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make OUString(OUStringLiteral) constructor constexpr

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with the intent that we can declare
    static const OUStringLiteral FOO = "bar";
    static const struct {
      OUString s;
    } xxx = { FOO };
and have the xxx laid out at compile time.

Kudos to mikekaganski for coming up with the idea of using
the rtl_uString struct inside OUStringLiteral.

We are using a union to defeat the array bounds checking
that the compiler does when it evaluates constexpr values,
so this might be brittle.

Change-Id: I9a6247a17afab2eb28df139d532a8d864c49cd62
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/114196
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
This commit is contained in:
Noel Grandin
2021-04-20 15:30:50 +02:00
parent 0ae159cc40
commit 21584b304b
1 changed files with 29 additions and 19 deletions
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include/rtl/ustring.hxx
+29 -19
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@@ -83,6 +83,7 @@ This class is not part of public API and is meant to be used only in LibreOffice
template<std::size_t N> class SAL_WARN_UNUSED OUStringLiteral {
static_assert(N != 0);
static_assert(N - 1 <= std::numeric_limits<sal_Int32>::max(), "literal too long");
friend class OUString;
public:
#if HAVE_CPP_CONSTEVAL
@@ -95,35 +96,44 @@ public:
assert(literal[N - 1] == '\0');
//TODO: Use C++20 constexpr std::copy_n (P0202R3):
for (std::size_t i = 0; i != N; ++i) {
buffer[i] = literal[i];
more.buffer[i] = literal[i];
}
}
constexpr sal_Int32 getLength() const { return length; }
constexpr sal_Int32 getLength() const { return more.length; }
constexpr sal_Unicode const * getStr() const SAL_RETURNS_NONNULL { return buffer; }
constexpr sal_Unicode const * getStr() const SAL_RETURNS_NONNULL { return more.buffer; }
constexpr operator std::u16string_view() const { return {buffer, sal_uInt32(length)}; }
constexpr operator std::u16string_view() const { return {more.buffer, sal_uInt32(more.length)}; }
private:
static constexpr void assertLayout() {
// These static_asserts verifying the layout compatibility with rtl_uString cannot be class
// member declarations, as offsetof requires a complete type, so defer them to here:
static_assert(offsetof(OUStringLiteral, refCount) == offsetof(rtl_uString, refCount));
static_assert(std::is_same_v<decltype(refCount), decltype(rtl_uString::refCount)>);
static_assert(offsetof(OUStringLiteral, length) == offsetof(rtl_uString, length));
static_assert(std::is_same_v<decltype(length), decltype(rtl_uString::length)>);
static_assert(offsetof(OUStringLiteral, buffer) == offsetof(rtl_uString, buffer));
static_assert(
std::is_same_v<
std::remove_extent_t<decltype(buffer)>,
std::remove_extent_t<decltype(rtl_uString::buffer)>>);
static_assert(offsetof(OUStringLiteral, str.refCount) == offsetof(OUStringLiteral, more.refCount));
static_assert(offsetof(OUStringLiteral, str.length) == offsetof(OUStringLiteral, more.length));
static_assert(offsetof(OUStringLiteral, str.buffer) == offsetof(OUStringLiteral, more.buffer));
}
// Same layout as rtl_uString (include/rtl/ustring.h):
oslInterlockedCount refCount = 0x40000000; // SAL_STRING_STATIC_FLAG (sal/rtl/strimp.hxx)
sal_Int32 length = N - 1;
sal_Unicode buffer[N] = {}; //TODO: drop initialization for C++20 (P1331R2)
#if defined(_WIN32)
#pragma pack(push, 4)
#endif
union {
rtl_uString str;
struct {
oslInterlockedCount refCount;
sal_Int32 length;
sal_Unicode buffer[N];
} more =
{
0x40000000, // SAL_STRING_STATIC_FLAG (sal/rtl/strimp.hxx)
N - 1,
{} //TODO: drop initialization for C++20 (P1331R2)
};
};
#if defined(_WIN32)
#pragma pack(pop)
#endif
};
#if defined RTL_STRING_UNITTEST
@@ -385,8 +395,8 @@ public:
@since LibreOffice 5.0
*/
template<std::size_t N> OUString(OUStringLiteral<N> const & literal):
pData(const_cast<rtl_uString *>(reinterpret_cast<rtl_uString const *>(&literal))) {}
template<std::size_t N> constexpr OUString(OUStringLiteral<N> const & literal):
pData(const_cast<rtl_uString *>(&literal.str)) {}
template<std::size_t N> OUString(OUStringLiteral<N> &&) = delete;
/// @endcond
#endif