[cfe-dev] clang, unknown identifiers, and ahead of time compilation

[Sebastian Redl]

On Sep 1, 2010, at 12:54 PM, Axel Naumann wrote:

> Our future interpreter will need to support variables that are defined
> from some context at runtime. They are unknown at compile time. Like this:
> 
> int f() {
>  int ret = 0;
>  ret = h->D(); // h is unknown at compile time
>  return ret
> }

It all comes down to C++ making it impossible to unambiguously parse past identifiers of unknown kind. That is, unless you know whether the identifier in question names a variable, type or template, you simply cannot continue parsing.
For example, the simple expression a(b) is a function call if a is a function or object, but an explicit cast if it's a type. Sema::isTypeName() is called by the parser to make the distinction.
Another example, a<b>(c)+d could be a rather weird but valid expression if all identifiers are variables (test if a is less than b, and test whether the result is greater than c plus d), or a call to a template function followed by an addition if a is a template function and b is a type or constant and c and d are variables, or an explicit type cast followed by an addition if a is a class template and b is a type or constant and c and d are variable, or another weird expression if a, b and d are variables and c is a type (check if a is less than b, and then compare the result to d after the unary + operator has been applied and the result was cast to c).

Of course, you could just defined any unknown identifier to be a variable name. In addition, you could tell the AST that its type is dependent, to suppress all type checks. Finally, add your runtime-evaluation bit and it would probably work. Just remember that you could be suppressing diagnostics in a way that results in VERY unexpected behavior. (In the above example, what if the user meant to refer to the type C, not the runtime variable c?)

Sebastian