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。要知道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值从int32_t转换为实际类型。有时我甚至想在代码中将无符号int32变量赋值给无符号int64变量。知道如果值足够大，unsigned int32返回为int32_t，那么它在该类型中表示为负值，如果我只是将其静态转换为uint64_t，那么符号位将扩展以填充uint64_t中所有较高的位，从而产生完全不同的unsigned值。

因此，我编写了一个强制转换函数，它应该负责将较大的int类型或较小的int类型强制转换为正确的值。然而，我觉得这可能是一个已知的问题，可能已经存在解决方案。下面是函数。请告知如果你认为这可以改进，或者如果有一个更好的解决方案(我已经使这个函数比我真正需要我的用例更通用)。

编辑:使此可移植关于Endianness。

编辑:删除编译器对有符号/无符号比较的警告。

#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);
}

union mergeint_t
{
  int8_t int8;
  int16_t int16;
  int32_t int32;
  int64_t int64;
  //..
};

  mergeval.int32=yourFunctionReturningInt32();

if (type==INT8_T) {
  int8val = mergeint.int8;
}

int main() {
  union mergeint_t mergeval;
  mergeval.int64=11;
  printf("mergeval int8=[%d]nn", mergeval.int8);
}