Re: Struct reference returning function and const members

2011-03-03 Thread Michel Fortin 

On 2011-03-03 01:42:18 -0500, Tom t...@nospam.com said:


I have...

int main(string[] args) {
auto s1 = f(); // MH MH
auto s2 = g(); // OK
s2.c = null; // OK
return 0;
}


I think the compiler complains because the s1.c member is not 
assignable (since it is const), so you can't assign to s1 as a whole. 
That said, it should probably be allowed for an initialization 
assignment like this one. I suggest you add it to the bug tracker.

http://d.puremagic.com/issues/


--
Michel Fortin
michel.for...@michelf.com
http://michelf.com/

Struct reference returning function and const members

2011-03-02 Thread Tom 

I have...

int main(string[] args) {
auto s1 = f(); // MH MH
auto s2 = g(); // OK
s2.c = null; // OK
return 0;
}

class C {}

struct StructWithConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
const(C) c;
}

struct StructWithoutConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
C c;
}

ref StructWithConstMember f() {
return * new StructWithConstMember(1, new C); // ERROR
}

ref StructWithoutConstMember g() {
return * new StructWithoutConstMember(1, new C); // OK
}


And get the error...
src\main.d(27): Error: *new StructWithConstMember(1,new C) is not mutable

So I can't return a struct that has a const member? Why? Am I missing 
something something?


Thanks,
Tom;

Re: Struct reference returning function and const members

2011-03-02 Thread Ali Çehreli 

On 03/02/2011 10:42 PM, Tom wrote:

I have...

int main(string[] args) {
auto s1 = f(); // MH MH
auto s2 = g(); // OK
s2.c = null; // OK
return 0;
}

class C {}

struct StructWithConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
const(C) c;
}

struct StructWithoutConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
C c;
}

ref StructWithConstMember f() {
return * new StructWithConstMember(1, new C); // ERROR
}

ref StructWithoutConstMember g() {
return * new StructWithoutConstMember(1, new C); // OK
}


And get the error...
src\main.d(27): Error: *new StructWithConstMember(1,new C) is not mutable

So I can't return a struct that has a const member? Why? Am I missing
something something?

Thanks,
Tom;


I don't know the full answer but returning a reference to const object 
works:


ref const(StructWithConstMember) f() {
return * new StructWithConstMember(1, new C); // now compiles
}

Ali

Re: Struct reference returning function and const members

2011-03-02 Thread Tom 

El 03/03/2011 03:47, Ali Çehreli escribió:

On 03/02/2011 10:42 PM, Tom wrote:

I have...

int main(string[] args) {
auto s1 = f(); // MH MH
auto s2 = g(); // OK
s2.c = null; // OK
return 0;
}

class C {}

struct StructWithConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
const(C) c;
}

struct StructWithoutConstMember {
this(int i, C c) { this.i=i; this.c=c; }
int i;
C c;
}

ref StructWithConstMember f() {
return * new StructWithConstMember(1, new C); // ERROR
}

ref StructWithoutConstMember g() {
return * new StructWithoutConstMember(1, new C); // OK
}


And get the error...
src\main.d(27): Error: *new StructWithConstMember(1,new C) is not mutable

So I can't return a struct that has a const member? Why? Am I missing
something something?

Thanks,
Tom;


I don't know the full answer but returning a reference to const object
works:

ref const(StructWithConstMember) f() {
return * new StructWithConstMember(1, new C); // now compiles
}

Ali



I know, but I don't want a const struct. It's like if const transitivity 
were going backwards, or something.


:(

Re: Struct reference returning function and const members

2011-03-02 Thread Jonathan M Davis 
On Wednesday 02 March 2011 22:42:18 Tom wrote:
 I have...
 
 int main(string[] args) {
   auto s1 = f(); // MH MH
   auto s2 = g(); // OK
   s2.c = null; // OK
   return 0;
 }
 
 class C {}
 
 struct StructWithConstMember {
   this(int i, C c) { this.i=i; this.c=c; }
   int i;
   const(C) c;
 }
 
 struct StructWithoutConstMember {
   this(int i, C c) { this.i=i; this.c=c; }
   int i;
   C c;
 }
 
 ref StructWithConstMember f() {
   return * new StructWithConstMember(1, new C); // ERROR
 }
 
 ref StructWithoutConstMember g() {
   return * new StructWithoutConstMember(1, new C); // OK
 }
 
 
 And get the error...
 src\main.d(27): Error: *new StructWithConstMember(1,new C) is not mutable
 
 So I can't return a struct that has a const member? Why? Am I missing
 something something?

Well, it has something do to with the ref, since passing be value works. I'm 
not 
sure if it's a bug or not. On the whole, I would _not_ advise that you have 
structs with const or immutable member variables. You can never reassign to 
them. And that may be related to why it's not working here. It may very well be 
a bug - I sure don't know why it isn't working here - but in general, having 
structs with const or immutable members probably isn't a good idea.

- Jonathan M Davis