http://d.puremagic.com/issues/show_bug.cgi?id=9588

           Summary: format prints "null" for struct functions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody@puremagic.com
        ReportedBy: andrej.mitrovich@gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-25 15:07:36 PST ---
import std.string;
import std.stdio;

void main()
{
    struct S { int x; bool empty() { return false; } }
    writeln( format("%s", S()) );
}

> S(0, null)

It shouldn't try to print the function. I think this should also apply to property functions since e.g. "empty" is typically a property function.

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Andrej Mitrovic <andrej.mitrovich@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|format prints "null" for    |format prints context
                   |struct functions            |pointer for struct


--- Comment #1 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-25 18:11:48 PST ---
(In reply to comment #0)
> import std.string;
> import std.stdio;
> 
> void main()
> {
>     struct S { int x; bool empty() { return false; } }
>     writeln( format("%s", S()) );
> }
> 
> > S(0, null)
> 
> It shouldn't try to print the function. I think this should also apply to property functions since e.g. "empty" is typically a property function.

Oh wait a minute, I know what this is. It's the hidden context pointer. If you change the struct to 'static' the null disappears. But I don't think format() should print that.

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hsteoh@quickfur.ath.cx changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hsteoh@quickfur.ath.cx


--- Comment #2 from hsteoh@quickfur.ath.cx 2013-02-26 16:23:47 PST ---
Is there a way to tell whether a field is the hidden context pointer? Does it have a specific name (that isn't compiler-dependent)?

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--- Comment #3 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-26 19:01:50 PST ---
(In reply to comment #2)
> Is there a way to tell whether a field is the hidden context pointer? Does it have a specific name (that isn't compiler-dependent)?

None that I know of, but it appears it's always the last member (at least in
DMD).

When we get the isNested[1] trait pulled we could simply do a test on the aggregate and just ignore the last field when writing its contents.

[1]: https://github.com/D-Programming-Language/dmd/pull/1362

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--- Comment #4 from hsteoh@quickfur.ath.cx 2013-02-26 20:19:22 PST ---
Hmm. Using __traits(allMembers, S) seems to indicate that the field name is 'this'. Since 'this' is a reserved keyword, that may be safer to check for than assuming that the compiler will always put it last.

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--- Comment #5 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-02-26 20:22:21 PST ---
(In reply to comment #4)
> Hmm. Using __traits(allMembers, S) seems to indicate that the field name is 'this'. Since 'this' is a reserved keyword, that may be safer to check for than assuming that the compiler will always put it last.

Nice catch.

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--- Comment #6 from Andrej Mitrovic <andrej.mitrovich@gmail.com> 2013-03-07 19:18:48 PST ---
(In reply to comment #4)
> Hmm. Using __traits(allMembers, S) seems to indicate that the field name is 'this'. Since 'this' is a reserved keyword, that may be safer to check for than assuming that the compiler will always put it last.

This won't work out that easy. The code in format uses .tupleof to get to the values, but .tupleof and allMembers return different results, for example:

void main()
{
    struct S { int x; this(int) { } }
}

allMembers:
tuple("x", "__ctor", "this")

.tupleof:
tuple(0, S(0).this)

So the indexes won't match. Since my new isNested trait was pulled, and assuming the context pointer is last, I could do:

immutable tupleLen = val.tupleof.length;
foreach (i, e; val.tupleof)
{
    // skip printing context pointer
    static if (__traits(isNested, T) && i+i == tupleLen) { }
}

It's hard to tell whether this will be safe.

Another idea is to change how .tupleof works internally (to remove exposing the context pointer), but this might be a bad idea, serialization might probably require it as well as other code.

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