Re: [PATCH 3/3] gdb: handle default argument in make-target-delegates.py

[Pedro Alves <pedro@palves.net>]

On 2023-11-14 09:41, Tankut Baris Aktemur wrote:

> Currently, target.h does not contain any arguments with default
> values.  The goal of this patch is to address potential future and
> downstream cases; we (Intel) have cases with default argument values.

Hmm, but default arguments in virtual methods is generally considered a bad idea,
because default arguments are a static property which doesn't propagate when
you override a method.  Like:

#include <stdio.h>

struct A
{
  virtual void func (int arg = 1) { printf ("%d\n", arg); }
};

struct B : A
{
  virtual void func (int arg = 2) { printf ("%d\n", arg); }
};

struct C : A
{
  virtual void func (int arg) { printf ("%d\n", arg); }
};

int
main ()
{
   B *b = new B ();
   A *a = b;
   a->func ();  // prints "1".
   b->func ();  // prints "2".

   C *c = new C ();
   c->func ();  // doesn't even compile.
}

Instead, it's better practice to put the default argument in a non-virtual method that
then calls the virtual method.  We don't have those in target_ops, but we have instead
the global target_foo wrapper functions.  So instead of putting the default argument here:

 -   virtual void rcmd (const char *command=0, struct ui_file *output)
 +   virtual void rcmd (const char *command=0, struct ui_file *output = nullptr)
       TARGET_DEFAULT_FUNC (default_rcmd);

You should put the default here:

 -extern void target_rcmd (const char *command, struct ui_file *outbuf);
 +extern void target_rcmd (const char *command, struct ui_file *outbuf = nullptr);