[boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions

Subject: [boost] [GSoC][Phoenix3] Regarding mutability of arguments passed to phoenix expressions
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2010-04-27 15:49:01

Regarding arguments to phoenix expressions there are two possibilities.
The first is: Arguments are mutable. The current phoenix is implemented to be 
able to this.
Example:

int i( 1 );
( ++phoenix::arg_names::arg1 )( i );

The result of ++phoenix::arg_names::arg1 will be 2, i will have the value 2 
after the expression is evaluated.

The other approach would follow a more functional programming style as the 
data will not be mutable

      int i( 1 );
[1]  ( ++phoenix::arg_names::arg1 )( i );
[2]  ( ++phoenix::arg_names::arg1 + phoenix::arg_names::arg1 )( i );

The result of [1] will be 2, however i does not get changed (it remains to be 
1).
The result of [2] will be 3. i still unchanged.
This example would be working "out-of-the-box" with the current prototypes.
To get the mutability back into phoenix i propose the use of phoenix::ref.

I favor the second approach. However, this would break API compatibility 
between the existing implementation.
The question i want to ask the current and prospective users if it feasible to 
break your existing code? Is there a lot of existing code depending on this 
behaviour? Does it make sense to break code in such a way?

Regards,
Thomas