[PATCH] D55337: NFC: Move dumpDeclRef to NodeDumper

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: include/clang/AST/TextNodeDumper.h:28
                                            const comments::FullComment *> {
+  TextTreeStructure &TreeStructure;
   raw_ostream &OS;
----------------
steveire wrote:
> aaron.ballman wrote:
> > This makes me a bit wary because you create a node dumper in the same situations you make a tree structure object, but now there's a strict ordering between the two object creations. If you're doing this construction local to a function, you wind up with a dangling reference unless you're careful (which is unfortunate, but not the end of the world). If you're doing this construction as part of a constructor's initializer list, you now have to properly order the member declarations within the class and that is also unfortunate. Given that those are the two common scenarios for how I envision constructing an ast dump of some kind, I worry about the fragility. e.g.,
> > ```
> > unique_ptr<ASTConsumer> createASTDumper(...) {
> >   TextTreeStructure TreeStructure;
> >   TextNodeDumper NodeDumper(TreeStructure); // Oops, dangling reference
> >   return make_unique<MySuperAwesomeASTDumper>(TreeStructure, NodeDumper, ...);
> > }
> > 
> > // vs
> > 
> > struct MySuperAwesomeASTDumper : ... {
> >   MySuperAwesomeASTDumper() : TreeStructure(...), NodeDumper(TreeStructure, ...) {}
> > private:
> >   TextTreeStructure TreeStructure; // This order is now SUPER important
> >   TextNodeDumper NodeDumper;
> > };
> > ```
> > There's a part of me that wonders if a better approach is to have this object passed to the `dumpFoo()` calls as a reference parameter. This way, the caller is still responsible for creating an object, but the creation order between the tree and the node dumper isn't as fragile.
> In your first snippet there is a dangling reference because the author of `MySuperAwesomeASTDumper` decided to make the members references. If the members are references, code like your first snippet will cause dangling references and nothing can prevent that. Adding `TreeStructure&` to Visit methods as you suggested does not prevent it.
> 
> The only solution is make the `MySuperAwesomeASTDumper` not use member references (ie your second snippet). The order is then in fact not problematic because "taking a reference to an uninitialized object is legal".
>  The order is then in fact not problematic because "taking a reference to an uninitialized object is legal".

This presumes that the constructors aren't using those references to the uninitialized object, which would be illegal. That's what I mean about this being very fragile -- if the stars line up correctly, everything works fine, but if the stars aren't aligned just right, you get really hard problems to track down.


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55337/new/

https://reviews.llvm.org/D55337