digitalmars.D.learn - full copies on assignment
- John Nixon (42/42) May 24 2016 1 import std.stdio;
- Adam D. Ruppe (6/8) May 24 2016 It's because the char[] being shallow copied still leads to
- John Nixon (3/12) May 24 2016 Thank you for this suggestion, perhaps a slightly neater
- Marc =?UTF-8?B?U2Now7x0eg==?= (8/22) May 25 2016 Or add an explicit constructor:
- John Nixon (3/10) May 26 2016 Unfortunately this results in "Error: cannot implicitly convert
- Marc =?UTF-8?B?U2Now7x0eg==?= (24/35) May 27 2016 Hmm... this is the full program that works for me:
- John Nixon (3/3) May 29 2016 On Friday, 27 May 2016 at 08:59:43 UTC, Marc Schütz wrote:
1 import std.stdio;
2
3 struct CS{
4 char[] t;
5 CS opAssign(const CS rhs){
6 writeln("CS.opAssign called");
7 this.t = rhs.t.dup;
8 return this;}
9 };
10 void test_fun(const ref CS rhs){
11 CS cs = rhs;//error cannot implicitly convert expression
(rhs) of type const(CS) to CS
12 CS cs;// these two lines in place of the line above
13 cs = rhs;//now give a program that runs correctly but needs
CS.opAssign
14 writeln("cs = ",cs);}
15
16 void main(){
17 CS rhs;
18 rhs.t="string".dup;
19 test_fun(rhs);
20 return;}
In the above program I noticed the impossibility of compiling a
declaration of a struct and its assignment in the same line (line
11) inside a function which has, as a parameter, a const ref to
the RHS of the assignment. This naively doesn’t seem right
because the RHS of an assignment should not be altered by it. On
investigating further, I could compile the program (with DMD64 D
Compiler v2.071.0) by adding CS.opAssign as indicated and
splitting line 11 into lines 12 and 13 as the comments suggest.
This has do to with D not making a full (deep) copy on assignment
of a CS because it contains a char[], instead giving a pointer to
it that could be used to change it. I think full copies should be
made on assignment regardless of type.
It seems to me that shallow copying of objects is partially in
conflict with the const system, and full copying should be the
default in assignment (perhaps over-ridable by using a keyword eg
alias) which would simplify the const system not giving access to
pointers, and would allow the above program to compile in the way
I first thought of (without CS.opAssign and lines 12 and 13).
I think this is almost the same issue as discussed in “Copying
structs with pointers” on 2011-07-20
May 24 2016
On Tuesday, 24 May 2016 at 14:29:53 UTC, John Nixon wrote:This naively doesn’t seem right because the RHS of an assignment should not be altered by it.It's because the char[] being shallow copied still leads to mutable stuff. What I typically do here is just add a method `dup` to the struct that deep copies. Then you do `CS cs = rhs.dup;` when you want to construct it, like you'd do with a copy of a naked array.
May 24 2016
On Tuesday, 24 May 2016 at 15:17:37 UTC, Adam D. Ruppe wrote:On Tuesday, 24 May 2016 at 14:29:53 UTC, John Nixon wrote:Thank you for this suggestion, perhaps a slightly neater workaround.This naively doesn’t seem right because the RHS of an assignment should not be altered by it.It's because the char[] being shallow copied still leads to mutable stuff. What I typically do here is just add a method `dup` to the struct that deep copies. Then you do `CS cs = rhs.dup;` when you want to construct it, like you'd do with a copy of a naked array.
May 24 2016
On Tuesday, 24 May 2016 at 20:58:11 UTC, John Nixon wrote:On Tuesday, 24 May 2016 at 15:17:37 UTC, Adam D. Ruppe wrote:Or add an explicit constructor: struct CS { // ... this(const CS rhs) { this = rhs; } } Then you can write: auto cs = CS(rhs);On Tuesday, 24 May 2016 at 14:29:53 UTC, John Nixon wrote:Thank you for this suggestion, perhaps a slightly neater workaround.This naively doesn’t seem right because the RHS of an assignment should not be altered by it.It's because the char[] being shallow copied still leads to mutable stuff. What I typically do here is just add a method `dup` to the struct that deep copies. Then you do `CS cs = rhs.dup;` when you want to construct it, like you'd do with a copy of a naked array.
May 25 2016
On Wednesday, 25 May 2016 at 15:44:34 UTC, Marc Schütz wrote:On Tuesday, 24 May 2016 at 20:58:11 UTC, John Nixon wrote:On Tuesday, 24 May 2016 at 15:17:37 UTC, Adam D. Ruppe wrote:On Tuesday, 24 May 2016 at 14:29:53 UTC, John Nixon wrote:Or add an explicit constructor: struct CS { // ... this(const CS rhs) { this = rhs; }Unfortunately this results in "Error: cannot implicitly convert expression (rhs) of type const(CS) to CS'.
May 26 2016
On Thursday, 26 May 2016 at 10:51:30 UTC, John Nixon wrote:On Wednesday, 25 May 2016 at 15:44:34 UTC, Marc Schütz wrote:Hmm... this is the full program that works for me: import std.stdio; struct CS { char[] t; this(const CS rhs) { this = rhs; } CS opAssign(const CS rhs) { writeln("CS.opAssign called"); this.t = rhs.t.dup; return this; } }; void test_fun(const ref CS rhs) { auto cs = CS(rhs); writeln("cs = ",cs); } void main() { CS rhs; rhs.t = "string".dup; test_fun(rhs); return; }On Tuesday, 24 May 2016 at 20:58:11 UTC, John Nixon wrote:On Tuesday, 24 May 2016 at 15:17:37 UTC, Adam D. Ruppe wrote:On Tuesday, 24 May 2016 at 14:29:53 UTC, John Nixon wrote:Or add an explicit constructor: struct CS { // ... this(const CS rhs) { this = rhs; }Unfortunately this results in "Error: cannot implicitly convert expression (rhs) of type const(CS) to CS'.
May 27 2016
John Nixon <john.h.nixon1 gmail.com> On Friday, 27 May 2016 at 08:59:43 UTC, Marc Schütz wrote: Yes indeed it does. Thanks. Something in my version must have been different.
May 29 2016