Re: "Value class instance" pattern?

[bearophile]

(Sorry, this post has gone in this newsgroup by mistake, but it's 
a small mistake.)

To change and understand your code a bit (now the ComposeClass 
constructor is a template) I have removed some of the tests and 
debug code, but they should be added back later:


/*
string listAvailableCtors(T)() {
    string result = "";
    foreach(t; __traits(getOverloads, T, "__ctor"))
        result ~= typeof(t).stringof ~ "\n";
    return result;
}
*/

struct DefaultCtor {} //call default ctor type
enum defaultCtor = DefaultCtor();

struct ComposeClass(T) if (is(T == class)) {
    void[__traits(classInstanceSize, T)] _classMemory = void;
    bool m_destructed = false;

    @property T _instance() {
        return cast(T)_classMemory.ptr;
    }

    @property const(T) _instance() const {
        return cast(const(T))_classMemory.ptr;
    }

    alias _instance this;

    @disable this();
    @disable this(this);

    this(DefaultCtor) {
        // _classMemory[] = typeid(T).init[]; // needed?
        _instance.__ctor;
    }

    this(Targs...)(Targs args) {
        _classMemory[] = typeid(T).init[];
        _instance.__ctor(args);
    }

    ~this() {
        if (!m_destructed) {
            _instance.destroy;
            m_destructed = true;
        }
    }
}

// Usage example ----------

class Foo {
    int i, j;
    this() {
        this.i = 5;
    }

    this(int ii) {
        this.i = ii;
    }
}

class Bar {
    ComposeClass!Foo f;

    this() {
        //f = typeof(f)(defaultCtor);
        f = typeof(f)(2); // alternative
    }
}

void main() {
    import std.stdio;
    auto bar = new Bar;
    writeln(bar.f.i);
    bar.f.i = 1;
    writeln(bar.f.i);
}


This code is unfinished.
Is the assignment needed in this(DefaultCtor)?

This code contains some sharp edges (for the D programmer and 
even for the compiler, I have opened a new bug report: 
http://d.puremagic.com/issues/show_bug.cgi?id=10629 ), for me 
it's easy to get wrong, hard to get right & good, and I believe 
it's of general usefulness, so I think it's fit for Phobos.

But isn't it a replacement for Phobos Scoped?

Bye,
bearophile