A function template about checked integer casting.

dlOuOlb.numeric.checked_cast.ixx

#if __cplusplus < 202002L
# error "This code is written in C++20."
#else

export module dlOuOlb.numeric:checked_cast;

import <format>; /* std::format */
import <limits>; /* std::numeric_limits */
import <stdexcept>; /* std::(overflow|underflow)_error */
import <string>; /* std::(to_)?string */

namespace dlOuOlb::numeric
{
	template<class Integer> requires
		std::numeric_limits<Integer>::is_integer
	static const std::string description =
		std::format
		(
			R"({1} {0}-digit integer)",
			std::numeric_limits<Integer>::digits,
			std::numeric_limits<Integer>::is_signed? R"(signed)": R"(unsigned)"
		);

	template<class Target, class Source> requires
		std::numeric_limits<Target>::is_integer &&
		std::numeric_limits<Source>::is_integer
	static auto explain( const Source Value ) noexcept( false )->std::string
	{
		return std::format
		(
			R"({2:+#X} cannot be preserved when being casted from {1} to {0}.)",
			description<Target>,
			description<Source>,
			Value
		);
	}

	export extern R"(C++)"
	template<class Target, class Source> requires
		std::numeric_limits<Target>::is_integer &&
		std::numeric_limits<Source>::is_integer
	constexpr auto checked_cast( const Source Value ) noexcept
	(
		std::numeric_limits<Source>::digits <= std::numeric_limits<Target>::digits &&
		(
			std::numeric_limits<Target>::is_signed ||
			!std::numeric_limits<Source>::is_signed
		)
	)->Target
	{
		if constexpr( !std::numeric_limits<Source>::is_signed )
			if constexpr( std::numeric_limits<Source>::digits <= std::numeric_limits<Target>::digits )
				return static_cast<Target>( Value );
			else if( static_cast<Source>( std::numeric_limits<Target>::max( ) ) < Value )
				throw std::overflow_error { explain<Target>( Value ) };
			else
				return static_cast<Target>( Value );
		else if constexpr( std::numeric_limits<Target>::is_signed )
			if constexpr( std::numeric_limits<Source>::digits <= std::numeric_limits<Target>::digits )
				return static_cast<Target>( Value );
			else if( Value < static_cast<Source>( std::numeric_limits<Target>::min( ) ) )
				throw std::underflow_error { explain<Target>( Value ) };
			else if( static_cast<Source>( std::numeric_limits<Target>::max( ) ) < Value )
				throw std::overflow_error { explain<Target>( Value ) };
			else
				return static_cast<Target>( Value );
		else
			if( Value < 0 )
				throw std::underflow_error { explain<Target>( Value ) };
			else if constexpr( std::numeric_limits<Source>::digits <= std::numeric_limits<Target>::digits )
				return static_cast<Target>( Value );
			else if( static_cast<Source>( std::numeric_limits<Target>::max( ) ) < Value )
				throw std::overflow_error { explain<Target>( Value ) };
			else
				return static_cast<Target>( Value );
	}
}

#endif

dlOuOlb.numeric.ixx

#if __cplusplus < 202002L
# error "This code is written in C++20."
#else

export module dlOuOlb.numeric;

export import:checked_cast;

#endif

main.cpp

#if __cplusplus < 202002L
# error "This code is written in C++20."
#else
# include <iostream> /* std::(cerr|cout|endl) */
# include <stdexcept> /* std::(overflow|underflow)_error */

import dlOuOlb.numeric; /* dlOuOlb::numeric */

extern R"(C++)" auto main( void ) noexcept( false )->signed int
{
	namespace S = std;
	namespace OuO = dlOuOlb;

	try
	{
		const constexpr signed long int Source = __cplusplus;
		const unsigned short int Target = OuO::numeric::checked_cast<decltype( Target )>( Source );

		static_cast<void>( S::cout << R"(__cplusplus : )" << Target << S::endl );
	}
	catch( const S::overflow_error &Error )
	{
		static_cast<void>( S::cerr << Error.what( ) << S::endl );
	}
	catch( const S::underflow_error &Error )
	{
		static_cast<void>( S::cerr << Error.what( ) << S::endl );
	}
	{
		return 0;
	}
}

#endif