[ADT] Add a fallible_iterator wrapper.

A fallible iterator is one whose increment or decrement operations may fail.
This would usually be supported by replacing the ++ and -- operators with
methods that return error:

    class MyFallibleIterator {
    public:
      // ...
      Error inc();
      Errro dec();
      // ...
    };

The downside of this style is that it no longer conforms to the C++ iterator
concept, and can not make use of standard algorithms and features such as
range-based for loops.

The fallible_iterator wrapper takes an iterator written in the style above
and adapts it to (mostly) conform with the C++ iterator concept. It does this
by providing standard ++ and -- operator implementations, returning any errors
generated via a side channel (an Error reference passed into the wrapper at
construction time), and immediately jumping the iterator to a known 'end'
value upon error. It also marks the Error as checked any time an iterator is
compared with a known end value and found to be inequal, allowing early exit
from loops without redundant error checking*.

Usage looks like:

    MyFallibleIterator I = ..., E = ...;

    Error Err = Error::success();
    for (auto &Elem : make_fallible_range(I, E, Err)) {
      // Loop body is only entered when safe.

      // Early exits from loop body permitted without checking Err.
      if (SomeCondition)
        return;

    }
    if (Err)
      // Handle error.

* Since failure causes a fallible iterator to jump to end, testing that a
  fallible iterator is not an end value implicitly verifies that the error is a
  success value, and so is equivalent to an error check.

Reviewers: dblaikie, rupprecht

Subscribers: mgorny, dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57618

llvm-svn: 353237

1 files changed, 4 insertions, 5 deletions

diff --git a/llvm/lib/Object/Archive.cpp b/llvm/lib/Object/Archive.cpp
index 3def30949fc..1d31feb714e 100644
--- a/llvm/lib/Object/Archive.cpp
+++ b/llvm/lib/Object/Archive.cpp
@@ -778,19 +778,18 @@ Archive::child_iterator Archive::child_begin(Error &Err,
     return child_end();
 
   if (SkipInternal)
-    return child_iterator(Child(this, FirstRegularData,
-                                FirstRegularStartOfFile),
-                          &Err);
+    return child_iterator::itr(
+        Child(this, FirstRegularData, FirstRegularStartOfFile), Err);
 
   const char *Loc = Data.getBufferStart() + strlen(Magic);
   Child C(this, Loc, &Err);
   if (Err)
     return child_end();
-  return child_iterator(C, &Err);
+  return child_iterator::itr(C, Err);
 }
 
 Archive::child_iterator Archive::child_end() const {
-  return child_iterator(Child(nullptr, nullptr, nullptr), nullptr);
+  return child_iterator::end(Child(nullptr, nullptr, nullptr));
 }
 
 StringRef Archive::Symbol::getName() const {