d的懒值啊

接上篇
在外界观察到之前赋值.看看当前语言能否实现.

struct LazyValue(T) {
    private class Impl
    {
        immutable T value;
        this(immutable T v) { value = v; }
    }
    private Impl impl;
    private T delegate() pure generate;

    this(T delegate() pure _generate)
    {
        generate = _generate;
    }

    immutable(T) get() {
        if (impl is null)
            impl = new Impl(generate());
        return impl.value;
    }
    alias get this;
}

void main() {
    import std;
    auto lazyInt = LazyValue!int({ return 123; });
    writeln(lazyInt.get);
}

当前的generate()必须为纯,限制了使用.如不能从文件加载值.但,理论上,该方法应可以.