// Copyright (c) 2017, The Monero Project
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#pragma once
#include <cstddef>
#include <memory>
#include <type_traits>
#include <utility>
namespace epee
{
/*!
\brief Non-owning sequence of data. Does not deep copy
Inspired by `gsl::span`. This class is intended to be used as a parameter
type for functions that need to take a writable or read-only sequence of
data. Most common cases are `span<char>` and `span<std::uint8_t>`. Using as
a class member is only recommended if clearly documented as not doing a
deep-copy. `std::vector`, `std::string`, `std::array`, and C-arrays are all
easily convertible to this type.
\note Conversion from C string literal to `view<char>` will include the
NULL-terminator.
\note `view<T>` is an alias for `span<const T>` which is read-only.
\note Never allows derived-to-base pointer conversion; an array of derived
types is not an array of base types.
*/
template<typename T>
class span
{
/* Supporting class types is tricky - the {ptr,len} and {container}
constructors will allow derived-to-base conversions. This is NOT
desireable because an array of derived types is not an array of base
types. It is possible to handle this case, implement when/if needed. */
static_assert(std::is_integral<T>::value, "only integral values currently for span");
// Disables ADL for these calls.
struct get {
// we only accept pointer and size_t return types, so use noexcept aggressively
template<std::size_t N>
static constexpr T* data(T (&src)[N]) noexcept { return src; }
template<typename C>
static constexpr auto
data(C& src) noexcept(noexcept(src.data())) -> decltype(src.data()) {
return src.data();
}
template<std::size_t N>
static constexpr std::size_t size(T (&)[N]) noexcept { return N; }
template<typename C>
static constexpr auto
size(const C& src) noexcept(noexcept(src.size())) -> decltype(src.size()) {
return src.size();
}
};
public:
using value_type = T;
using pointer = T*;
using reference = T&;
using iterator = pointer;
using const_iterator = typename std::add_const<value_type>::type *;
constexpr span() noexcept : ptr(nullptr), len(0) {}
constexpr span(std::nullptr_t) noexcept : span() {}
constexpr span(T* const src_ptr, const std::size_t src_len) noexcept
: ptr(src_ptr), len(src_len) {
}
/*!
A `span` is implicitly convertible from any type where `get::data(src)`
returns a pointer convertible to T* and `get::size(src)` returns a
`std::size_t`. This constructor is disabled for all other types.
*/
template<
typename C,
typename U = decltype(get::data(std::declval<C&>())),
typename S = decltype(get::size(std::declval<C&>())),
typename = typename std::enable_if<
// Do not allow proxy iterators, but allow valid cv conversions
std::is_pointer<U>::value && std::is_convertible<U, pointer>::value &&
// Prevent underflow/overflow on size storage
std::is_same<S, std::size_t>::value
>::type
>
constexpr span(C&& src) noexcept(noexcept(get::data(src), get::size(src)))
: span(get::data(src), get::size(src)) {
}
constexpr span(span&&) noexcept = default;
constexpr span(span&) noexcept = default;
constexpr span(const span&) noexcept = default;
span& operator=(const span&) noexcept = default;
constexpr iterator begin() const noexcept { return ptr; }
constexpr const_iterator cbegin() const noexcept { return ptr; }
constexpr iterator end() const noexcept { return begin() + size(); }
constexpr const_iterator cend() const noexcept { return cbegin() + size(); }
constexpr bool empty() const noexcept { return size() == 0; }
constexpr pointer data() const noexcept { return ptr; }
constexpr std::size_t size() const noexcept { return len; }
constexpr std::size_t size_bytes() const noexcept { return size() * sizeof(value_type); }
private:
T* ptr;
std::size_t len;
};
//! A `view` is a `span` over immutable data. See `span` for more info.
template<typename T>
using view = span<typename std::add_const<T>::type>;
//! \return `src` aliased as a view of type `T`.
template<typename T, typename U>
view<T> view_cast(const span<U>& src) noexcept
{
using normalized = typename std::remove_cv<T>::type;
static_assert(
std::is_same<normalized, char>::value || std::is_same<normalized, std::uint8_t>::value,
"only valid aliasing allowed - char or unsigned char target types"
);
static_assert(std::is_integral<U>::value, "only integral source types");
return {reinterpret_cast<typename std::add_const<T>::type *>(src.data()), src.size_bytes()};
}
//! \return `src` aliased as a span of type `T`.
template<typename T, typename U>
span<T> pod_cast(U&& src) noexcept
{
using normalized = typename std::remove_cv<T>::type;
static_assert(
std::is_same<normalized, char>::value || std::is_same<normalized, std::uint8_t>::value,
"only valid aliasing allowed - char or unsigned char target types"
);
using decayed = typename std::decay<U>::type;
static_assert(std::is_pod<decayed>::value, "only POD types allowed for conversion");
static_assert(alignof(decayed) == 1, "only types with 1 byte alignment allowed for conversion");
return {reinterpret_cast<T*>(std::addressof(src)), sizeof(decayed)};
}
}
