libutils: Introduce StaticString16
This is a backward compatible implementation of compile time
constructed String16 support.
As much as we'd like a regular constexpr constructor for String16, we
want to make sure the regular non-static String16 does not regress.
We also need to make sure prebuilts built with previous version of
String16 still works with new libutils. This means we cannot change
the size of String16 objects and we cannot make anything virtual.
To add a flag to indicate whether a String16 is static without
increasing the size of non-static String16 objects, we repurpose a
reserved field in SharedBuffer as "for client use". With this, we can
tag every String16 and perform memory operation differently based on
how the underlying buffers are allocated.
By using StaticString16, we are able to eliminate the runtime
construction of a String16 and move it out of .bss section.
Bug: 138856262
Test: Run newly added unit tests.
Change-Id: I72bb8dc27a59b9ef34e0d934bc1e00b0f675855a
diff --git a/libutils/Android.bp b/libutils/Android.bp
index 8be4dd0..30798dd 100644
--- a/libutils/Android.bp
+++ b/libutils/Android.bp
@@ -205,6 +205,7 @@
"Mutex_test.cpp",
"SharedBuffer_test.cpp",
"String8_test.cpp",
+ "String16_test.cpp",
"StrongPointer_test.cpp",
"Unicode_test.cpp",
"Vector_test.cpp",
diff --git a/libutils/SharedBuffer.cpp b/libutils/SharedBuffer.cpp
index 7910c6e..3e703db 100644
--- a/libutils/SharedBuffer.cpp
+++ b/libutils/SharedBuffer.cpp
@@ -41,6 +41,7 @@
// The following is OK on Android-supported platforms.
sb->mRefs.store(1, std::memory_order_relaxed);
sb->mSize = size;
+ sb->mClientMetadata = 0;
}
return sb;
}
diff --git a/libutils/SharedBuffer.h b/libutils/SharedBuffer.h
index fdf13a9..476c842 100644
--- a/libutils/SharedBuffer.h
+++ b/libutils/SharedBuffer.h
@@ -102,7 +102,12 @@
// Must be sized to preserve correct alignment.
mutable std::atomic<int32_t> mRefs;
size_t mSize;
- uint32_t mReserved[2];
+ uint32_t mReserved;
+public:
+ // mClientMetadata is reserved for client use. It is initialized to 0
+ // and the clients can do whatever they want with it. Note that this is
+ // placed last so that it is adjcent to the buffer allocated.
+ uint32_t mClientMetadata;
};
static_assert(sizeof(SharedBuffer) % 8 == 0
diff --git a/libutils/String16.cpp b/libutils/String16.cpp
index 818b171..5c3cf32 100644
--- a/libutils/String16.cpp
+++ b/libutils/String16.cpp
@@ -24,21 +24,21 @@
namespace android {
+static const StaticString16 emptyString(u"");
static inline char16_t* getEmptyString() {
- static SharedBuffer* gEmptyStringBuf = [] {
- SharedBuffer* buf = SharedBuffer::alloc(sizeof(char16_t));
- char16_t* str = static_cast<char16_t*>(buf->data());
- *str = 0;
- return buf;
- }();
-
- gEmptyStringBuf->acquire();
- return static_cast<char16_t*>(gEmptyStringBuf->data());
+ return const_cast<char16_t*>(emptyString.string());
}
// ---------------------------------------------------------------------------
-static char16_t* allocFromUTF8(const char* u8str, size_t u8len)
+void* String16::alloc(size_t size)
+{
+ SharedBuffer* buf = SharedBuffer::alloc(size);
+ buf->mClientMetadata = kIsSharedBufferAllocated;
+ return buf;
+}
+
+char16_t* String16::allocFromUTF8(const char* u8str, size_t u8len)
{
if (u8len == 0) return getEmptyString();
@@ -49,7 +49,7 @@
return getEmptyString();
}
- SharedBuffer* buf = SharedBuffer::alloc(sizeof(char16_t)*(u16len+1));
+ SharedBuffer* buf = static_cast<SharedBuffer*>(alloc(sizeof(char16_t) * (u16len + 1)));
if (buf) {
u8cur = (const uint8_t*) u8str;
char16_t* u16str = (char16_t*)buf->data();
@@ -66,13 +66,13 @@
return getEmptyString();
}
-static char16_t* allocFromUTF16(const char16_t* u16str, size_t u16len) {
+char16_t* String16::allocFromUTF16(const char16_t* u16str, size_t u16len) {
if (u16len >= SIZE_MAX / sizeof(char16_t)) {
android_errorWriteLog(0x534e4554, "73826242");
abort();
}
- SharedBuffer* buf = SharedBuffer::alloc((u16len + 1) * sizeof(char16_t));
+ SharedBuffer* buf = static_cast<SharedBuffer*>(alloc((u16len + 1) * sizeof(char16_t)));
ALOG_ASSERT(buf, "Unable to allocate shared buffer");
if (buf) {
char16_t* str = (char16_t*)buf->data();
@@ -97,8 +97,8 @@
// having run. In this case we always allocate an empty string. It's less
// efficient than using getEmptyString(), but we assume it's uncommon.
- char16_t* data = static_cast<char16_t*>(
- SharedBuffer::alloc(sizeof(char16_t))->data());
+ SharedBuffer* buf = static_cast<SharedBuffer*>(alloc(sizeof(char16_t)));
+ char16_t* data = static_cast<char16_t*>(buf->data());
data[0] = 0;
mString = data;
}
@@ -106,7 +106,7 @@
String16::String16(const String16& o)
: mString(o.mString)
{
- SharedBuffer::bufferFromData(mString)->acquire();
+ acquire();
}
String16::String16(const String16& o, size_t len, size_t begin)
@@ -136,26 +136,30 @@
String16::~String16()
{
- SharedBuffer::bufferFromData(mString)->release();
+ release();
}
size_t String16::size() const
{
- return SharedBuffer::sizeFromData(mString)/sizeof(char16_t)-1;
+ if (isStaticString()) {
+ return staticStringSize();
+ } else {
+ return SharedBuffer::sizeFromData(mString) / sizeof(char16_t) - 1;
+ }
}
void String16::setTo(const String16& other)
{
- SharedBuffer::bufferFromData(other.mString)->acquire();
- SharedBuffer::bufferFromData(mString)->release();
+ release();
mString = other.mString;
+ acquire();
}
status_t String16::setTo(const String16& other, size_t len, size_t begin)
{
const size_t N = other.size();
if (begin >= N) {
- SharedBuffer::bufferFromData(mString)->release();
+ release();
mString = getEmptyString();
return OK;
}
@@ -184,8 +188,7 @@
abort();
}
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((len+1)*sizeof(char16_t));
+ SharedBuffer* buf = static_cast<SharedBuffer*>(editResize((len + 1) * sizeof(char16_t)));
if (buf) {
char16_t* str = (char16_t*)buf->data();
memmove(str, other, len*sizeof(char16_t));
@@ -212,8 +215,8 @@
abort();
}
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((myLen+otherLen+1)*sizeof(char16_t));
+ SharedBuffer* buf =
+ static_cast<SharedBuffer*>(editResize((myLen + otherLen + 1) * sizeof(char16_t)));
if (buf) {
char16_t* str = (char16_t*)buf->data();
memcpy(str+myLen, other, (otherLen+1)*sizeof(char16_t));
@@ -238,8 +241,8 @@
abort();
}
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((myLen+otherLen+1)*sizeof(char16_t));
+ SharedBuffer* buf =
+ static_cast<SharedBuffer*>(editResize((myLen + otherLen + 1) * sizeof(char16_t)));
if (buf) {
char16_t* str = (char16_t*)buf->data();
memcpy(str+myLen, chrs, otherLen*sizeof(char16_t));
@@ -273,8 +276,8 @@
len, myLen, String8(chrs, len).string());
#endif
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((myLen+len+1)*sizeof(char16_t));
+ SharedBuffer* buf =
+ static_cast<SharedBuffer*>(editResize((myLen + len + 1) * sizeof(char16_t)));
if (buf) {
char16_t* str = (char16_t*)buf->data();
if (pos < myLen) {
@@ -338,23 +341,87 @@
return strstr16(mString, chrs) != nullptr;
}
+void* String16::edit() {
+ SharedBuffer* buf;
+ if (isStaticString()) {
+ buf = static_cast<SharedBuffer*>(alloc((size() + 1) * sizeof(char16_t)));
+ if (buf) {
+ buf->acquire();
+ memcpy(buf->data(), mString, (size() + 1) * sizeof(char16_t));
+ }
+ } else {
+ buf = SharedBuffer::bufferFromData(mString)->edit();
+ buf->mClientMetadata = kIsSharedBufferAllocated;
+ }
+ return buf;
+}
+
+void* String16::editResize(size_t newSize) {
+ SharedBuffer* buf;
+ if (isStaticString()) {
+ size_t copySize = (size() + 1) * sizeof(char16_t);
+ if (newSize < copySize) {
+ copySize = newSize;
+ }
+ buf = static_cast<SharedBuffer*>(alloc(newSize));
+ if (buf) {
+ buf->acquire();
+ memcpy(buf->data(), mString, copySize);
+ }
+ } else {
+ buf = SharedBuffer::bufferFromData(mString)->editResize(newSize);
+ buf->mClientMetadata = kIsSharedBufferAllocated;
+ }
+ return buf;
+}
+
+void String16::acquire()
+{
+ if (!isStaticString()) {
+ SharedBuffer::bufferFromData(mString)->acquire();
+ }
+}
+
+void String16::release()
+{
+ if (!isStaticString()) {
+ SharedBuffer::bufferFromData(mString)->release();
+ }
+}
+
+bool String16::isStaticString() const {
+ // See String16.h for notes on the memory layout of String16::StaticData and
+ // SharedBuffer.
+ static_assert(sizeof(SharedBuffer) - offsetof(SharedBuffer, mClientMetadata) == 4);
+ const uint32_t* p = reinterpret_cast<const uint32_t*>(mString);
+ return (*(p - 1) & kIsSharedBufferAllocated) == 0;
+}
+
+size_t String16::staticStringSize() const {
+ // See String16.h for notes on the memory layout of String16::StaticData and
+ // SharedBuffer.
+ static_assert(sizeof(SharedBuffer) - offsetof(SharedBuffer, mClientMetadata) == 4);
+ const uint32_t* p = reinterpret_cast<const uint32_t*>(mString);
+ return static_cast<size_t>(*(p - 1));
+}
+
status_t String16::makeLower()
{
const size_t N = size();
const char16_t* str = string();
- char16_t* edit = nullptr;
+ char16_t* edited = nullptr;
for (size_t i=0; i<N; i++) {
const char16_t v = str[i];
if (v >= 'A' && v <= 'Z') {
- if (!edit) {
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)->edit();
+ if (!edited) {
+ SharedBuffer* buf = static_cast<SharedBuffer*>(edit());
if (!buf) {
return NO_MEMORY;
}
- edit = (char16_t*)buf->data();
- mString = str = edit;
+ edited = (char16_t*)buf->data();
+ mString = str = edited;
}
- edit[i] = tolower((char)v);
+ edited[i] = tolower((char)v);
}
}
return OK;
@@ -364,18 +431,18 @@
{
const size_t N = size();
const char16_t* str = string();
- char16_t* edit = nullptr;
+ char16_t* edited = nullptr;
for (size_t i=0; i<N; i++) {
if (str[i] == replaceThis) {
- if (!edit) {
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)->edit();
+ if (!edited) {
+ SharedBuffer* buf = static_cast<SharedBuffer*>(edit());
if (!buf) {
return NO_MEMORY;
}
- edit = (char16_t*)buf->data();
- mString = str = edit;
+ edited = (char16_t*)buf->data();
+ mString = str = edited;
}
- edit[i] = withThis;
+ edited[i] = withThis;
}
}
return OK;
@@ -385,7 +452,7 @@
{
const size_t N = size();
if (begin >= N) {
- SharedBuffer::bufferFromData(mString)->release();
+ release();
mString = getEmptyString();
return OK;
}
@@ -395,8 +462,7 @@
}
if (begin > 0) {
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((N+1)*sizeof(char16_t));
+ SharedBuffer* buf = static_cast<SharedBuffer*>(editResize((N + 1) * sizeof(char16_t)));
if (!buf) {
return NO_MEMORY;
}
@@ -404,8 +470,7 @@
memmove(str, str+begin, (N-begin+1)*sizeof(char16_t));
mString = str;
}
- SharedBuffer* buf = SharedBuffer::bufferFromData(mString)
- ->editResize((len+1)*sizeof(char16_t));
+ SharedBuffer* buf = static_cast<SharedBuffer*>(editResize((len + 1) * sizeof(char16_t)));
if (buf) {
char16_t* str = (char16_t*)buf->data();
str[len] = 0;
diff --git a/libutils/String16_test.cpp b/libutils/String16_test.cpp
new file mode 100644
index 0000000..f1f24c3
--- /dev/null
+++ b/libutils/String16_test.cpp
@@ -0,0 +1,218 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <utils/String16.h>
+#include <utils/String8.h>
+
+#include <gtest/gtest.h>
+
+namespace android {
+
+::testing::AssertionResult Char16_tStringEquals(const char16_t* a, const char16_t* b) {
+ if (strcmp16(a, b) != 0) {
+ return ::testing::AssertionFailure()
+ << "\"" << String8(a).c_str() << "\" not equal to \"" << String8(b).c_str() << "\"";
+ }
+ return ::testing::AssertionSuccess();
+}
+
+#define EXPECT_STR16EQ(a, b) EXPECT_TRUE(Char16_tStringEquals(a, b))
+
+TEST(String16Test, FromChar16_t) {
+ String16 tmp(u"Verify me");
+ EXPECT_STR16EQ(u"Verify me", tmp);
+}
+
+TEST(String16Test, FromChar16_tSized) {
+ String16 tmp(u"Verify me", 7);
+ EXPECT_STR16EQ(u"Verify ", tmp);
+}
+
+TEST(String16Test, FromChar) {
+ String16 tmp("Verify me");
+ EXPECT_STR16EQ(u"Verify me", tmp);
+}
+
+TEST(String16Test, FromCharSized) {
+ String16 tmp("Verify me", 7);
+ EXPECT_STR16EQ(u"Verify ", tmp);
+}
+
+TEST(String16Test, Copy) {
+ String16 tmp("Verify me");
+ String16 another = tmp;
+ EXPECT_STR16EQ(u"Verify me", tmp);
+ EXPECT_STR16EQ(u"Verify me", another);
+}
+
+TEST(String16Test, Move) {
+ String16 tmp("Verify me");
+ String16 another(std::move(tmp));
+ EXPECT_STR16EQ(u"Verify me", another);
+}
+
+TEST(String16Test, Size) {
+ String16 tmp("Verify me");
+ EXPECT_EQ(9U, tmp.size());
+}
+
+TEST(String16Test, setTo) {
+ String16 tmp("Verify me");
+ tmp.setTo(u"New content");
+ EXPECT_EQ(11U, tmp.size());
+ EXPECT_STR16EQ(u"New content", tmp);
+}
+
+TEST(String16Test, Append) {
+ String16 tmp("Verify me");
+ tmp.append(String16("Hello"));
+ EXPECT_EQ(14U, tmp.size());
+ EXPECT_STR16EQ(u"Verify meHello", tmp);
+}
+
+TEST(String16Test, Insert) {
+ String16 tmp("Verify me");
+ tmp.insert(6, u"Insert");
+ EXPECT_EQ(15U, tmp.size());
+ EXPECT_STR16EQ(u"VerifyInsert me", tmp);
+}
+
+TEST(String16Test, Remove) {
+ String16 tmp("Verify me");
+ tmp.remove(2, 6);
+ EXPECT_EQ(2U, tmp.size());
+ EXPECT_STR16EQ(u" m", tmp);
+}
+
+TEST(String16Test, MakeLower) {
+ String16 tmp("Verify Me!");
+ tmp.makeLower();
+ EXPECT_EQ(10U, tmp.size());
+ EXPECT_STR16EQ(u"verify me!", tmp);
+}
+
+TEST(String16Test, ReplaceAll) {
+ String16 tmp("Verify verify Verify");
+ tmp.replaceAll(u'r', u'!');
+ EXPECT_STR16EQ(u"Ve!ify ve!ify Ve!ify", tmp);
+}
+
+TEST(String16Test, Compare) {
+ String16 tmp("Verify me");
+ EXPECT_EQ(String16(u"Verify me"), tmp);
+}
+
+TEST(String16Test, StaticString) {
+ String16 nonStaticString("NonStatic");
+ StaticString16 staticString(u"Static");
+
+ EXPECT_TRUE(staticString.isStaticString());
+ EXPECT_FALSE(nonStaticString.isStaticString());
+}
+
+TEST(String16Test, StaticStringCopy) {
+ StaticString16 tmp(u"Verify me");
+ String16 another = tmp;
+ EXPECT_STR16EQ(u"Verify me", tmp);
+ EXPECT_STR16EQ(u"Verify me", another);
+ EXPECT_TRUE(tmp.isStaticString());
+ EXPECT_TRUE(another.isStaticString());
+}
+
+TEST(String16Test, StaticStringMove) {
+ StaticString16 tmp(u"Verify me");
+ String16 another(std::move(tmp));
+ EXPECT_STR16EQ(u"Verify me", another);
+ EXPECT_TRUE(another.isStaticString());
+}
+
+TEST(String16Test, StaticStringSize) {
+ StaticString16 tmp(u"Verify me");
+ EXPECT_EQ(9U, tmp.size());
+}
+
+TEST(String16Test, StaticStringSetTo) {
+ StaticString16 tmp(u"Verify me");
+ tmp.setTo(u"New content");
+ EXPECT_EQ(11U, tmp.size());
+ EXPECT_STR16EQ(u"New content", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringAppend) {
+ StaticString16 tmp(u"Verify me");
+ tmp.append(String16("Hello"));
+ EXPECT_EQ(14U, tmp.size());
+ EXPECT_STR16EQ(u"Verify meHello", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringInsert) {
+ StaticString16 tmp(u"Verify me");
+ tmp.insert(6, u"Insert");
+ EXPECT_EQ(15U, tmp.size());
+ EXPECT_STR16EQ(u"VerifyInsert me", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringRemove) {
+ StaticString16 tmp(u"Verify me");
+ tmp.remove(2, 6);
+ EXPECT_EQ(2U, tmp.size());
+ EXPECT_STR16EQ(u" m", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringMakeLower) {
+ StaticString16 tmp(u"Verify me!");
+ tmp.makeLower();
+ EXPECT_EQ(10U, tmp.size());
+ EXPECT_STR16EQ(u"verify me!", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringReplaceAll) {
+ StaticString16 tmp(u"Verify verify Verify");
+ tmp.replaceAll(u'r', u'!');
+ EXPECT_STR16EQ(u"Ve!ify ve!ify Ve!ify", tmp);
+ EXPECT_FALSE(tmp.isStaticString());
+}
+
+TEST(String16Test, StaticStringCompare) {
+ StaticString16 tmp(u"Verify me");
+ EXPECT_EQ(String16(u"Verify me"), tmp);
+}
+
+TEST(String16Test, StringSetToStaticString) {
+ StaticString16 tmp(u"Verify me");
+ String16 another(u"nonstatic");
+ another = tmp;
+ EXPECT_STR16EQ(u"Verify me", tmp);
+ EXPECT_STR16EQ(u"Verify me", another);
+}
+
+TEST(String16Test, StringMoveFromStaticString) {
+ StaticString16 tmp(u"Verify me");
+ String16 another(std::move(tmp));
+ EXPECT_STR16EQ(u"Verify me", another);
+}
+
+TEST(String16Test, EmptyStringIsStatic) {
+ String16 tmp("");
+ EXPECT_TRUE(tmp.isStaticString());
+}
+
+} // namespace android
diff --git a/libutils/include/utils/String16.h b/libutils/include/utils/String16.h
index afbc2ed..adc3e7d 100644
--- a/libutils/include/utils/String16.h
+++ b/libutils/include/utils/String16.h
@@ -37,13 +37,17 @@
class String8;
+template <size_t N>
+class StaticString16;
+
// DO NOT USE: please use std::u16string
//! This is a string holding UTF-16 characters.
class String16
{
public:
- /* use String16(StaticLinkage) if you're statically linking against
+ /*
+ * Use String16(StaticLinkage) if you're statically linking against
* libutils and declaring an empty static String16, e.g.:
*
* static String16 sAStaticEmptyString(String16::kEmptyString);
@@ -123,8 +127,76 @@
inline operator const char16_t*() const;
-private:
- const char16_t* mString;
+ // Static and non-static String16 behave the same for the users, so
+ // this method isn't of much use for the users. It is public for testing.
+ bool isStaticString() const;
+
+ private:
+ /*
+ * A flag indicating the type of underlying buffer.
+ */
+ static constexpr uint32_t kIsSharedBufferAllocated = 0x80000000;
+
+ /*
+ * alloc() returns void* so that SharedBuffer class is not exposed.
+ */
+ static void* alloc(size_t size);
+ static char16_t* allocFromUTF8(const char* u8str, size_t u8len);
+ static char16_t* allocFromUTF16(const char16_t* u16str, size_t u16len);
+
+ /*
+ * edit() and editResize() return void* so that SharedBuffer class
+ * is not exposed.
+ */
+ void* edit();
+ void* editResize(size_t new_size);
+
+ void acquire();
+ void release();
+
+ size_t staticStringSize() const;
+
+ const char16_t* mString;
+
+protected:
+ /*
+ * Data structure used to allocate static storage for static String16.
+ *
+ * Note that this data structure and SharedBuffer are used interchangably
+ * as the underlying data structure for a String16. Therefore, the layout
+ * of this data structure must match the part in SharedBuffer that is
+ * visible to String16.
+ */
+ template <size_t N>
+ struct StaticData {
+ // The high bit of 'size' is used as a flag.
+ static_assert(N - 1 < kIsSharedBufferAllocated, "StaticString16 too long!");
+ constexpr StaticData() : size(N - 1), data{0} {}
+ const uint32_t size;
+ char16_t data[N];
+
+ constexpr StaticData(const StaticData<N>&) = default;
+ };
+
+ /*
+ * Helper function for constructing a StaticData object.
+ */
+ template <size_t N>
+ static constexpr const StaticData<N> makeStaticData(const char16_t (&s)[N]) {
+ StaticData<N> r;
+ // The 'size' field is at the same location where mClientMetadata would
+ // be for a SharedBuffer. We do NOT set kIsSharedBufferAllocated flag
+ // here.
+ for (size_t i = 0; i < N - 1; ++i) r.data[i] = s[i];
+ return r;
+ }
+
+ template <size_t N>
+ explicit constexpr String16(const StaticData<N>& s) : mString(s.data) {}
+
+public:
+ template <size_t N>
+ explicit constexpr String16(const StaticString16<N>& s) : mString(s.mString) {}
};
// String16 can be trivially moved using memcpy() because moving does not
@@ -132,6 +204,42 @@
ANDROID_TRIVIAL_MOVE_TRAIT(String16)
// ---------------------------------------------------------------------------
+
+/*
+ * A StaticString16 object is a specialized String16 object. Instead of holding
+ * the string data in a ref counted SharedBuffer object, it holds data in a
+ * buffer within StaticString16 itself. Note that this buffer is NOT ref
+ * counted and is assumed to be available for as long as there is at least a
+ * String16 object using it. Therefore, one must be extra careful to NEVER
+ * assign a StaticString16 to a String16 that outlives the StaticString16
+ * object.
+ *
+ * THE SAFEST APPROACH IS TO USE StaticString16 ONLY AS GLOBAL VARIABLES.
+ *
+ * A StaticString16 SHOULD NEVER APPEAR IN APIs. USE String16 INSTEAD.
+ */
+template <size_t N>
+class StaticString16 : public String16 {
+public:
+ constexpr StaticString16(const char16_t (&s)[N]) : String16(mData), mData(makeStaticData(s)) {}
+
+ constexpr StaticString16(const StaticString16<N>& other)
+ : String16(mData), mData(other.mData) {}
+
+ constexpr StaticString16(const StaticString16<N>&&) = delete;
+
+ // There is no reason why one would want to 'new' a StaticString16. Delete
+ // it to discourage misuse.
+ static void* operator new(std::size_t) = delete;
+
+private:
+ const StaticData<N> mData;
+};
+
+template <typename F>
+StaticString16(const F&)->StaticString16<sizeof(F) / sizeof(char16_t)>;
+
+// ---------------------------------------------------------------------------
// No user servicable parts below.
inline int compare_type(const String16& lhs, const String16& rhs)