On 2/1/20 8:39 AM, Saurabh Das wrote:
I faced this issue while working with custom formatting for a struct. I
have reduced the error down to this test program:
import std.format, std.stdio, std.array;
struct Test1
{
void toString(W, C)(ref W w, scope const ref FormatSpec!C fmt)
{
pragma(msg, "Test1 function compiled with W=" ~ W.stringof);
// formatValue(w, this, fmt);
}
}
struct Test2
{
void toString(W, C)(ref W w, scope const ref FormatSpec!C fmt)
{
pragma(msg, "Test2 function compiled with W=" ~ W.stringof);
formatValue(w, this, fmt);
}
}
void main()
{
Test1 t1;
Test2 t2;
Appender!string writer;
auto ff = singleSpec("%s");
formatValue(writer, t1, ff);
formatValue(writer, t2, ff);
}
When compiled, the output is:
Test1 function compiled with W=S
Test1 function compiled with W=Appender!string
Test2 function compiled with W=S
1. Why was Test2 never compiled with W=Appender!string?
2. What is "S"?
Essentially, my custom struct was not being formatted using the toString
method that I had written. Reducing the issue, it seems like a call to
formatValue with the same type caused the issue. If someone can explain
what I am doing wrong here, it would really help a lot.
Thanks,
Saurabh
Something very weird is happening.
I switched to fullyQualifiedName!W, and I get *no output*.
The "S" comes from hasToString
here:https://github.com/dlang/phobos/blob/9fe5cd354f0166b11d32a5c1214932757d8e7eba/std/format.d#L3876-L3899
I tried copying the implementation to a local file, and as expected, I
get customPutWriterFormatSpec for both types.
But it's only calling one of them in the implementation.
I think the only true way to diagnose this is to instrument std.format
and see what it's doing with more pragma(msg) calls. Don't have the time
right now.
-Steve
