) cause it all to crash


It seems that writefln doesn't know how to convert a static array to a 
hex number. It does, however, know how to write a pointer in hex, so 
using .ptr works.


 b) Is the spec web page incorrect (or is it reasonable to be mislead by its
wording) when it seems to imply that one can simply pass the name of the array
around, rather than .ptr?


The spec is incorrect. D was changed a few versions ago to disallow this 
implicit conversion from arrays to pointers.

-- 
Kirk McDonald
Pyd: Wrapping Python with D
http://pyd.dsource.org
Jan 27 2007
parent Rick Mann <rmann-d-lang latencyzero.com> writes:
Kirk McDonald Wrote:


 It seems that writefln doesn't know how to convert a static array to a 
 hex number. It does, however, know how to write a pointer in hex, so 
 using .ptr works.


I should've been more clear. I wasn't actually trying to write the contents of
the array, just the address of the first element in it. But your note about the
spec helps me to clear things up.

Thanks!
Jan 28 2007
prev sibling parent torhu <fake address.dude> writes:
Rick Mann wrote:

 I had a little hiccup earlier when I tried to do this:
 
 	static EventTypeSpec[1] events =
 	[
 		{ EventClass.kEventClassWindow, kEventWindowBoundsChanged }
 	];
 	writefln("events: 0x%x", events);
 
 (EventTypeSpec is a struct with two uint members).
 
 This resulted in a bus error inside writefln(). Someone pointed out that I
should pass events.ptr, which works. However, from the examples in
(http://www.digitalmars.com/d/arrays.html). For example, it says that static
arrays are analogous to C arrays. It also suggests this:
 
 int* p;
 int[3] s;
 int[] a;
 
 p = s;		// p points to the first element of the array s.
 p = a;		// p points to the first element of the array a.
 
 In both cases, assigning from s or a results in something pointing to the
first element of s and a. 
 
 I guess I have two questions:
 
 a) why would passing "events" to writefln() (with that specific format
specifier) cause it all to crash
 


I don't know, you would have to look at the source code that comes with 
dmd to see why it crashes.  "%x" is definitely not the correct format 
specifier for an array, though.  The real problem is that writefln 
doesn't seem to know what to do with structs.  Crashing is not a great 
solution, though.

You can either do this:
writefln("events: 0x%x, 0x%x", events[0].field1, events[0].field2);

Or you can give the struct a toString() member.  Then you can use "%s" 
as the format, or just nothing at all.  writefln (which I seem to 
remember uses std.format.doFormat) will call your toString for each element.

char[] toString()
{
     return "{" ~ std.string.toString(field1) ~ ", " ~
                     std.string.toString(field2)  "}";
}

  b) Is the spec web page incorrect (or is it reasonable to be mislead 
by its wording) when it seems to imply that one can simply pass the name 
of the array around, rather than .ptr?

 


The name of the array doesn't get implicitly converted to a pointer to 
the data anymore. Other than that, you can pass arrays around as you 
please.  Be aware that static arrays are also passed as length + pointer 
when used as variadic arguments, which messes up things when used with a 
C api.
Jan 27 2007