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digitalmars.D.learn - Error: function `...` without `this` cannot be `const`

reply someone <someone somewhere.com> writes:
I do not understand the compiler error when I add the const 
keyword in the following function which works (and compiles) as 
expected without the const keyword:

```d
public string getAmountSI(
    in float lnumAmount
    ) const {

    /// (1) given amount

    string lstrAmount;

    if (lnumAmount < 1_000f) {

       lstrAmount = r"1K"c;

    } else {

       if (lnumAmount < 1_000_000f) {

          lstrAmount = format(r"%.0fK"c, lnumAmount / 1_000f);

       } else {

          lstrAmount = format(r"%.0fM"c, lnumAmount / 1_000_000f);

       }

    }

    return lstrAmount;

}
```

I used to put all attributes BEFORE the function name which now I 
understand is completely wrong since they should follow the 
parameter declaration section because putting them before affects 
the function in other ways.

I first noted this while browsing the DUB package DB and came 
accross https://code.dlang.org/packages/dscanner which states 
(among a lot of checks):

- Placement of const, immutable, or inout before a function 
return type instead of after the parameters

Is it because const for a function is intended to be used ONLY 
within a structure/class (ie: a method) and thus the this error 
(http://ddili.org/ders/d.en/const_member_functions.html) ?

Can anyone explain please ?

PS: I think I should re-check all the code I've written so far 
for things like this that obviously I quite not completely 
understand yet.
Jun 30 2021
next sibling parent reply Alexandru Ermicioi <alexandru.ermicioi gmail.com> writes:
On Wednesday, 30 June 2021 at 17:47:05 UTC, someone wrote:

 ...


That is because const/immutable/shared are being applied on the 
object hence 'this' variable inside function body if function is 
a member of a struct or class. It doesn't make sense to have a 
const modifier on a simple function. What will that const mean 
then in context of that function? To what it will be applied?

Best regards,
alexandri.
Jun 30 2021
parent reply someone <someone somewhere.com> writes:
On Wednesday, 30 June 2021 at 18:10:52 UTC, Alexandru Ermicioi 
wrote:


 That is because const/immutable/shared are being applied on the 
  object hence 'this' variable inside function body if function 
 is a member of a struct or class.


So this will make sense ONLY for an object's method right ?


 It doesn't make sense to have a const modifier on a simple 
 function. What will that const mean then in context of that 
 function? To what it will be applied?


I think of all the code I have I choose the worst example for 
asking advice on attribute placement !

Let's use  property instead of const:

```d
struct Foo
{
      property int data() { return m_data; } // read property

      property int data(int value) { return m_data = value; } // 
write property

   private:
     int m_data;
}
```

In the above example 
(https://dlang.org/spec/function.html#property-functions) 
 property is placed before the return type of the property and 
not after the parameters section.

At first I started to do the same, say, by intuition, but then I 
saw lots of examples like:

```d
string something()  property { return this.whatever; }
```

... and changed them accordingly.

Now I am not sure which is the correct way.

Thanks alexandri
Jun 30 2021
next sibling parent reply "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Wed, Jun 30, 2021 at 07:40:40PM +0000, someone via Digitalmars-d-learn wrote:
[...]

      property int data() { return m_data; } // read property


[...]

 string something()  property { return this.whatever; }


[...]

 Now I am not sure which is the correct way.


[...]

Both are correct. :-)  It's up to personal style preference.


T

-- 
2+2=4. 2*2=4. 2^2=4. Therefore, +, *, and ^ are the same operation.
Jun 30 2021
next sibling parent jfondren <julian.fondren gmail.com> writes:
On Wednesday, 30 June 2021 at 20:12:29 UTC, H. S. Teoh wrote:

 On Wed, Jun 30, 2021 at 07:40:40PM +0000, someone via 
 Digitalmars-d-learn wrote: [...]

      property int data() { return m_data; } // read property


 [...]

 string something()  property { return this.whatever; }


 [...]

 Now I am not sure which is the correct way.


 [...]

 Both are correct. :-)  It's up to personal style preference.


 T


 nogc unittest { } // works
unittest  nogc { } // doesn't work

https://dlang.org/dstyle.html doesn't suggest either way, but
does suggest alphabetical ordering(!) which I hadn't noticed.
Jun 30 2021
prev sibling parent Alexandru Ermicioi <alexandru.ermicioi gmail.com> writes:
On Wednesday, 30 June 2021 at 20:12:29 UTC, H. S. Teoh wrote:

 On Wed, Jun 30, 2021 at 07:40:40PM +0000, someone via 
 Digitalmars-d-learn wrote: [...]

      property int data() { return m_data; } // read property


 [...]

 string something()  property { return this.whatever; }


 [...]

 Now I am not sure which is the correct way.


 [...]

 Both are correct. :-)  It's up to personal style preference.


 T


Just to remark here, if you want to apply const to a return type 
put it inside brackets like: const(MyClass) foo(); otherwise 
compiler will try to apply it to 'this' parameter.

Best regards,
Alexandru.
Jun 30 2021
prev sibling parent reply Johan Lermer <johann.lermer elvin.eu> writes:
On Wednesday, 30 June 2021 at 19:40:40 UTC, someone wrote:

 Let's use  property instead of const:

 ```d
 struct Foo
 {
      property int data() { return m_data; } // read property

      property int data(int value) { return m_data = value; } // 
 write property

   private:
     int m_data;
 }


Isn't  property kind of deprecated?
Jul 01 2021
parent someone <someone somewhere.com> writes:
On Thursday, 1 July 2021 at 19:15:50 UTC, Johan Lermer wrote:


 Isn't  property kind of deprecated?


The docs state "experimental" not deprecated ... but what this 
means today I don't know.
Jul 01 2021
prev sibling parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
On Wed, Jun 30, 2021 at 05:47:05PM +0000, someone via Digitalmars-d-learn wrote:
[...]

 ```d
 public string getAmountSI(
    in float lnumAmount
    ) const {


[...]

 }
 ```
 
 I used to put all attributes BEFORE the function name which now I
 understand is completely wrong since they should follow the parameter
 declaration section because putting them before affects the function
 in other ways.


The `const` here is being applied to the implicit `this` parameter to
your function.  If your function is not a member function (does not have
an implicit `this` parameter), then the attribute is meaningless.


T

-- 
Give me some fresh salted fish, please.
Jun 30 2021