On Sunday, 22 August 2021 at 07:58:12 UTC, james.p.leblanc wrote:
Hello,
Question about a possible strategy to avoid problems with
undesired/unintended copying of a structure:
1) We have a struct, call this **Foo**.
2) We instantiate it with, **x = Foo(a,b,c);**
a. our constructor will initialize a field:
**this.myadd = &this**
b. this capture the address of our original "x" in
x.myadd.
Sorry, I don't follow this at all.
Consider:
```d
struct Foo {
int a, b, c;
Foo* myadd;
this(int a, int b, int c) {
myadd = &this;
this.a = a;
this.b = b;
this.c = c;
}
}
void main() {
import std.stdio : writeln;
Foo x; // initialized with (0, 0, 0, null)
writeln(&x); // a memory location
x = Foo(1, 2, 3); // (1, 2, 3, &x)
writeln(&x); // the exact same memory location
writeln(x.myadd.a); // 1, rather than 0, because &x==x.myadd
}
```
There's no saving the original "x" here, it's overwritten by the
assignment. Are you sure you don't want classes instead, to get
reference semantics?
4) Our goal is to avoid problems with any accidental copying
of x ... say by doing: **auto y=x;**
5) the copy of step 4 **does not use the constructor**, thus
y.myadd
would contain the address of x (and not y)
`auto y=x;` actually does call object lifetime functions.
```d
struct CountCopies {
int copies;
this(this) { // postblit
copies++;
}
}
unittest {
CountCopies x;
auto y = x;
auto z = y;
assert(x.copies == 0);
assert(y.copies == 1);
assert(z.copies == 2);
}
```
or with a copy constructor,
```d
struct CountCopies {
int copies;
this(ref return scope const CountCopies rhs) { // copy
constructor
copies = rhs.copies + 1;
}
@disable this(this); // helps pre-copy-constructor compilers
reject this code
}
unittest {
CountCopies x;
auto y = x;
auto z = y;
assert(x.copies == 0);
assert(y.copies == 1);
assert(z.copies == 2);
}
```
Some initial experiments lead me to believe this may acheive
part of what I would like. But, it feels very "hackish" and
ugly.
Is there a better way?
If you don't get an answer that you like, I suggesting posting
functional code and then stating your dissastisfactions with it.
