Constructors (starstruck noob from C++)

[Jacob Carlborg]

On 2011-01-21 04:02, Andrei Alexandrescu wrote:
> On 1/20/11 7:18 PM, Luke J. West wrote:
>> Hi to all from a total noob.
>>
>> first of all, I'd like to say how impressed I am with D. In fact, I keep
>> pinching myself. Have I *really* found a language worth leaving C++ for
>> after two decades? It's beginning to look that way. Obviously I'm
>> devouring the 2.0 documentation right now, but have not yet found out
>> how to create a new instance of an existing class object. What I mean
>> is, given...
>>
>> auto a = new A;
>>
>> how do I, in c++ speak, do the D for...
>>
>> A b(a); // or if you prefer...
>> A* b = new A(a);
>>
>> I'm sure this must be trivial.
>>
>> Many many thanks,
>>
>> Luke
>
> Hopefully this won't mark a demise of your newfound interest :o). If A
> were a struct, auto b = new A(*a) would do. For classes, D does not
> provide automatic copy constructors; you need to define and follow a
> sort of cloning protocol.
>
> That being said, it's not difficult to define a generic function that
> copies fields over from one class object to another. Here's a start:
>
> import std.stdio;
>
> void copyMembers(A)(A src, A tgt) if (is(A == class)) {
> foreach (e; __traits(allMembers, A)) {
> static if (!is(typeof(__traits(getMember, src, e)) == function)
> && e != "Monitor")
> {
> __traits(getMember, tgt, e) = __traits(getMember, src, e);
> }
> }
> }
>
> class A {
> int x = 42;
> string y = "hello";
> final void fun1() {}
> void fun2() {}
> static void fun3(){}
> }
>
> void main() {
> auto a = new A;
> a.x = 43;
> auto b = new A;
> copyMembers(a, b);
> assert(b.x == 43);
> }
>
> I think copyMembers belongs to the standard library. I wanted to define
> a family of functions like it but never got around to it.
>
>
> Andrei

Or you can use a serialization library, serialize the struct, then 
deserialize it and you have a deep copy. For example using Orange: 
http://dsource.org/projects/orange/ . Although it would not be very 
efficient to use XML as an intermediate format.

-- 
/Jacob Carlborg