Re: [RFA 02/22] Use RAII to save and restore scalars

[Trevor Saunders <tbsaunde at tbsaunde dot org>]

On Mon, Sep 26, 2016 at 10:08:30PM -0600, Tom Tromey wrote:
> This patch replaces many (but not all) uses of
> make_cleanup_restore_integer with a simple RAII-based template class.
> It also removes the similar restore_execution_direction cleanup in
> favor of this new class.  Subsequent patches will replace other
> similar cleanups with this class.
> 
> I chose the name "scoped_restore" for this class.  I considered that
> perhaps "auto_restore" might be clearer.  Or maybe something else;
> it's simple enough to change.

Its worth noting dmalcolm wants to add something similar to gcc for his
rtl front end work.

> I had hoped that template parameter deduction would work here, but it
> did not, and so the patch uses explicit template parameters
> everywhere.

yeah, you can't deduce template args from a constructor in C++

> +  // Create a new scoped_restore object that saves the current value
> +  // of *VAR, and then sets *VAR to VALUE.  *VAR will be restored when
> +  // this scoped_restore object is destroyed.
> +  scoped_restore (T *var, T value)
> +    : saved_var (var),
> +      saved_value (*var)
> +  {
> +    *var = value;

I wonder if it isn't clearer to create the scoped_restore and then set
the variable yourself.

> +  // The saved variable.
> +  T *saved_var;
> +
> +  // The saved value.
> +  T saved_value;

 I think you could use const T, but I doubt it matters, these objects
 really shouldn't have there address taken so the compiler should see
 everything about them.

Trev