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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
* Safely access the contents of a string even as GC can cause the string's
* contents to move around in memory.
#ifndef js_StableStringChars_h
#define js_StableStringChars_h
#include "mozilla/Assertions.h" // MOZ_ASSERT
#include "mozilla/Attributes.h" // MOZ_INIT_OUTSIDE_CTOR, MOZ_STACK_CLASS
#include "mozilla/Maybe.h" // mozilla::Maybe
#include "mozilla/Range.h" // mozilla::Range
#include <stddef.h> // size_t
#include <stdint.h> // uint8_t
#include "jstypes.h" // JS_PUBLIC_API
#include "js/AllocPolicy.h"
#include "js/RootingAPI.h" // JS::Handle, JS::Rooted
#include "js/String.h" // JS::GetStringLength
#include "js/TypeDecls.h" // JSContext, JS::Latin1Char, JSString
#include "js/Vector.h" // js::Vector
class JSLinearString;
namespace JS {
* This class provides safe access to a string's chars across a GC. If we ever
* nursery allocate strings' out of line chars, this class will have to make a
* copy, so it's best to avoid using this class unless you really need it. It's
* usually more efficient to use the latin1Chars/twoByteChars JSString methods
* and often the code can be rewritten so that only indexes instead of char
* pointers are used in parts of the code that can GC.
class MOZ_STACK_CLASS JS_PUBLIC_API AutoStableStringChars final {
* When copying string char, use this many bytes of inline storage. This is
* chosen to allow the inline string types to be copied without allocating.
* This is asserted in AutoStableStringChars::allocOwnChars.
static const size_t InlineCapacity = 24;
/* Ensure the string is kept alive while we're using its chars. */
Rooted<JSString*> s_;
const char16_t* twoByteChars_;
const Latin1Char* latin1Chars_;
mozilla::Maybe<js::Vector<uint8_t, InlineCapacity>> ownChars_;
enum State { Uninitialized, Latin1, TwoByte };
State state_;
explicit AutoStableStringChars(JSContext* cx)
: s_(cx), state_(Uninitialized) {}
[[nodiscard]] bool init(JSContext* cx, JSString* s);
/* Like init(), but Latin1 chars are inflated to TwoByte. */
[[nodiscard]] bool initTwoByte(JSContext* cx, JSString* s);
bool isLatin1() const { return state_ == Latin1; }
bool isTwoByte() const { return state_ == TwoByte; }
const Latin1Char* latin1Chars() const {
MOZ_ASSERT(state_ == Latin1);
return latin1Chars_;
const char16_t* twoByteChars() const {
MOZ_ASSERT(state_ == TwoByte);
return twoByteChars_;
mozilla::Range<const Latin1Char> latin1Range() const {
MOZ_ASSERT(state_ == Latin1);
return mozilla::Range<const Latin1Char>(latin1Chars_, GetStringLength(s_));
mozilla::Range<const char16_t> twoByteRange() const {
MOZ_ASSERT(state_ == TwoByte);
return mozilla::Range<const char16_t>(twoByteChars_, GetStringLength(s_));
/* If we own the chars, transfer ownership to the caller. */
bool maybeGiveOwnershipToCaller() {
MOZ_ASSERT(state_ != Uninitialized);
if (!ownChars_.isSome() || !ownChars_->extractRawBuffer()) {
return false;
state_ = Uninitialized;
return true;
AutoStableStringChars(const AutoStableStringChars& other) = delete;
void operator=(const AutoStableStringChars& other) = delete;
bool baseIsInline(Handle<JSLinearString*> linearString);
template <typename T>
T* allocOwnChars(JSContext* cx, size_t count);
bool copyLatin1Chars(JSContext* cx, Handle<JSLinearString*> linearString);
bool copyTwoByteChars(JSContext* cx, Handle<JSLinearString*> linearString);
bool copyAndInflateLatin1Chars(JSContext*,
Handle<JSLinearString*> linearString);
} // namespace JS
#endif /* js_StableStringChars_h */