steveire marked an inline comment as done.
steveire added inline comments.
================
Comment at: include/clang/AST/TextNodeDumper.h:28
const comments::FullComment *> {
+ TextTreeStructure &TreeStructure;
raw_ostream &OS;
----------------
steveire wrote:
> steveire wrote:
> > aaron.ballman wrote:
> > > 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.
> > Actually 'the stars would have to line up in a particular way' in order to
> > reach the scenario you are concerned about. What would have to happen is:
> >
> > * This patch gets in as-is
> > * Someone in the future reorders the members
> > * But they don't reorder the init-list
> > * They build on a platform without -Wreorder (only MSVC?) enabled in the
> > build.
> > * That passes review
> > * Other users update their checkout and everyone else also ignores the
> > -Wreorder warning.
> >
> > That is a vanishingly likely scenario. It's just unreasonable to consider
> > that as a reason to create a broken interface.
> >
> > And it would be a broken interface.
> >
> > After the refactoring is complete, we have something like
> >
> > ```
> > class ASTDumper
> > : public ASTDumpTraverser<ASTDumper, TextTreeStructure, TextNodeDumper>
> > {
> > TextTreeStructure TreeStructure;
> > TextNodeDumper NodeDumper;
> > public:
> > TextTreeStructure &getTreeStructure() { return TreeStructure; }
> > TextNodeDumper &getNodeDumper() { return NodeDumper; }
> >
> > ASTDumper(raw_ostream &OS, const SourceManager *SM)
> > : TreeStructure(OS),
> > NodeDumper(TreeStructure, OS, SM) {}
> > };
> >
> > ```
> >
> > In the case, of the `ASTDumper`, the `TextNodeDumper` uses the
> > `TextTreeStructure`.
> >
> > However, in the case of any other subclass of `ASTDumpTraverser`, the
> > `NodeDumper` type does not depend on the `TreeStructure` type. The
> > `ASTDumpTraverser` should not pass the `TreeStructure` to the
> > `NodeDumper`because the `ASTDumpTraverser` should not assume the
> > `NodeDumper` needs the `ASTDumpTraverser`.
> >
> > That is true in one concrete case (the `TextNodeDumper`), but not in
> > general. You would be encoding an assumption about a concrete `NodeDumper`
> > implementation in the generic `ASTDumpTraverser`.
> >
> > That is an interface violation which is definitely not justified by your
> > far-fetched scenario of someone re-ordering the members in the future and
> > ignoring `-Wreorder`.
> Should be "should not assume the `NodeDumper` needs the `TreeStructure`",
> sorry.
I believe if something like https://reviews.llvm.org/D56407 is accepted, then
* The generic traverser will not artificially couple the `TreeStructure` and
the `NodeVisitor`
* The end-result ASTDumper will not have two members with
reference-relationship and there will be no ordering issue.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D55337/new/
https://reviews.llvm.org/D55337
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