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◆ main openmsx / src / utils / small_buffer.hh
2.1 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
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#ifndef SMALL_BUFFER_HH
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#define SMALL_BUFFER_HH
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#include "stl.hh"
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#include <algorithm>
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#include <array>
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#include <cassert>
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#include <iterator>
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#include <memory>
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#include <span>
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#include <type_traits>
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// A run-time-sized buffer of (possibly) uninitialized data.
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// Sizes upto 'BUF_SIZE' are stored internally in this object. Larger sizes use
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// heap-allocation. In other words: this implements the small-buffer-optimization.
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// Typically objects of this type are used as local variables. Not as member
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// variables of a larger class.
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// If you're sure the run-time size will _always_ be <= BUF_SIZE, then it may be
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// better to use 'inplace_buffer'.
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template<typename T, size_t BUF_SIZE>
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class small_buffer
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{
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	static_assert(std::is_trivially_constructible_v<T>);
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public:
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	explicit small_buffer(uninitialized_tag, size_t size)
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	{
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		if (size <= BUF_SIZE) {
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			sp = std::span<T>(inplaceBuf.data(), size);
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		} else {
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			extBuf = std::make_unique_for_overwrite<T[]>(size);
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			sp = std::span<T>(extBuf.get(), size);
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		}
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	}
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	explicit small_buffer(size_t size, const T& t)
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		: small_buffer(uninitialized_tag{}, size)
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	{
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		std::ranges::fill(sp, t);
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	}
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	template<std::ranges::forward_range Range>
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	explicit small_buffer(const Range& range)
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		: small_buffer(uninitialized_tag{}, std::ranges::distance(range))
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	{
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		std::ranges::copy(range, begin());
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	}
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	[[nodiscard]] explicit(false) operator std::span<T>() noexcept { return sp; }
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	[[nodiscard]] auto size()  const noexcept { return sp.size(); }
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	[[nodiscard]] auto empty() const noexcept { return sp.empty(); }
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	[[nodiscard]] auto data()  noexcept { return sp.data(); }
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	[[nodiscard]] auto begin() noexcept { return sp.begin(); }
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	[[nodiscard]] auto end()   noexcept { return sp.end(); }
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	[[nodiscard]] T& operator[](size_t index) {
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		assert(index < size());
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		return sp[index];
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	}
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	[[nodiscard]] T& front() {
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		assert(!empty());
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		return sp.front();
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	}
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	[[nodiscard]] T& back() {
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		assert(!empty());
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		return sp.back();
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	}
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private:
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	std::span<T> sp; // span representing the active buffer
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	std::array<T, BUF_SIZE> inplaceBuf;
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	std::unique_ptr<T[]> extBuf;
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};
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#endif