🚀 go-pugleaf

In article gjm@sdaw04.seinf.abb.se, alindbac@sw.seisy.abb.se (Anders Lindback) writes:
>In article <4gognf$et@news.bridge.net>,
>David Byrden  <100101.2547@compuserve.com> wrote:
>><<<<<<<

>>Why is it not allowed to delegate the initialisation to another
>>constructor

>>class X
>>{
>>public:
>>    X (int i) : i_(i) {}
>>
>>    X () : X(34) {}             // Not allowed
>>>>>>>>>>>
>>
>>     Why not use      X (int i = 34 ) : i_(i) {}

>>[ I think he was concerned with the more general case of a complicated
>>  initialization (not assignment) that must be repeated for each
>>  constructor. -sdc, moderator
>>]

>Well, one could use a similar thing using a member function:
>
>X (int i) { init(i); }
>X () { init(34); }
>
>where init is a member function for class X.

That does not solve the *initialization* problem. Consider this example:

class X {
	const int i;
	...
public:
	X() : i(0) { ... more stuff }
	X(int k) : i(k) { ... more stuff }
	X(const X& rhs) : i(rhs.i) { ... more stuff }
};

Because member 'i' is const, it cannot be assigned to, but must be
initialized. You cannot break out its initialization into a common
subroutine, but must repeat the initialization on every constructor.
In this toy example that is not a hardship, but suppose there were
many items, and each had a more complicated initialization.

Similarly, suppose 'i' were a class type that did not have a no-argument
constructor, or a reference. You would have to provide an initializer on
every constructor.

Finally, suppose 'i' were a class type that did have a no-argument
constructor, but you did not want the default initialization in
this class. You could let the default initialization occur then fix
up the value in a common subroutine. That approach is often wasteful,
particularly if it involves allocating and freeing memory, opening
and closing files, or setting and clearing locks.

I think the original question is really about a way to break out
a member-init-list into a common bit of code somehow, which cannot
currently be done. It seems to me it is (only) a convenience and ease-of-
maintenance issue.
---
Steve Clamage, stephen.clamage@eng.sun.com
---
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