[Issue 5931] New: keyword new insists on calling a struct's (non-zero argument) constructor--in other words, it won't allow default initialization; also postblit this(this) constructor dosen't get called from "new <structname>(<struct_instance>)"

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5931

           Summary: keyword new insists on calling a struct's (non-zero
                    argument) constructor--in other words, it won't allow
                    default initialization; also postblit this(this)
                    constructor dosen't get called from "new
                    <structname>(<struct_instance>)"
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Mac OS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: ultimatemacfanatic at gmail.com


--- Comment #0 from Christopher the Magnificent <ultimatemacfanatic at gmail.com> 2011-05-05 12:08:38 PDT ---
So I need to do this:

class Tokenizer
{
    File file;

    this(File f)
    {
        File f2; // default initialized

        // problem 1
        this.file = new File; // should pointer to return default initialized
File struct but compiler complains
        *this.file = f;

        // problem 2
        this.file = new File(f);  // this doesn't work either, notwithstanding
File has a this(this) post-blitter!
    }
}

unfortunately, on the line where I make the call to "new File", it gives me
this error:

Error: constructor std.stdio.File.this (string name, in const(char[])
stdioOpenmode = "rb") is not callable using argument types ()

Shouldn't I be able to allocate a default-initialized struct?  Or if not,
shouldn't it be allow to have empty-argument struct constructors?  
http://www.digitalmars.com/d/2.0/struct.html says empty parameter-list
constructors are not allowed for structs, but why not?  Can someone explain
this language decision to me and make a good argument for why it should not be
changed?

when I try "new File(f)" then I get this error message:

tokenizer.d(199): Error: constructor std.stdio.File.this (string name, in
const(char[]) stdioOpenmode = "rb") is not callable using argument types (File)
tokenizer.d(199): Error: cannot implicitly convert expression (file) of type
File to string

It seems to me that the post-blit constructor should be called automatically
when a struct  is "function-called" with an instance of the same struct as the
sole parameter like "new S(s_instance)"  Don't you agree?  You may think this
is a feature request, but when functionality is a departure from what one would
expect, I would tend to call that a bug instead.  :-)

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