narrow cast のお試し。

narrow-cast-test.cpp

#include "pch.h"

#include <limits>
#include <stdexcept>
#include <type_traits>

#if !defined(__cpp_if_constexpr) || !defined(__cpp_static_assert)
#error
#endif

template<typename T, typename U>
constexpr T narrow_cast(U && u)
{
	// 参照を取り除いた型を基に判断
	using ValueT = std::remove_reference_t<T>;
	using ValueU = std::remove_reference_t<U>;

	// 大きい型への変換はコンパイルエラーとする
	constexpr int DigitsT = std::numeric_limits<ValueT>::digits + std::is_signed<ValueT>::value;
	constexpr int DigitsU = std::numeric_limits<ValueU>::digits + std::is_signed<ValueU>::value;

	static_assert(DigitsT <= DigitsU);

	if constexpr (DigitsT < DigitsU)
	{
		// 変換先の最小値と最大値
		constexpr auto min = std::numeric_limits<ValueT>::min();
		constexpr auto max = std::numeric_limits<ValueT>::max();

		// 変換先と変換元が、それぞれ符号付きかどうか
		if constexpr (std::is_signed_v<ValueT> && std::is_signed_v<ValueU>)
		{
			if (u < min || max < u)
			{
				throw std::bad_cast();
			}
		}
		else if constexpr (std::is_unsigned_v<ValueT> && std::is_unsigned_v<ValueU>)
		{
			if (max < u)
			{
				throw std::bad_cast();
			}
		}
		else if constexpr (std::is_signed_v<ValueU>)
		{
			if (u < 0 || ((ValueU) max) < u)
			{
				throw std::bad_cast();
			}
		}
		else if constexpr (std::is_signed_v<ValueT>)
		{
			if (((ValueU) max) < u)
			{
				throw std::bad_cast();
			}
		}
	}

	return static_cast<T>(std::forward<U>(u));
}

TEST(NarrowCastTest, None)
{
	EXPECT_EQ(0, narrow_cast<long>((long) 0));
	EXPECT_EQ(0, narrow_cast<long>((unsigned long) 0));
	EXPECT_EQ(0, narrow_cast<unsigned long>((long) 0));
	EXPECT_EQ(0, narrow_cast<unsigned long>((unsigned long) 0));

	EXPECT_EQ(-1, narrow_cast<long>((long) -1));
	EXPECT_EQ(-1, narrow_cast<long>((unsigned long) -1));
	EXPECT_EQ(-1, narrow_cast<unsigned long>((long) -1));
	EXPECT_EQ(-1, narrow_cast<unsigned long>((unsigned long) -1));
#if 0
	// コンパイルエラーとなる
	EXPECT_EQ(0, narrow_cast<long long>((long) 0));
	EXPECT_EQ(0, narrow_cast<long long>((unsigned long) 0));
	EXPECT_EQ(0, narrow_cast<unsigned long long>((long) 0));
	EXPECT_EQ(0, narrow_cast<unsigned long long>((unsigned long) 0));
#endif
}

TEST(NarrowCastTest, Signed)
{
	EXPECT_EQ(LONG_MIN, narrow_cast<long>((long long) LONG_MIN));
	EXPECT_EQ(LONG_MAX, narrow_cast<long>((long long) LONG_MAX));

	EXPECT_THROW(narrow_cast<long>((long long) LONG_MIN - 1), std::bad_cast);
	EXPECT_THROW(narrow_cast<long>((long long) LONG_MAX + 1), std::bad_cast);
}

TEST(NarrowCastTest, Unsigned)
{
	EXPECT_EQ(0, narrow_cast<unsigned long>((unsigned long long) 0));
	EXPECT_EQ(ULONG_MAX, narrow_cast<unsigned long>((unsigned long long) ULONG_MAX));

	EXPECT_THROW(narrow_cast<unsigned long>((unsigned long long) -1), std::bad_cast);
	EXPECT_THROW(narrow_cast<unsigned long>((unsigned long long) ULONG_MAX + 1), std::bad_cast);
}

TEST(NarrowCastTest, SignedToUnsigned)
{
	EXPECT_EQ(0, narrow_cast<unsigned long>((long long) 0));
	EXPECT_EQ(ULONG_MAX, narrow_cast<unsigned long>((long long) ULONG_MAX));

	EXPECT_THROW(narrow_cast<unsigned long>((long long) -1), std::bad_cast);
	EXPECT_THROW(narrow_cast<unsigned long>((long long) ULONG_MAX + 1), std::bad_cast);
}

TEST(NarrowCastTest, UnsignedToSigned)
{
	EXPECT_EQ(0, narrow_cast<long>((unsigned long long) 0));
	EXPECT_EQ(LONG_MAX, narrow_cast<long>((unsigned long long) LONG_MAX));

	EXPECT_THROW(narrow_cast<long>((unsigned long long) - 1), std::bad_cast);
	EXPECT_THROW(narrow_cast<long>((unsigned long long) LONG_MAX + 1), std::bad_cast);
}