按位整数转换

我必须使用返回int32_t的API . 但实际值可能是较小的有符号/无符号类型 . 要了解API返回ENUM值的确切类型 . 它看起来像这样：

typedef enum Int{
    Int8,
    Int16,
    Int32,
    Uint8,
    Uint16,
    Uint32
}IntT;

typedef struct{
    IntT (*getType)();
    void (*getInt)(int32_t* out);

}IntegerT;

我想通过知道枚举值将int32_t的值转换为实际类型 . 有时我甚至想在我的代码中将unsigned int32分配给unsigned int64变量 . 知道unsigned int32作为int32_t返回，如果值足够大，那么它在此类型中表示为负值，如果我只是static_cast它到uint64_t那么符号位被扩展以填充uint64_t中的所有高位这产生了与预期完全不同的无符号值 .

因此，我编写了一个强制转换函数，应该将更大的int类型或更小的int类型转换为正确的值 . 但是，我觉得这可能是一个已知问题，并且可能存在已有的解决方案 . 以下是功能 . 如果您认为可以改进或者有更好的解决方案，请通知我（虽然我已经使这个功能比我真正需要的更通用） .

编辑：使这个便携式的Endianness .

编辑：删除了有关签名/未签名比较的编译器警告 .

#include <limits>
#include <boost/static_assert.hpp> //BOOST_STATIC_ASSERT
#include <stdexcept>
#include <cstring>
#include <boost/type_traits/make_unsigned.hpp>


namespace Detail
{
/** This a implementation helper class for bitwise_int_cast function */
template<bool IsToTypeSigned, bool IsFromTypeSigned>
class Converter
{
public:
    template<typename ToIntType, typename FromIntType>
    ToIntType convert(FromIntType from) {
        BOOST_STATIC_ASSERT(sizeof(from) == 0); //This prevents this generic implementation being compiled
        return from;
    }
};

/** From signed to signed */
template<>
template<typename ToIntType, typename FromIntType>
ToIntType Converter<true, true>::convert(FromIntType from)
{
    BOOST_STATIC_ASSERT(std::numeric_limits<ToIntType>::is_signed && std::numeric_limits<FromIntType>::is_signed);
    if((from < std::numeric_limits<ToIntType>::min()) ||
            (from > std::numeric_limits<ToIntType>::max())
      ) {
        throw std::runtime_error("Integer overflow in casting from large signed rvalue into smaller signed lvalue");
    }
    return static_cast<ToIntType>(from);
}

/** From signed to unsigned */
template<>
template<typename ToIntType, typename FromIntType>
ToIntType Converter<false, true>::convert(FromIntType from)
{
    BOOST_STATIC_ASSERT(!std::numeric_limits<ToIntType>::is_signed && std::numeric_limits<FromIntType>::is_signed);
   typedef typename boost::make_unsigned<FromIntType>::type unsignedType;
   unsignedType unsignedIn = from;

   if(std::numeric_limits<FromIntType>::digits < std::numeric_limits<ToIntType>::digits) {
       if(from < 0) {
           return unsignedIn;
       }
    } else {
        if(from > 0) {
            if (unsignedIn > std::numeric_limits<ToIntType>::max()) {
                throw std::runtime_error("Integer overflow in casting from large signed rvalue into smaller unsigned lvalue");
            }
        } else if (from < 0) {
            throw std::runtime_error("Integer overflow in casting from large signed rvalue into smaller unsigned lvalue");
        }
    }
    return unsignedIn;
}

/** From unsigned to signed */
template<>
template<typename ToIntType, typename FromIntType>
ToIntType Converter<true, false>::convert(FromIntType from)
{
    BOOST_STATIC_ASSERT(std::numeric_limits<ToIntType>::is_signed && !std::numeric_limits<FromIntType>::is_signed);
    if(std::numeric_limits<ToIntType>::digits < std::numeric_limits<FromIntType>::digits) {
        typename boost::make_unsigned<ToIntType>::type allBitsSet = -1; //e.g. 0xFFFF
        if( from > allBitsSet) {
            throw std::runtime_error("Integer overflow in casting from large unsigned rvalue into smaller signed lvalue");
        }
    }
    return static_cast<ToIntType>(from);
}

/** From unsigned to unsigned */
template<>
template<typename ToIntType, typename FromIntType>
ToIntType Converter<false, false>::convert(FromIntType from)
{

    if(from > std::numeric_limits<ToIntType>::max()) {
        throw std::runtime_error("Integer overflow in casting from large unsigned rvalue into smaller unsigned lvalue");
    }
    return static_cast<ToIntType>(from);
}


}


/**
 * This cast only cares about integer sizes not sign mismatch
 * works only on two's complement (Big or Little Endian) Machines
 */
template<typename ToIntType, typename FromIntType>
inline ToIntType bitwise_int_cast(FromIntType from)
{
    BOOST_STATIC_ASSERT(std::numeric_limits<ToIntType>::is_integer && std::numeric_limits<FromIntType>::is_integer);
    Detail::Converter<std::numeric_limits<ToIntType>::is_signed, std::numeric_limits<FromIntType>::is_signed> converter;
    return converter.template convert<ToIntType>(from);
}