On 6/13/18 1:08 PM, Michał wrote:
On Wednesday, 13 June 2018 at 16:40:51 UTC, Steven Schveighoffer wrote:
On 6/13/18 10:43 AM, Michał wrote:
When I pass my struct to function something is going wrong. I don't
know how to fix it.
Code:
import std.stdio;
void print(ref Vector v, string s){
writefln("%s==%s %s", &v.x, v.ptr, s);
}
struct Vector {
int x;
int* ptr;
this(this) {
ptr = &x;
print(this, "postblit");
}
}
void someFunc(Vector t) {
print(t, "in someFunc");
}
void main() {
auto tmpA = Vector();
tmpA.ptr = &tmpA.x;
print(tmpA, "start");
someFunc(tmpA);
}
Result on my machine:
7FFF7D70BC00==7FFF7D70BC00 start
7FFF7D70BBF0==7FFF7D70BBF0 postblit
7FFF7D70BBD0==7FFF7D70BBF0 in someFunc
In the last line pointers are not matching. I thought that postblit
will do the thing but it is not the case. How to make 'ptr' to be
null or '&this.x' all the time?
D allows moving any struct instance without calling postblit, as long
as the original is no longer used.
The optimizer is likely seeing here that the memory can be copied
without calling postblit, because nobody is using tmpA after the call.
In general, you should NOT store an internal pointer in a struct,
unless you allocate it on the heap.
-Steve
I need internal pointer because I want to implement vector(like in C++)
with 'small vector optimization', when i have internal pointer it is
very easy and functions like 'add' don't have additional checks.
If storing internal pointers is forbidden do you know some way to
implement 'small vector optimization' without additional checks in 'add'
function?
Hm... the only way to do it in D is to provide a function that checks
whether the small vector optimization is in play, and return a
pointer/slice to itself.
With D it is possible to alias the getter function that provides the
actual data to allow code to look nicer.
For example (crude example):
struct Vector(T)
{
bool svo; // small vector optimization
union
{
T[4] local;
T[] heap;
}
inout(T)[] get() inout { return svo ? local[], heap; }
alias get this;
... // implement specialized append, concat operators, etc.
}
Now, you can use Vector as if it were an array, and it just works.
-Steve
