#949 Base class missing default constructor

[Rule]

Consider the following example:

Type B
    Declare Constructor (ByVal i As Integer)
    Value As Integer
End Type

Constructor B (ByVal i As Integer)
    This.Value = i
End Constructor

Type D Extends B
    Declare Constructor
End Type

Constructor D
    Base(0)
End Constructor

When the compiler reaches End Type of type D, it issues an error 188: Missing UDT.constructor(byref as const UDT) implementation (base UDT without default constructor requires manual initialization), found 'End' in 'End Type'

The error is superfluous because the default constructor of type B is never called. Could it be removed?

[Rule]

Tested with Version 1.08.1 (2021-07-08), built for linux-x86_64 (64bit)

[fxm (freebasic.net)]

Default-constructor and copy-constructor:

Dim As UDT u1  '' default-constructor called
Dim As UDT u2 = u1  '' copy-constructor called

If an UDT is missing any of the two, the compiler has to generate them automatically if a base or field has any constructors, in order to ensure the base/field are always getting constructed properly. However, if the base/field only has a non-default constructor, then the auto-generation isn't possible and the coder must help out (if any non default-constructors are declared, the compiler does not provide a default-constructor and the user is forced to declare one if necessary).

---

See in changelog.txt:

Version 0.24.0:
[fixed]
- Implicit default and copy constructors (and also a Let overload and a destructor, if needed) will now be generated for derived UDTs which don't have either of them yet, if their base has a default constructor. If it has only non-default constructors, an error will be shown.

Before version 0.24.0, a code like yours was allowed, but the copy-construction of a derived instance could be bugged (for a string field in the base for example: shallow copy instead of deep copy because string field badly constructed).

---

See in the Programmer's Guide "2. Rules of good manners (for constructors, copy-constructors, copy-assignment operators, and destructors)":
https://www.freebasic.net/wiki/wikka.php?wakka=ProPgCtorsAssignDtors2#ProPgCtorsAssignDtors22

---

See also "#578 Non-default base type ctor not autocalled, yet no error":
https://sourceforge.net/p/fbc/bugs/578/

---

Simplest workaround:
The non-default base constructor modified to allow optional parameters:

Type B
    Declare Constructor (ByVal i As Integer = 0)
    Value As Integer
End Type

Other workarounds:

• add a default-constructor for the B type: Constructor B (),
• or add a constant copy-constructor for the D type: Constructor D (Byref As Const D).

[Rule]

fxm, thanks for your extensive explanation. I found it also works to declare a default constructor for type B without defining it, as long as it is not actually called, in which case it is required anyway.

[Rule]

P.S. I guess this ticket can be closed now.