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digitalmars.D.learn - assigment to null class object member compiled? is this a bug?

reply dangbinghoo <dangbinghoo gmail.com> writes:
hi,

why the code bellow compiles?

---
import std.stdio;
class A {
     int m;
}

void main() {
     A a;
     a.m = 1;
}
---

and running this code get:

`segmentation fault (core dumped)  ./test`

I consider this couldn't be compiled according to book <The D 
Programming Language>.

The latest dmd (2.082) and LDC2 behaves the same.
Oct 18 2018
next sibling parent reply Vijay Nayar <madric gmail.com> writes:
On Friday, 19 October 2018 at 06:53:32 UTC, dangbinghoo wrote:

 hi,

 why the code bellow compiles?

 ---
 import std.stdio;
 class A {
     int m;
 }

 void main() {
     A a;
     a.m = 1;
 }
 ---

 and running this code get:

 `segmentation fault (core dumped)  ./test`

 I consider this couldn't be compiled according to book <The D 
 Programming Language>.

 The latest dmd (2.082) and LDC2 behaves the same.


Technically the code you have is syntactically correct.  You are 
permitted to create a class variable without assigning it to a 
class object.  (Assigning it to a class object would look like "A 
a = new A();")

Which section of The D Programming Language book makes you think 
this would not compile?  I have the book as well, but I'm not 
quite sure what part of the book you're referring to.
Oct 19 2018
parent reply dangbinghoo <dangbinghoo gmail.com> writes:
On Friday, 19 October 2018 at 09:08:32 UTC, Vijay Nayar wrote:

 Technically the code you have is syntactically correct.  You 
 are permitted to create a class variable without assigning it 
 to a class object.  (Assigning it to a class object would look 
 like "A a = new A();")

 Which section of The D Programming Language book makes you 
 think this would not compile?  I have the book as well, but I'm 
 not quite sure what part of the book you're referring to.


the section 6.2, which is
---
A a;
a.x = 5;
---

the book explained this should be refused to compile.


thanks!

--
dangbinghoo
Oct 21 2018
parent reply Alex <sascha.orlov gmail.com> writes:
On Monday, 22 October 2018 at 01:39:48 UTC, dangbinghoo wrote:

 On Friday, 19 October 2018 at 09:08:32 UTC, Vijay Nayar wrote:

 Technically the code you have is syntactically correct.  You 
 are permitted to create a class variable without assigning it 
 to a class object.  (Assigning it to a class object would look 
 like "A a = new A();")

 Which section of The D Programming Language book makes you 
 think this would not compile?  I have the book as well, but 
 I'm not quite sure what part of the book you're referring to.


 the section 6.2, which is
 ---
 A a;
 a.x = 5;
 ---

 the book explained this should be refused to compile.


 thanks!

 --
 dangbinghoo


You are wrong, actually:
In the book, site 179 it states:
"If you try to access a non-static member of a reference and the 
compiler can prove statically that the reference would definitely 
be null, it will refuse to compile the code."

But the compiler is not able to prove that the reference is 
definitely null. At least in this case, with compiler, I think of.
That's why it called runtime error.
Oct 22 2018
parent dangbinghoo <dangbinghoo gmail.com> writes:
On Monday, 22 October 2018 at 15:51:03 UTC, Alex wrote:

 On Monday, 22 October 2018 at 01:39:48 UTC, dangbinghoo wrote:

 On Friday, 19 October 2018 at 09:08:32 UTC, Vijay Nayar wrote:

 Technically the code you have is syntactically correct.  You 
 are permitted to create a class variable without assigning it 
 to a class object.  (Assigning it to a class object would 
 look like "A a = new A();")

 Which section of The D Programming Language book makes you 
 think this would not compile?  I have the book as well, but 
 I'm not quite sure what part of the book you're referring to.


 the section 6.2, which is
 ---
 A a;
 a.x = 5;
 ---

 the book explained this should be refused to compile.


 thanks!

 --
 dangbinghoo


 You are wrong, actually:
 In the book, site 179 it states:
 "If you try to access a non-static member of a reference and 
 the compiler can prove statically that the reference would 
 definitely be null, it will refuse to compile the code."

 But the compiler is not able to prove that the reference is 
 definitely null. At least in this case, with compiler, I think 
 of.
 That's why it called runtime error.


no, I'm not, the code

---
A a;
a.x = 5
--

is the example explaining that 'the compiler can prove statically 
that the reference would
  definitely be null, it will refuse to compile the code.'

and for `is not able to prove that the reference is definitely 
null` the books has another example code:

---
A a;

if (<condition>) {
   a = new A;
}

...

if (<condition>) {
   a.x = 43; // correct
}
---

I think Adam is giving a better reason: the envolving D now 
behaves different by default about the null flow catch with the 
book.

thanks!
Oct 22 2018
prev sibling parent Adam D. Ruppe <destructionator gmail.com> writes:
On Friday, 19 October 2018 at 06:53:32 UTC, dangbinghoo wrote:

 why the code bellow compiles?


D compilers are allowed to make that an error, but it might not.

With the current implementation,

dmd that.d

will compile, but

dmd -O that.d

will fail with an error. Yes, turning on optimizations happens to 
catch the null flow at compile time.

It is just a matter of compiler implementation happening to catch 
it or not.
Oct 22 2018