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Don't rely on SSO zeroing in basic_string::_Construct #3235

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Nov 19, 2022
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Don't rely on SSO zeroing in basic_string::_Construct #3235

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9 changes: 6 additions & 3 deletions stl/inc/xstring
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Original file line number Diff line number Diff line change
Expand Up @@ -2335,8 +2335,9 @@ public:
}

union _Bxty { // storage for small buffer or pointer to larger one
// This constructor previously initialized _Ptr. Don't rely on the new behavior without
// renaming `_String_val` (and fixing the visualizer).
_CONSTEXPR20 _Bxty() noexcept : _Buf() {} // user-provided, for fancy pointers

_CONSTEXPR20 ~_Bxty() noexcept {} // user-provided, for fancy pointers

value_type _Buf[_BUF_SIZE];
Expand Down Expand Up @@ -2723,11 +2724,11 @@ public:

private:
enum class _Construct_strategy : uint8_t { _From_char, _From_ptr, _From_string };

template <_Construct_strategy _Strat, class _Char_or_ptr>
_CONSTEXPR20 void _Construct(const _Char_or_ptr _Arg, _CRT_GUARDOVERFLOW const size_type _Count) {
auto& _My_data = _Mypair._Myval2;
_STL_INTERNAL_CHECK(!_My_data._Large_string_engaged());
_STL_INTERNAL_CHECK(_STD count(_My_data._Bx._Buf, _My_data._Bx._Buf + _BUF_SIZE, _Elem()) == _BUF_SIZE);

if constexpr (_Strat == _Construct_strategy::_From_char) {
_STL_INTERNAL_STATIC_ASSERT(is_same_v<_Char_or_ptr, _Elem>);
Expand All @@ -2748,11 +2749,13 @@ private:
_My_data._Myres = _BUF_SIZE - 1;
if constexpr (_Strat == _Construct_strategy::_From_char) {
_Traits::assign(_My_data._Bx._Buf, _Count, _Arg);
_Traits::assign(_My_data._Bx._Buf[_Count], _Elem());
} else if constexpr (_Strat == _Construct_strategy::_From_ptr) {
_Traits::move(_My_data._Bx._Buf, _Arg, _Count);
_Traits::assign(_My_data._Bx._Buf[_Count], _Elem());
} else { // _Strat == _Construct_strategy::_From_string
#ifdef _INSERT_STRING_ANNOTATION
_Traits::move(_My_data._Bx._Buf, _Arg, _Count);
_Traits::move(_My_data._Bx._Buf, _Arg, _Count + 1);
#else // ^^^ _INSERT_STRING_ANNOTATION ^^^ // vvv !_INSERT_STRING_ANNOTATION vvv
_Traits::move(_My_data._Bx._Buf, _Arg, _BUF_SIZE);
#endif // !_INSERT_STRING_ANNOTATION
Expand Down