[cfe-dev] Ownership of Stmts and Exprs, and destroying them

[Chris Lattner]

On Nov 23, 2008, at 8:33 AM, Howard Hinnant wrote:
>> friends. That way, the type system enforces our ownership model
>> (transfer into Sema for the parameters of ActOn*, transfer out of  
>> Sema
>> on return), and we're safe from early exits.
>
> If it helps, here's a C++03 emulation of the C++0X std::unique_ptr
> (what Doug refers to as move_ptr):
>
> http://home.twcny.rr.com/hinnant/cpp_extensions/unique_ptr_03.html
>
> You can set its "deleter" to call Action::DestroyExpr.  It might look
> something like:
>
>
>                           unique_ptr<ExprTy, CallActionDestroyExpr>
> lhs,
>                           unique_ptr<ExprTy, CallActionDestroyExpr>
> rhs) {
>  BinaryOperator::Opcode Opc = ConvertTokenKindToBinaryOpcode(Kind);
>  ...

This doesn't work.  The problem is that the C++ type system is  
insufficiently powerful (!!) to express what we need here.  The parser  
needs to be independent of any specific actions implementation and AST  
form.  This is why it needs to call callbacks on Actions to delete  
expressions.

Sema, on the other hand, implements these actions but does know the  
specific concrete types.  It isn't acceptable to encode "Expr" into  
the interface used by the parser, that would be a severe layering  
violation.

-Chris