Question

在以下代码中

template<unsigned bits_count, typename ut_t, typename udt_t, template <typename> class meta_t>
struct uint_t
{
    typedef uint_t<bits_count, ut_t, udt_t, meta_t> t;

    typedef ut_t     ut; 
    typedef udt_t   udt; 

    typedef meta_t<t> meta;

    ut comp[meta::array_count];
};

namespace mxi
{

template<typename type>
struct basic_info
{
    static const bool is_native = true;
    static const bool is_signed = (type(-1) < type(0));

    static const int num_of_bytes = sizeof(type);
    static const int num_of_bits = sizeof(type) * 8;
};

template<unsigned bits_count, typename ut_t, typename udt_t, template <typename> class meta_t>
struct basic_info< uint_t<bits_count, ut_t, udt_t, meta_t> >
{
    static const bool is_native = false;
    static const bool is_signed = false;

    static const int num_of_bytes = bits_count / 8;
    static const int num_of_bits = bits_count;
};

template<typename t>
struct isizeof
{
    static const int value = sizeof(t) * 8;
};

template<typename t>
struct num_of_dec_digits_in_type
{
    static const int value = int(isizeof<t>::value * 0.301f);
};

template <typename type>
struct default_meta
{
    static const int array_count = basic_info<type>::num_of_bits / isizeof<typename type::ut>::value;

    static const int dec_digits_count = num_of_dec_digits_in_type<type>::value;
};

}

int main()
{
    typedef uint_t<128, unsigned short int, unsigned int, mxi::default_meta> uint128_t;

    int a = uint128_t::meta::dec_digits_count;

    int b = mxi::num_of_dec_digits_in_type<uint128_t>::value;
}

GCC 4.8.1编译没有问题，但Visual C ++ 2008 SP1编译时出现以下错误：

line (40): error C2027: use of undefined type 'uint_t<bits_count,ut_t,udt_t,meta_t>'
    with
    [
        bits_count=128,
        ut_t=unsigned short,
        udt_t=unsigned int,
        meta_t=mxi::default_meta
    ]
    line(46) : see reference to class template instantiation 'mxi::isizeof<t>' being compiled
    with
    [
        t=uint_t<128,unsigned short,unsigned int,mxi::default_meta>
    ]
    line(54) : see reference to class template instantiation 'mxi::num_of_dec_digits_in_type<t>' being compiled
    with
    [
        t=uint_t<128,unsigned short,unsigned int,mxi::default_meta>
    ]
    line(11) : see reference to class template instantiation 'mxi::default_meta<type>' being compiled
    with
    [
        type=uint_t<128,unsigned short,unsigned int,mxi::default_meta>
    ]
    line(63) : see reference to class template instantiation 'uint_t<bits_count,ut_t,udt_t,meta_t>' being compiled
    with
    [
        bits_count=128,
        ut_t=unsigned short,
        udt_t=unsigned int,
        meta_t=mxi::default_meta
    ]

如果uint_t是文件中定义的第一个类型，它是如何定义的？

注意：我已经将大量代码剥离到只有错误的部分，即这里的大多数数字是在编译时计算的，但是我在这里直接使用它们来关注问题

Answer 1

当我使用完整类型时，我不明白为什么类型不完整 - 即uint128_t - 但我能找到两个解决方案：

1-在num_of_dec_digits_in_type中将isizeof<t>::value替换为basic_info<t>::num_of_bits即可。

2-创建新类型num_of_dec_digits_in_type_size并使用它代替num_of_dec_digits_in_type：

template<int size>
struct num_of_dec_digits_in_type_size
{
    static const int value = int(size * 0.301f);
};

template <typename type>
struct default_meta
{
    .
    .

    static const int dec_digits_count = num_of_dec_digits_in_type_size< basic_info<type>::num_of_bits >::value;
};

第二种解决方案只是对第一种解决方案的修改。

使用未定义的模板类型

1 个答案: