digitalmars.D.learn - Understanding opAssign and 'alias this'
- Maurice (29/29) Oct 13 2013 Hello,
- Kozzi (2/31) Oct 13 2013 It is a bug from my POV
- Maurice (5/6) Oct 14 2013 It sure seems like it. But what would be the correct behaviour?
- John Colvin (19/48) Oct 14 2013 Everything is working fine except for the error on [2] when xxx
- Maxim Fomin (4/22) Oct 14 2013 This does not work (and need not) because compiler generates
- Maurice (15/42) Oct 14 2013 Then why does it work when replacing "opAssign(A a)" with
- Maurice (3/46) Oct 14 2013 Or when just not defining any opAssign: struct A {}, that also
- Maxim Fomin (8/51) Oct 14 2013 OK, it seems to be what actually causes the problem is implicit
Hello,
Can anybody explain my what is happening here?
enum xxx = true;
struct A {
static if (xxx) ref A opAssign(ref const A a) { return this; }
ref A opAssign(int v) { return this; }
}
struct B {
A a;
alias a this;
}
void main() {
A a;
B b;
a = b; // [1]
b = a; // [2]
}
When xxx is false:
[1] Gives an error
[2] Compiles fine
When xxx is true:
[1] Compiles fine
[2] Gives an error
What exactly is happening here? When xxx is false, what does 'b =
a' do that is wrong when xxx is true?
I can't find any clear documentation about opAssign and its
'default' implementation.
Thanks!
-Maurice-
Oct 13 2013
On Sunday, 13 October 2013 at 21:37:31 UTC, Maurice wrote:
Hello,
Can anybody explain my what is happening here?
enum xxx = true;
struct A {
static if (xxx) ref A opAssign(ref const A a) { return this; }
ref A opAssign(int v) { return this; }
}
struct B {
A a;
alias a this;
}
void main() {
A a;
B b;
a = b; // [1]
b = a; // [2]
}
When xxx is false:
[1] Gives an error
[2] Compiles fine
When xxx is true:
[1] Compiles fine
[2] Gives an error
What exactly is happening here? When xxx is false, what does 'b
= a' do that is wrong when xxx is true?
I can't find any clear documentation about opAssign and its
'default' implementation.
Thanks!
-Maurice-
It is a bug from my POV
Oct 13 2013
"Maurice" <m-ou.se m-ou.se> On Sunday, 13 October 2013 at 22:47:12 UTC, Kozzi wrote:It is a bug from my POVIt sure seems like it. But what would be the correct behaviour? Should [1] and [2] both work in both cases? What are the rules around the default opAssign? Does the default one disappear by adding a opAssign(int)?
Oct 14 2013
On Sunday, 13 October 2013 at 21:37:31 UTC, Maurice wrote:
Hello,
Can anybody explain my what is happening here?
enum xxx = true;
struct A {
static if (xxx) ref A opAssign(ref const A a) { return this; }
ref A opAssign(int v) { return this; }
}
struct B {
A a;
alias a this;
}
void main() {
A a;
B b;
a = b; // [1]
b = a; // [2]
}
When xxx is false:
[1] Gives an error
[2] Compiles fine
When xxx is true:
[1] Compiles fine
[2] Gives an error
What exactly is happening here? When xxx is false, what does 'b
= a' do that is wrong when xxx is true?
I can't find any clear documentation about opAssign and its
'default' implementation.
Thanks!
-Maurice-
Everything is working fine except for the error on [2] when xxx
== true, which I think is a bug.
minimised test:
struct A
{
void opAssign(A a) {}
}
struct B {
A a;
alias a this;
}
void main() {
A a;
B b;
b = a;
}
Error: function assign.B.opAssign (B p) is not callable using
argument types (A)
Oct 14 2013
On Monday, 14 October 2013 at 09:17:12 UTC, John Colvin wrote:
Everything is working fine except for the error on [2] when xxx
== true, which I think is a bug.
minimised test:
struct A
{
void opAssign(A a) {}
}
struct B {
A a;
alias a this;
}
void main() {
A a;
B b;
b = a;
}
Error: function assign.B.opAssign (B p) is not callable using
argument types (A)
This does not work (and need not) because compiler generates
default function B.opAssign(B) which is really not callable using
argument types (A).
Oct 14 2013
On Monday, 14 October 2013 at 09:32:15 UTC, Maxim Fomin wrote:On Monday, 14 October 2013 at 09:17:12 UTC, John Colvin wrote:Then why does it work when replacing "opAssign(A a)" with "opAssign(int)"? struct A { void opAssign(int) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; // This now compiles fine... }Everything is working fine except for the error on [2] when xxx == true, which I think is a bug. minimised test: struct A { void opAssign(A a) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; } Error: function assign.B.opAssign (B p) is not callable using argument types (A)This does not work (and need not) because compiler generates default function B.opAssign(B) which is really not callable using argument types (A).
Oct 14 2013
On Monday, 14 October 2013 at 10:45:26 UTC, Maurice wrote:On Monday, 14 October 2013 at 09:32:15 UTC, Maxim Fomin wrote:Or when just not defining any opAssign: struct A {}, that also works. I don't get how adding a opAssign(A) disables 'b = a'.On Monday, 14 October 2013 at 09:17:12 UTC, John Colvin wrote:Then why does it work when replacing "opAssign(A a)" with "opAssign(int)"? struct A { void opAssign(int) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; // This now compiles fine... }Everything is working fine except for the error on [2] when xxx == true, which I think is a bug. minimised test: struct A { void opAssign(A a) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; } Error: function assign.B.opAssign (B p) is not callable using argument types (A)This does not work (and need not) because compiler generates default function B.opAssign(B) which is really not callable using argument types (A).
Oct 14 2013
On Monday, 14 October 2013 at 10:45:26 UTC, Maurice wrote:On Monday, 14 October 2013 at 09:32:15 UTC, Maxim Fomin wrote:OK, it seems to be what actually causes the problem is implicit conversion from B to A. Situation depends on presence of A.opAssign(A ) because when compiler tries to resolve function call, it is confused between calling B.opAssign(B) and casting through alias this B to A and calling A.opAssign(A). When it realizes that there is no best match it throws error. Since argument B is casted to A, the error message looks so weird.On Monday, 14 October 2013 at 09:17:12 UTC, John Colvin wrote:Then why does it work when replacing "opAssign(A a)" with "opAssign(int)"? struct A { void opAssign(int) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; // This now compiles fine... }Everything is working fine except for the error on [2] when xxx == true, which I think is a bug. minimised test: struct A { void opAssign(A a) {} } struct B { A a; alias a this; } void main() { A a; B b; b = a; } Error: function assign.B.opAssign (B p) is not callable using argument types (A)This does not work (and need not) because compiler generates default function B.opAssign(B) which is really not callable using argument types (A).
Oct 14 2013