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//===--- fallible_iterator.h - Wrapper for fallible iterators ---*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ADT_FALLIBLE_ITERATOR_H
#define LLVM_ADT_FALLIBLE_ITERATOR_H
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Support/Error.h"
#include <type_traits>
namespace llvm {
/// A wrapper class for fallible iterators.
///
/// The fallible_iterator template wraps an underlying iterator-like class
/// whose increment and decrement operations are replaced with fallible versions
/// like:
///
/// @code{.cpp}
/// Error inc();
/// Error dec();
/// @endcode
///
/// It produces an interface that is (mostly) compatible with a traditional
/// c++ iterator, including ++ and -- operators that do not fail.
///
/// Instances of the wrapper are constructed with an instance of the
/// underlying iterator and (for non-end iterators) a reference to an Error
/// instance. If the underlying increment/decrement operations fail, the Error
/// is returned via this reference, and the resulting iterator value set to an
/// end-of-range sentinel value. This enables the following loop idiom:
///
/// @code{.cpp}
/// class Archive { // E.g. Potentially malformed on-disk archive
/// public:
/// fallible_iterator<ArchiveChildItr> children_begin(Error &Err);
/// fallible_iterator<ArchiveChildItr> children_end();
/// iterator_range<fallible_iterator<ArchiveChildItr>>
/// children(Error &Err) {
/// return make_range(children_begin(Err), children_end());
/// //...
/// };
///
/// void walk(Archive &A) {
/// Error Err = Error::success();
/// for (auto &C : A.children(Err)) {
/// // Loop body only entered when increment succeeds.
/// }
/// if (Err) {
/// // handle error.
/// }
/// }
/// @endcode
///
/// The wrapper marks the referenced Error as unchecked after each increment
/// and/or decrement operation, and clears the unchecked flag when a non-end
/// value is compared against end (since, by the increment invariant, not being
/// an end value proves that there was no error, and is equivalent to checking
/// that the Error is success). This allows early exits from the loop body
/// without requiring redundant error checks.
template <typename Underlying> class fallible_iterator {
private:
template <typename T>
using enable_if_struct_deref_supported = std::enable_if_t<
!std::is_void<decltype(std::declval<T>().operator->())>::value,
decltype(std::declval<T>().operator->())>;
public:
/// Construct a fallible iterator that *cannot* be used as an end-of-range
/// value.
///
/// A value created by this method can be dereferenced, incremented,
/// decremented and compared, providing the underlying type supports it.
///
/// The error that is passed in will be initially marked as checked, so if the
/// iterator is not used at all the Error need not be checked.
static fallible_iterator itr(Underlying I, Error &Err) {
(void)!!Err;
return fallible_iterator(std::move(I), &Err);
}
/// Construct a fallible iterator that can be used as an end-of-range value.
///
/// A value created by this method can be dereferenced (if the underlying
/// value points at a valid value) and compared, but not incremented or
/// decremented.
static fallible_iterator end(Underlying I) {
return fallible_iterator(std::move(I), nullptr);
}
/// Forward dereference to the underlying iterator.
decltype(auto) operator*() { return *I; }
/// Forward const dereference to the underlying iterator.
decltype(auto) operator*() const { return *I; }
/// Forward structure dereference to the underlying iterator (if the
/// underlying iterator supports it).
template <typename T = Underlying>
enable_if_struct_deref_supported<T> operator->() {
return I.operator->();
}
/// Forward const structure dereference to the underlying iterator (if the
/// underlying iterator supports it).
template <typename T = Underlying>
enable_if_struct_deref_supported<const T> operator->() const {
return I.operator->();
}
/// Increment the fallible iterator.
///
/// If the underlying 'inc' operation fails, this will set the Error value
/// and update this iterator value to point to end-of-range.
///
/// The Error value is marked as needing checking, regardless of whether the
/// 'inc' operation succeeds or fails.
fallible_iterator &operator++() {
assert(getErrPtr() && "Cannot increment end iterator");
if (auto Err = I.inc())
handleError(std::move(Err));
else
resetCheckedFlag();
return *this;
}
/// Decrement the fallible iterator.
///
/// If the underlying 'dec' operation fails, this will set the Error value
/// and update this iterator value to point to end-of-range.
///
/// The Error value is marked as needing checking, regardless of whether the
/// 'dec' operation succeeds or fails.
fallible_iterator &operator--() {
assert(getErrPtr() && "Cannot decrement end iterator");
if (auto Err = I.dec())
handleError(std::move(Err));
else
resetCheckedFlag();
return *this;
}
/// Compare fallible iterators for equality.
///
/// Returns true if both LHS and RHS are end-of-range values, or if both are
/// non-end-of-range values whose underlying iterator values compare equal.
///
/// If this is a comparison between an end-of-range iterator and a
/// non-end-of-range iterator, then the Error (referenced by the
/// non-end-of-range value) is marked as checked: Since all
/// increment/decrement operations result in an end-of-range value, comparing
/// false against end-of-range is equivalent to checking that the Error value
/// is success. This flag management enables early returns from loop bodies
/// without redundant Error checks.
friend bool operator==(const fallible_iterator &LHS,
const fallible_iterator &RHS) {
// If both iterators are in the end state they compare
// equal, regardless of whether either is valid.
if (LHS.isEnd() && RHS.isEnd())
return true;
assert(LHS.isValid() && RHS.isValid() &&
"Invalid iterators can only be compared against end");
bool Equal = LHS.I == RHS.I;
// If the iterators differ and this is a comparison against end then mark
// the Error as checked.
if (!Equal) {
if (LHS.isEnd())
(void)!!*RHS.getErrPtr();
else
(void)!!*LHS.getErrPtr();
}
return Equal;
}
/// Compare fallible iterators for inequality.
///
/// See notes for operator==.
friend bool operator!=(const fallible_iterator &LHS,
const fallible_iterator &RHS) {
return !(LHS == RHS);
}
private:
fallible_iterator(Underlying I, Error *Err)
: I(std::move(I)), ErrState(Err, false) {}
Error *getErrPtr() const { return ErrState.getPointer(); }
bool isEnd() const { return getErrPtr() == nullptr; }
bool isValid() const { return !ErrState.getInt(); }
void handleError(Error Err) {
*getErrPtr() = std::move(Err);
ErrState.setPointer(nullptr);
ErrState.setInt(true);
}
void resetCheckedFlag() {
*getErrPtr() = Error::success();
}
Underlying I;
mutable PointerIntPair<Error *, 1> ErrState;
};
/// Convenience wrapper to make a fallible_iterator value from an instance
/// of an underlying iterator and an Error reference.
template <typename Underlying>
fallible_iterator<Underlying> make_fallible_itr(Underlying I, Error &Err) {
return fallible_iterator<Underlying>::itr(std::move(I), Err);
}
/// Convenience wrapper to make a fallible_iterator end value from an instance
/// of an underlying iterator.
template <typename Underlying>
fallible_iterator<Underlying> make_fallible_end(Underlying E) {
return fallible_iterator<Underlying>::end(std::move(E));
}
template <typename Underlying>
iterator_range<fallible_iterator<Underlying>>
make_fallible_range(Underlying I, Underlying E, Error &Err) {
return make_range(make_fallible_itr(std::move(I), Err),
make_fallible_end(std::move(E)));
}
} // end namespace llvm
#endif // LLVM_ADT_FALLIBLE_ITERATOR_H
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