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digitalmars.D.learn - DMD 2.092 and DIP 25

reply Mike Parker <aldacron gmail.com> writes:
The following declarations now give a deprecation warning:

```d
struct ErrorInfo {
private:
     char[32] _error;
     char[96] _message;

public  nogc nothrow  property:
     /**
         Returns the string "Missing Symbol" to indicate a symbol 
load failure, and
         the name of a library to indicate a library load failure.
     */
     const(char)* error() const { return _error.ptr; }

     /**
         Returns a symbol name for symbol load failures, and a 
system-specific error
         message for library load failures.
     */
     const(char)* message() const { return _message.ptr; }
}
```

I find it rather annoying, as I'm returning `const(char)*` and 
not `char*`, but it is what it is. My question is, if I add 
`return` to the function declarations, will this compile all the 
way back to DMD 2.067 *without* `-preview=dip25`? It works on 
2.091.0. I always assumed a preview feature's syntax wasn't 
supported without the preview switch.
May 30 2020
next sibling parent reply Max Samukha <maxsamukha gmail.com> writes:
On Saturday, 30 May 2020 at 07:00:07 UTC, Mike Parker wrote:


 I find it rather annoying, as I'm returning `const(char)*` and 
 not `char*`, but it is what it is. My question is, if I add 
 `return` to the function declarations, will this compile all 
 the way back to DMD 2.067 *without* `-preview=dip25`? It works 
 on 2.091.0. I always assumed a preview feature's syntax wasn't 
 supported without the preview switch.


https://run.dlang.io/is/aOZqww

Since      2.067.1: Success and no output
May 30 2020
next sibling parent Mike Parker <aldacron gmail.com> writes:
On Saturday, 30 May 2020 at 07:30:17 UTC, Max Samukha wrote:

 On Saturday, 30 May 2020 at 07:00:07 UTC, Mike Parker wrote:

 https://run.dlang.io/is/aOZqww

 Since      2.067.1: Success and no output


Thanks! I forgot that run.dlang.io supports all those old 
compilers.
May 30 2020
prev sibling parent Mike Parker <aldacron gmail.com> writes:
On Saturday, 30 May 2020 at 07:30:17 UTC, Max Samukha wrote:

 On Saturday, 30 May 2020 at 07:00:07 UTC, Mike Parker wrote:


 https://run.dlang.io/is/aOZqww

 Since      2.067.1: Success and no output


Thanks, Max (and you, too, Seb). I had forgotten that 
run.dlang.io supports compilers going so far back.
May 30 2020
prev sibling next sibling parent Seb <seb wilzba.ch> writes:
On Saturday, 30 May 2020 at 07:00:07 UTC, Mike Parker wrote:

 The following declarations now give a deprecation warning:

 ```d
 struct ErrorInfo {
 private:
     char[32] _error;
     char[96] _message;

 public  nogc nothrow  property:
     /**
         Returns the string "Missing Symbol" to indicate a 
 symbol load failure, and
         the name of a library to indicate a library load 
 failure.
     */
     const(char)* error() const { return _error.ptr; }

     /**
         Returns a symbol name for symbol load failures, and a 
 system-specific error
         message for library load failures.
     */
     const(char)* message() const { return _message.ptr; }
 }
 ```

 I find it rather annoying, as I'm returning `const(char)*` and 
 not `char*`, but it is what it is. My question is, if I add 
 `return` to the function declarations, will this compile all 
 the way back to DMD 2.067 *without* `-preview=dip25`? It works 
 on 2.091.0. I always assumed a preview feature's syntax wasn't 
 supported without the preview switch.


Return is actually pretty old, so it will compile:

https://run.dlang.io/is/DgbYU9

Typically -preview flags are just looked at during the semantic 
phases.

Another solution could be to turn the functions into templates 
and let the compiler do its attribute inference.
May 30 2020
prev sibling parent reply Steven Schveighoffer <schveiguy gmail.com> writes:
On 5/30/20 3:00 AM, Mike Parker wrote:

 The following declarations now give a deprecation warning:
 
 ```d
 struct ErrorInfo {
 private:
      char[32] _error;
      char[96] _message;
 
 public  nogc nothrow  property:
      /**
          Returns the string "Missing Symbol" to indicate a symbol load 
 failure, and
          the name of a library to indicate a library load failure.
      */
      const(char)* error() const { return _error.ptr; }
 
      /**
          Returns a symbol name for symbol load failures, and a 
 system-specific error
          message for library load failures.
      */
      const(char)* message() const { return _message.ptr; }
 }
 ```
 
 I find it rather annoying, as I'm returning `const(char)*` and not 
 `char*`, but it is what it is. My question is, if I add `return` to the 
 function declarations, will this compile all the way back to DMD 2.067 
 *without* `-preview=dip25`? It works on 2.091.0. I always assumed a 
 preview feature's syntax wasn't supported without the preview switch.


I had to run the code to see the warning to understand what you meant.

Here is the warning:

onlineapp.d(11): Deprecation: returning &this._error escapes a reference 
to parameter this, perhaps annotate with return
onlineapp.d(17): Deprecation: returning &this._message escapes a 
reference to parameter this, perhaps annotate with return

This is not about const or not, it's about lifetime management.

For example, this would return a pointer to a stack frame that is about 
to go away:

const(char)* foo()
{
    ErrorInfo info;
    return info.message;
}

I know that you have already fixed the problem, but I wanted to make 
sure you understood why the compiler is complaining.

-Steve
May 30 2020
parent Mike Parker <aldacron gmail.com> writes:
On Saturday, 30 May 2020 at 16:14:34 UTC, Steven Schveighoffer 
wrote:


 This is not about const or not, it's about lifetime management.

 For example, this would return a pointer to a stack frame that 
 is about to go away:

 const(char)* foo()
 {
    ErrorInfo info;
    return info.message;
 }

 I know that you have already fixed the problem, but I wanted to 
 make sure you understood why the compiler is complaining.


Yes, that was me having never actually read DIP 25 and 
misunderstanding what it was about. I've since been educated. 
Thanks!
May 30 2020