How to allocate an element of type T with value x in generic code?

[Ali Çehreli]

On 04/03/2013 10:19 AM, John Colvin wrote:
> On Wednesday, 3 April 2013 at 16:39:18 UTC, Ali Çehreli wrote:
>
>> That assignment will fail in general when the left-hand side has those
>> undetermined bits.
>
> Could you expand on this? I don't fully understand.

In short, there is no object on the left-hand side.

GC.malloc() does not initialize the memory that it allocates. Assignment 
involves destroying the lhs object. (It is two operations packaged 
together: copy the right-hand side and destroy the left-hand side.)

If the uninitialized bits in the newly-allocated memory were invalid for 
T, then the destruction will fail or do something wrong.

The following program is sure to fail because 'fileName' happens to be a 
bad string when opAssign() is entered:

// WARNING: Your system may become unresponsive if you execute this program
import std.stdio;
import core.memory;
import std.exception;
import std.array;

struct S
{
     string fileName;

     this(string fileName)
     {
         enforce(!fileName.empty);
     }

     ref S opAssign(S rhs)
     {
         writefln("Stop using file %s", fileName);

         this.fileName = rhs.fileName;
         writefln("Start using file %s", fileName);

         return this;
     }

     ~this()
     {
         writefln("Destroying S with %s", fileName);
     }
}

void main()
{
     auto x = S("abc");

     S* pt = cast(S*)GC.malloc(S.sizeof);
     *pt = x;
}

Ali