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author | Ryan Ofsky <ryan@ofsky.org> | 2023-09-28 12:58:51 -0400 |
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committer | Ryan Ofsky <ryan@ofsky.org> | 2023-10-20 10:30:16 -0400 |
commit | 6d43aad742c7ea28303cf2799528188938e7ce32 (patch) | |
tree | 66e55937ac2dfe171f26269493d04c00f19fe4d9 /src/span.h | |
parent | 8062c3bdb9dd3062597ed8299e99151b612d32b7 (diff) | |
download | bitcoin-6d43aad742c7ea28303cf2799528188938e7ce32.tar.xz |
span: Make Span template deduction guides work in SFINAE context
Also add test to make sure this doesn't get broken in the future.
This was breaking vector<bool> serialization in multiprocess code because
template current deduction guides would make it appear like vector<bool> could
be converted to a span, but then the actual conversion to span would fail.
Diffstat (limited to 'src/span.h')
-rw-r--r-- | src/span.h | 21 |
1 files changed, 19 insertions, 2 deletions
diff --git a/src/span.h b/src/span.h index 7209d21a58..2e8da27cde 100644 --- a/src/span.h +++ b/src/span.h @@ -222,15 +222,32 @@ public: template <typename O> friend class Span; }; +// Return result of calling .data() method on type T. This is used to be able to +// write template deduction guides for the single-parameter Span constructor +// below that will work if the value that is passed has a .data() method, and if +// the data method does not return a void pointer. +// +// It is important to check for the void type specifically below, so the +// deduction guides can be used in SFINAE contexts to check whether objects can +// be converted to spans. If the deduction guides did not explicitly check for +// void, and an object was passed that returned void* from data (like +// std::vector<bool>), the template deduction would succeed, but the Span<void> +// object instantiation would fail, resulting in a hard error, rather than a +// SFINAE error. +// https://stackoverflow.com/questions/68759148/sfinae-to-detect-the-explicitness-of-a-ctad-deduction-guide +// https://stackoverflow.com/questions/16568986/what-happens-when-you-call-data-on-a-stdvectorbool +template<typename T> +using DataResult = std::remove_pointer_t<decltype(std::declval<T&>().data())>; + // Deduction guides for Span // For the pointer/size based and iterator based constructor: template <typename T, typename EndOrSize> Span(T*, EndOrSize) -> Span<T>; // For the array constructor: template <typename T, std::size_t N> Span(T (&)[N]) -> Span<T>; // For the temporaries/rvalue references constructor, only supporting const output. -template <typename T> Span(T&&) -> Span<std::enable_if_t<!std::is_lvalue_reference_v<T>, const std::remove_pointer_t<decltype(std::declval<T&&>().data())>>>; +template <typename T> Span(T&&) -> Span<std::enable_if_t<!std::is_lvalue_reference_v<T> && !std::is_void_v<DataResult<T&&>>, const DataResult<T&&>>>; // For (lvalue) references, supporting mutable output. -template <typename T> Span(T&) -> Span<std::remove_pointer_t<decltype(std::declval<T&>().data())>>; +template <typename T> Span(T&) -> Span<std::enable_if_t<!std::is_void_v<DataResult<T&>>, DataResult<T&>>>; /** Pop the last element off a span, and return a reference to that element. */ template <typename T> |