diff options
Diffstat (limited to 'src/span.h')
-rw-r--r-- | src/span.h | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/src/span.h b/src/span.h index d5d7ebccc4..830164514b 100644 --- a/src/span.h +++ b/src/span.h @@ -31,6 +31,62 @@ /** A Span is an object that can refer to a contiguous sequence of objects. * * It implements a subset of C++20's std::span. + * + * Things to be aware of when writing code that deals with Spans: + * + * - Similar to references themselves, Spans are subject to reference lifetime + * issues. The user is responsible for making sure the objects pointed to by + * a Span live as long as the Span is used. For example: + * + * std::vector<int> vec{1,2,3,4}; + * Span<int> sp(vec); + * vec.push_back(5); + * printf("%i\n", sp.front()); // UB! + * + * may exhibit undefined behavior, as increasing the size of a vector may + * invalidate references. + * + * - One particular pitfall is that Spans can be constructed from temporaries, + * but this is unsafe when the Span is stored in a variable, outliving the + * temporary. For example, this will compile, but exhibits undefined behavior: + * + * Span<const int> sp(std::vector<int>{1, 2, 3}); + * printf("%i\n", sp.front()); // UB! + * + * The lifetime of the vector ends when the statement it is created in ends. + * Thus the Span is left with a dangling reference, and using it is undefined. + * + * - Due to Span's automatic creation from range-like objects (arrays, and data + * types that expose a data() and size() member function), functions that + * accept a Span as input parameter can be called with any compatible + * range-like object. For example, this works: +* + * void Foo(Span<const int> arg); + * + * Foo(std::vector<int>{1, 2, 3}); // Works + * + * This is very useful in cases where a function truly does not care about the + * container, and only about having exactly a range of elements. However it + * may also be surprising to see automatic conversions in this case. + * + * When a function accepts a Span with a mutable element type, it will not + * accept temporaries; only variables or other references. For example: + * + * void FooMut(Span<int> arg); + * + * FooMut(std::vector<int>{1, 2, 3}); // Does not compile + * std::vector<int> baz{1, 2, 3}; + * FooMut(baz); // Works + * + * This is similar to how functions that take (non-const) lvalue references + * as input cannot accept temporaries. This does not work either: + * + * void FooVec(std::vector<int>& arg); + * FooVec(std::vector<int>{1, 2, 3}); // Does not compile + * + * The idea is that if a function accepts a mutable reference, a meaningful + * result will be present in that variable after the call. Passing a temporary + * is useless in that context. */ template<typename C> class Span @@ -124,6 +180,7 @@ public: return m_data[m_size - 1]; } constexpr std::size_t size() const noexcept { return m_size; } + constexpr bool empty() const noexcept { return size() == 0; } CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept { ASSERT_IF_DEBUG(size() > pos); @@ -179,4 +236,16 @@ T& SpanPopBack(Span<T>& span) return back; } +// Helper functions to safely cast to unsigned char pointers. +inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; } +inline unsigned char* UCharCast(unsigned char* c) { return c; } +inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; } +inline const unsigned char* UCharCast(const unsigned char* c) { return c; } + +// Helper function to safely convert a Span to a Span<[const] unsigned char>. +template <typename T> constexpr auto UCharSpanCast(Span<T> s) -> Span<typename std::remove_pointer<decltype(UCharCast(s.data()))>::type> { return {UCharCast(s.data()), s.size()}; } + +/** Like MakeSpan, but for (const) unsigned char member types only. Only works for (un)signed char containers. */ +template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(MakeSpan(std::forward<V>(v)))) { return UCharSpanCast(MakeSpan(std::forward<V>(v))); } + #endif |