Constructor Overloading
NameList header syntax is used to define different versions of a Constructor each with a different number of parameters, referred to as its signature. See Namelists for details. The illustrates this principle.
In deciding which Constructor to call, APL matches the shape of the Constructor argument with the signature of each of the Constructors that are defined. If a constructor with the same number of arguments exists (remembering that 0 arguments will match a niladic Constructor), it is called. If there is no exact match, and there is a Constructor with a general signature (an un-parenthesised right argument), it is called. If no suitable constructor is found, a LENGTH ERROR is reported.
There may be one and only one constructor with a particular signature.
The only way a Constructor function should be invoked is by ⎕NEW. See Base Constructors for further details. If you attempt to call a Constructor function from outside its Class, it will cause a SYNTAX ERROR. A Constructor function should not call another Constructor function within the same Class, although it will not generate an error. This would cause the Base Constructor to be called twice, with unpredictable consequences.
In the example Class, the following Constructors are defined:
| Constructor | Implied argument |
|---|---|
Make1 |
1-item vector |
Make2 |
2-item vector |
Make3 |
3-item vector |
Make0 |
No argument |
MakeAny |
Any array accepted |
In the following examples, the Make function (see for details) displays:
<shape of argument> <name of Constructor called><argument>
(see function make)
Creating a new Instance of Clover with a 1-element vector as the Constructor argument, causes the system to choose the Make1 Constructor. Note that, although the argument to Make1 is a 1-element vector, this is disclosed as the list of arguments is unpacked into the (single) variable arg1.
(⎕NEW Clover(,1)).Con
Make1 1
Creating a new Instance of Clover with a 2- or 3-element vector as the Constructor argument causes the system to choose Make2, or Make3 respectively.
(⎕NEW Clover(1 2)).Con
2 Make2 1 2
(⎕NEW Clover(1 2 3)).Con
3 Make3 1 2 3
Creating an Instance with any other Constructor argument causes the system to choose MakeAny.
(⎕NEW Clover(⍳10)).Con
10 MakeAny 1 2 3 4 5 6 7 8 9 10
(⎕NEW Clover(2 2⍴⍳4)).Con
2 2 MakeAny 1 2
3 4
Note that a scalar argument will call MakeAny and not Make1.
(⎕NEW Clover 1).Con
MakeAny 1
and finally, creating an Instance without a Constructor argument causes the system to choose Make0.
(⎕NEW Clover).Con
Make0 0