On 05/23/2010 05:06 PM, bearophile wrote:
Andrei Alexandrescu:
1. Using a class makes implementing members easier because there's no
need to do work through an additional member. With a struct, you need a
pimpl approach. For example:
struct Array {
struct Impl {
...
}
Impl * impl;
...
@property length() const { return impl->length; }
...
}
I am far from being an expert as you, and while I have read about the PImpl in
C++, I don't understand what you are saying here. I am sorry.
If you want to define a class you go like this:
class CBlah
{
private int payload_;
@property int payload() { return payload_; }
@property void payload(int p) { payload_ = p; }
}
Right? The class has by birth reference semantics:
auto obj = new CBlah;
obj.payload = 42;
auto obj1 = obj;
obj1.payload = 43;
assert(obj.payload == 43);
If you want to define a struct with ref semantics you define it like this:
struct SBlah
{
private struct Impl
{
private int payload_;
}
private Impl * pImpl;
@property int payload() { return pImpl->payload_; }
@property void payload(int p) { pImpl->payload_ = p; }
}
auto obj = SBlah();
obj.pImpl = new SBlah.Impl; // meh, a crappy detail that we need to
// discuss later
obj.payload = 42;
auto obj1 = obj;
obj1.payload = 43;
assert(obj.payload == 43);
That's what I said.
3. The creation syntaxes are different.
In C# you use 'new' for both structs allocated on the stack and classes
allocated on the heap:
http://msdn.microsoft.com/en-us/library/51y09td4%28VS.71%29.aspx
That's pretty retarded (sorry retard). There was opportunity there to
get rid of new altogether.
Let me note that I have reached the conclusion that containers should
be at best reference types, with a meta-constructor Value!(C) that takes
a container C and makes it into a value type.
Does Value!() use static introspection and placement new to instantiate the
given class on the stack? :-)
No. It does copy it automatically. There should be no notable difference
between Value!(Array!int) and std::vector<int>.
Andrei
