Struct with default ctor (Was: [dmd-beta] dmd 2.064 beta take 2)

[Andrej Mitrovic]

On 5/17/13, Walter Bright <walter at digitalmars.com> wrote:
> I oppose this. D has a lot of nice features because of the .init property.
> Default constructors wreck that.

Would they? I'm thinking the process would be:

struct S
{
    int x;
    int y = void;

    this()  // hypothetical
    {
        // x would already be initialized to int.init here
        assert(x == int.init);

        // y is left uninitialized here
    }
}

Maybe that's already clear. But why is .init actually such a big
problem? If it becomes arbitrarily expensive to call .init of a
struct, well it's because it has to be - if the user really provided
an expensive default ctor. But it's entirely the user's
responsibility. So then .init can even throw, but throwing exceptions
isn't a big deal. Is there some other problem?

A custom default ctor in a struct is one of the most asked for
features. Just yesterday we spent several hours explaining to a C++
user why a default ctor doesn't work, and what .init is for. The whole
conversation could have been avoided if D had support for custom
default ctors for structs. This topic comes up very often in IRC and
the forums.