Re: D Classes Passed By Reference or Value?
On Sunday, 16 August 2015 at 22:31:02 UTC, Brandon Ragland wrote:
Hi All, I'm a bit confused as to how Classes in D are passed in
arguments and returns.
Take this for example:
class MyClass{
int x = 2;
}
And then in app.d
ref MyClass doStuff(){
MyClass mc = new MyClass() // Heap allocation, using new
return mc;
}
The above fails, as escaping reference to local variable
however, this was created using new Not understanding what
the deal is, this should be a valid heap allocated object, and
therefore, why can I not pass this by reference? I don't want
this to be a local variable...
So this begs the question: Are Classes (Objects) passed by
reference already?
-Brandon
Classes are reference types, a la Java/C#.
class MyClass {
int x = 5;
}
void main() {
auto c1 = new MyClass();
auto c2 = c1;
c1.x = 123;
assert(c2.x == 123);
}
Re: D Classes Passed By Reference or Value?
On Sunday, 16 August 2015 at 22:31:02 UTC, Brandon Ragland wrote:
ref MyClass doStuff(){
MyClass mc = new MyClass() // Heap allocation, using new
return mc;
}
This attempts to return a reference to the _variable_ `mc`, not a
reference to the class. Just remove `ref` from the function
signature.
Re: D Classes Passed By Reference or Value?
On Sunday, 16 August 2015 at 22:35:15 UTC, Alex Parrill wrote:
On Sunday, 16 August 2015 at 22:31:02 UTC, Brandon Ragland
wrote:
Hi All, I'm a bit confused as to how Classes in D are passed
in arguments and returns.
Take this for example:
class MyClass{
int x = 2;
}
And then in app.d
ref MyClass doStuff(){
MyClass mc = new MyClass() // Heap allocation, using new
return mc;
}
The above fails, as escaping reference to local variable
however, this was created using new Not understanding what
the deal is, this should be a valid heap allocated object, and
therefore, why can I not pass this by reference? I don't want
this to be a local variable...
So this begs the question: Are Classes (Objects) passed by
reference already?
-Brandon
Classes are reference types, a la Java/C#.
class MyClass {
int x = 5;
}
void main() {
auto c1 = new MyClass();
auto c2 = c1;
c1.x = 123;
assert(c2.x == 123);
}
Thanks,
That makes more sense. Though it does make the ref method
signature unclear, as it only applies to literals at this point?
Would you still need the ref signature for method parameters for
classes to avoid a copy? Such that I could work on the class
itself, and not a copy.
//This is reference?
void doStuff(ref MyClass mc){
return;
}
or would this also be a valid reference type:
//This is a copy?
void doStuff(MyClass mc){
return;
}
Re: D Classes Passed By Reference or Value?
On 08/16/2015 04:13 PM, Brandon Ragland wrote:
That makes more sense. Though it does make the ref method
signature unclear, as it only applies to literals at this
point?
As long as the returned object will be valid after the function leaves,
it can be anything: one of the ref parameters, a module-level variable, etc.
Would you still need the ref signature for method parameters
for classes to avoid a copy? Such that I could work on the
class itself, and not a copy.
Obviously, you meant the object itself.
//This is reference?
void doStuff(ref MyClass mc){
return;
}
Yes, that's a reference to a class variable. Since class variables are
references anyway, unless intended, there is one too many level of
indirection there. (Although, it is valid and it may exactly be what is
needed.)
or would this also be a valid reference type:
//This is a copy?
void doStuff(MyClass mc){
return;
}
That's the normal way of doing it. mc is class reference to an object
that was presumably created somewhere else.
Ali
Re: D Classes Passed By Reference or Value?
On Sunday, 16 August 2015 at 23:31:46 UTC, Ali Çehreli wrote:
On 08/16/2015 04:13 PM, Brandon Ragland wrote:
That makes more sense. Though it does make the ref method
signature unclear, as it only applies to literals at this
point?
As long as the returned object will be valid after the function
leaves, it can be anything: one of the ref parameters, a
module-level variable, etc.
Would you still need the ref signature for method parameters
for classes to avoid a copy? Such that I could work on the
class itself, and not a copy.
Obviously, you meant the object itself.
//This is reference?
void doStuff(ref MyClass mc){
return;
}
Yes, that's a reference to a class variable. Since class
variables are references anyway, unless intended, there is one
too many level of indirection there. (Although, it is valid and
it may exactly be what is needed.)
or would this also be a valid reference type:
//This is a copy?
void doStuff(MyClass mc){
return;
}
That's the normal way of doing it. mc is class reference to an
object that was presumably created somewhere else.
Ali
If I understand you correctly than:
void doStuff(MyClass mc){
mc.x = 7;
}
Would be working with the reference to the object instantiated
elsewhere. This would NOT be a copy of the object.
That would mean that (ref MyClass mc) is the equivalent to a
pointer to a pointer (sorta, though these are references, same
idea follows).
-Brandon
Re: D Classes Passed By Reference or Value?
On Sunday, 16 August 2015 at 23:40:41 UTC, Brandon Ragland wrote:
On Sunday, 16 August 2015 at 23:31:46 UTC, Ali Çehreli wrote:
On 08/16/2015 04:13 PM, Brandon Ragland wrote:
That makes more sense. Though it does make the ref method
signature unclear, as it only applies to literals at this
point?
As long as the returned object will be valid after the
function leaves, it can be anything: one of the ref
parameters, a module-level variable, etc.
Would you still need the ref signature for method parameters
for classes to avoid a copy? Such that I could work on the
class itself, and not a copy.
Obviously, you meant the object itself.
//This is reference?
void doStuff(ref MyClass mc){
return;
}
Yes, that's a reference to a class variable. Since class
variables are references anyway, unless intended, there is one
too many level of indirection there. (Although, it is valid
and it may exactly be what is needed.)
or would this also be a valid reference type:
//This is a copy?
void doStuff(MyClass mc){
return;
}
That's the normal way of doing it. mc is class reference to an
object that was presumably created somewhere else.
Ali
If I understand you correctly than:
void doStuff(MyClass mc){
mc.x = 7;
}
Would be working with the reference to the object instantiated
elsewhere. This would NOT be a copy of the object.
That would mean that (ref MyClass mc) is the equivalent to a
pointer to a pointer (sorta, though these are references, same
idea follows).
-Brandon
Yes.
void doStuff(ref MyClass a, ref MyClass b, ref MyClass c, bool d)
{
if (d)
b = a;
else
c = a;
}
void main(string[] args)
{
auto a = new MyClass(); a.x = 1;
auto b = new MyClass(); b.x = 2;
auto c = new MyClass(); c.x = 3;
assert(a !is b);
assert(a.x == 1);
assert(a !is c);
assert(b.x == 2);
assert(c !is b);
assert(c.x == 3);
doStuff(a,b,c,true);
assert(a is b);
assert(b.x == 1);
b.x = 2;
assert( a.x == 2);
assert(a !is c);
assert(c.x == 3);
doStuff(a,b,c,false);
assert(a is c);
assert(c.x == 2);
}
