[Pretty printers] Can the name or type of a child value change?

[Pretty printers] Can the name or type of a child value change?

[Elmenthaler, Jens]

Consider the following code example:

  struct JAny {
    union {
      char *text;
      int number;
    } val;

    // 1 -> text, 2 -> number
    int type;
  };

And the following pretty printer:
class AnyPrinter:
    class _iterator:
        def __init__ (self, name, value):
            self.count = 0
            self.name = name
            self.value = value

        def __iter__(self):
            return self

        def next(self):
            if self.count != 0:
                raise StopIteration
            
            self.count = self.count + 1
            return (self.name, self.value)    

    def __init__(self, val):
        self.type = val['type']
        
        if self.type == 1:
            self.value = val['val']['text']
            self.name = "text"
            
        if self.type == 2:
            self.value = val['val']['number']
            self.name = "number"
            
    def children(self):
        return self._iterator(self.name, self.value)

    def to_string(self):
        return "JAny"

When stepping in the debugger over a line that changes JAny.type, the name and the type of the child value returned by the pretty printer change.

Are the MI variable objects meant to handle this?

Greetings, Jens.

Re: [Pretty printers] Can the name or type of a child value change?

[Tom Tromey]

>>>>> "Jens" == Elmenthaler, Jens <jens.elmenthaler@verigy.com> writes:

Jens> Consider the following code example:
[...]

Jens> When stepping in the debugger over a line that changes JAny.type,
Jens> the name and the type of the child value returned by the pretty
Jens> printer change.

Jens> Are the MI variable objects meant to handle this?

Yes.  The MI consumer is expected to understand that dynamic varobjs are
dynamic: the number and names of children can change in arbitrary ways.
This is incompatible with previous expectations, which is why consumers
have to explicitly request this feature from gdb.

Tom

RE: [Pretty printers] Can the name or type of a child value change?

[Elmenthaler, Jens]

This actual is a return to a discussion that goes back to 1 1/2 years ago. The question was whether a pretty printer can change the name and type of the returned children when stepping. Tom's answer (see bottom this email) was yes.

I'm now using this feature, but stumble over a very basic question: if the name of a child changes, how does the MI variable object return this fact in -var-update? I don't see a tuple telling the frontend that this happened. It seems it should be the "exp" field that is returned by -var-list-children, but neither documentation nor gdb's code seems to consider it for -var-update.

Greetings, Jens.

p.s For references, here the original example:

Consider the following code example: 

  struct JAny { 
    union { 
      char *text; 
      int number; 
    } val; 

    // 1 -> text, 2 -> number 
    int type; 
  }; 

And the following pretty printer: 
class AnyPrinter: 
    class _iterator: 
        def __init__ (self, name, value): 
            self.count = 0 
            self.name = name 
            self.value = value 

        def __iter__(self): 
            return self 

        def next(self): 
            if self.count != 0: 
                raise StopIteration 
            
            self.count = self.count + 1 
            return (self.name, self.value)     

    def __init__(self, val): 
        self.type = val['type'] 
        
        if self.type == 1: 
            self.value = val['val']['text'] 
            self.name = "text" 
            
        if self.type == 2: 
            self.value = val['val']['number'] 
            self.name = "number" 
            
    def children(self): 
        return self._iterator(self.name, self.value) 

    def to_string(self): 
        return "JAny" 

When stepping in the debugger over a line that changes JAny.type, the name and the type of the child value returned by the pretty printer change. 

Are the MI variable objects meant to handle this? 


-----Original Message-----
From: Tom Tromey [mailto:tromey@redhat.com] 
Sent: Monday, August 16, 2010 10:54 PM
To: Elmenthaler, Jens
Cc: gdb@sourceware.org
Subject: Re: [Pretty printers] Can the name or type of a child value change?

>>>>> "Jens" == Elmenthaler, Jens <jens.elmenthaler@verigy.com> writes:

Jens> Consider the following code example:
[...]

Jens> When stepping in the debugger over a line that changes JAny.type, 
Jens> the name and the type of the child value returned by the pretty 
Jens> printer change.

Jens> Are the MI variable objects meant to handle this?

Yes.  The MI consumer is expected to understand that dynamic varobjs are
dynamic: the number and names of children can change in arbitrary ways.
This is incompatible with previous expectations, which is why consumers have to explicitly request this feature from gdb.

Tom