This is an alternative approach to the following diff: D111658: Add JSON output skeleton to llvm-readelf
The major difference is that in this change JSONScoperPrinter inherits from ScopedPrinter rather
than creating a ScopedPrinterBase base class which both JSONScopedPrinter and ScopedPrinter
inherit from.
In comment {D111658#3121940} I cited 3 reasons for why I didn't think this was possible. I'll go over
how this change address each problem:
Problem 1: ScopedPrinter has template methods which are not able to be virtual.
Solution: Add private internal non-template methods which can be virtual. The
public template method then transforms all template parameters into concrete types
and then used to call the internal non-template method. This enables JSONScopedPrinter
to override the functionality of all of ScopedPrinter's methods.
Problem 2: ScopedPrinter provides a method startLine() which returns the underlying
raw_ostream. It would be impossible to ensure printing directly to raw_ostream results
in valid JSON.
Solution: There is still no easy solution to the problem. Though since this change is to
enable json output for llvm-readelf. The planned solution to this problem is to have a
JSONELFDumper class similar to D111658: Add JSON output skeleton to llvm-readelf and override
methods which call startLine() to use another interface.
Problem 3: A common usage pattern inside llvm-readelf (calling ListScope(W, "attr1")
and then DictScope(W, "attr2")) produces invalid json since you can't have attributes inside
of lists. (The example would product the following json output)
ListScope(W, "attr1");
DictScope(W, "attr2");
"attr1" : [
"attr2" : { ... }
]Solution: Add a "history" stack similar to the one inside json::OStream conditionally
output an encompassing {} if inside the context of a list. The following are code examples
and their respective outputs:
ListScope(W, "attr1");
DictScope(W, "attr2");
W.printString("attr3", "val");
"attr1" : [
{
"attr2" : {
"attr3" : "val"
}
}
]DictScope(W, "attr1");
DictScope(W, "attr2");
W.printString("attr3", "val");
"attr1" : {
"attr2" : {
"attr3" : "val"
}
}Note: There is also another approach which handles Problem 3 differently D114053: Add JSONScopedPrinter as a subclass to ScopedPrinter with array output