http://d.puremagic.com/issues/show_bug.cgi?id=4047

           Summary: [CTFE] class/struct heap allocation
           Product: D
           Version: future
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-04-02 14:31:22 PDT ---
struct Foo {}
class Bar {}
int spam() {
    Foo* f = new Foo();
    Bar b = new Bar();
    return 0;
}
enum int _ = spam();
void main() {}


dmd 2.042 gives:

test.d(4): Error: Cannot interpret new Foo at compile time
test.d(8): Error: cannot evaluate spam() at compile time
test.d(8): Error: cannot evaluate spam() at compile time

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Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug@yahoo.com.au


--- Comment #1 from Don <clugdbug@yahoo.com.au> 2010-04-03 11:29:04 PDT ---
This is because pointers are not supported in CTFE. (Nor are classes).

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--- Comment #2 from bearophile_hugs@eml.cc 2010-04-04 07:56:58 PDT ---
This is a (possibly silly) idea for a low-priority enhancement, to allow the allocation of class instances at compile-time. This can be useful because their creation time can be moved from run-time to compile-time.

When a class is instantiated at compile-time, it can be used and it can even get garbage collected. When the compile-time computation ends, the compile-time GC must collect all unused class instances (otherwise you can't tell what class instance the compiled program will contain. The compile-time GC can be a reference counter too, that is more deterministic) and put the active ones in a data segment of the binary.

If a class instance is mutable, but the data segment is read-only, then at the start of run time the runtime has to copy it into the heap and to update the internal pointers and references.

To reduce implementation complexity, the compile-time class instances can be immutable only, so there's no need to copy them to the heap and change the pointers. If the the class instance is immutable but its reference is mutable then the class instance may need a mutable boolean somewhere to tell if it's it's garbage-collected at runtime (because it can't be really deleted from the ROM).


This whole idea is related to the section "Pointer Hydration" written by Walter, the main difference is that it's done by the compiler (and the data is stored inside the binary itself), so for the programmer it's safer and simpler to use:

http://www.artima.com/cppsource/backyard.html

Such loading of already initialized class instances is commonly done in games written in C++, see for example the "Fast data load trick" in the Game Programming Gems 1 book.

Many things in D are designed taking the idea from a common C++ idiom, where it is usually implemented in a more or less manual way, and change it into a explicit D construct, that is semantically clean and possibly safer. So this compile-time class instances can be another exaple of such kind of idiom conversions.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla@digitalmars.com
         Resolution|                            |FIXED


--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> 2011-11-02 13:06:23 PDT ---
https://github.com/D-Programming-Language/dmd/commit/3055cc6a8b36a2e00c270b0e48e1d1bd2c931f37

https://github.com/D-Programming-Language/dmd/commit/72e750209b370b66157f320d3517c5816a3951d7

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