为什么这个专门用于basic_ifstream模板的char_traits <uint8_t>和codecvt <uint8_t>会抛出std :: bad_cast?</uint8_t> </uint8_t>

时间:2013-10-06 04:47:12

标签: c++ visual-studio c++11 g++ char-traits

Stackoverflow上的

There are already questions询问为什么 basic_fstream<uint8_t>不起作用。答案说,char_traits仅适用于charwchar_t(加上char16_t,{+ 1}}在C ++ 11中),你应该坚持char32_t 1}}读取二进制数据并在需要时投射它。

好吧,这还不够好! :)

没有一个答案(我能找到)说如何专门化basic_fstream<char>并将其与char_traits<uint8_t>模板一起使用,或者甚至可能。所以我想我会尝试自己实现它。

在Windows 7 64位上使用Visual Studio Express 2013 RC并在Kubuntu GNU / Linux 13.04 64位上使用g ++ - 4.7时,以下编译没有错误。但是它会在运行时抛出std :: bad_cast异常。我无法访问使用libc ++的clang ++来测试该组合。

basic_fstream

使用g ++和GNU / Linux编译并运行:

#include <cinttypes>
#include <cstring>

#include <algorithm>
#include <fstream>
#include <iostream>
#include <locale>

#ifdef _WIN32
    #define constexpr
    #define NOEXCEPT throw()
#else
    #define NOEXCEPT noexcept
#endif

// Change this to char and it works.
using byte_type = std::uint8_t;

namespace std
{
    // Specialization of std::char_traits
    template <> struct char_traits< std::uint8_t >
    {
        using char_type = std::uint8_t;
        using int_type = int;
        using off_type = std::streamoff;
        using pos_type = std::streampos;
        using state_type = std::mbstate_t;

        static void assign(char_type& value1, const char_type& value2)
        {
            value1 = value2;
        }

        static char_type* assign(char_type* ptr, std::size_t count, char_type value)
        {
            return static_cast<char_type*>(std::memset(ptr, value, count));
        }

        static constexpr bool eq(const char_type& value1, const char_type& value2) NOEXCEPT
        {
            return value1 == value2;
        }

        static constexpr bool lt(const char_type value1, const char_type value2) NOEXCEPT
        {
            return value1 < value2;
        }

        static std::size_t length(const char_type* ptr)
        {
            std::size_t i = 0;
            while (!eq(ptr[i], char_type()))
            {
                ++i;
            }
            return i;
        }

        static int compare(const char_type* ptr1, const char_type* ptr2, std::size_t count)
        {
            return std::memcmp(ptr1, ptr2, count);
        }

        static const char_type* find(const char_type* ptr, std::size_t count, const char_type& value)
        {
            return static_cast<const char_type*>(std::memchr(ptr, value, count));
        }

        static char_type* move(char_type* dest, const char_type* src, std::size_t count)
        {
            return static_cast<char_type*>(std::memmove(dest, src, count));
        }

        static char_type* copy(char_type* dest, const char_type* src, std::size_t count)
        {
            return static_cast<char_type*>(std::memcpy(dest, src, count));
        }

        static constexpr char_type to_char_type(const int_type& value) NOEXCEPT
        {
            return static_cast<char_type>(value);
        }

        static constexpr int_type to_int_type(const char_type& value) NOEXCEPT
        {
            return static_cast<int_type>(value);
        }

        static constexpr bool eq_int_type(const int_type& value1, const int_type& value2) NOEXCEPT
        {
            return value1 == value2;
        }

        static constexpr int_type eof() NOEXCEPT
        {
            return static_cast<int_type>(std::char_traits<char>::eof());
        }

        static constexpr int_type not_eof(const int_type& value) NOEXCEPT
        {
            return (value == eof()) ? 0 : value;
        }
    };

    // Specialization of std::codecvt
    template<> class codecvt< std::uint8_t, char, std::mbstate_t > : public locale::facet, public codecvt_base
    {
    public:
        using internal_type = std::uint8_t;
        using external_type = char;
        using state_type = std::mbstate_t;

        static std::locale::id id;

        codecvt(std::size_t refs = 0)
            : locale::facet(refs)
        {}

        std::codecvt_base::result out(state_type& state, const internal_type* from, const internal_type* from_end, const internal_type*& from_next, external_type* to, external_type* to_end, external_type*& to_next) const
        {
            return do_out(state, from, from_end, from_next, to, to_end, to_next);
        }

        std::codecvt_base::result in(state_type& state, const external_type* from, const external_type* from_end, const external_type*& from_next, internal_type* to, internal_type* to_end, internal_type*& to_next) const
        {
            return do_in(state, from, from_end, from_next, to, to_end, to_next);
        }

        std::codecvt_base::result unshift(state_type& state, external_type* to, external_type* to_end, external_type*& to_next) const
        {
            return do_unshift(state, to, to_end, to_next);
        }

        int length(state_type& state, const external_type* from, const external_type* from_end, std::size_t max) const
        {
            return do_length(state, from, from_end, max);
        }

        int max_length() const NOEXCEPT
        {
            return do_max_length();
        }

        int encoding() const NOEXCEPT
        {
            return do_encoding();
        }

        bool always_noconv() const NOEXCEPT
        {
            return do_always_noconv();
        }

    protected:
        virtual ~codecvt() {}
        virtual std::codecvt_base::result do_out(state_type& state, const internal_type* from, const internal_type* from_end, const internal_type*& from_next, external_type* to, external_type* to_end, external_type*& to_next) const;
        virtual std::codecvt_base::result do_in(state_type& state, const external_type* from, const external_type* from_end, const external_type*& from_next, internal_type* to, internal_type* to_end, internal_type*& to_next) const;
        virtual std::codecvt_base::result do_unshift(state_type& state, external_type* to, external_type* to_end, external_type*& to_next) const;
        virtual int do_length(state_type& state, const external_type* from, const external_type* from_end, std::size_t max) const;
        virtual int do_max_length() const NOEXCEPT;
        virtual int do_encoding() const NOEXCEPT;
        virtual bool do_always_noconv() const NOEXCEPT;
    }; // class codecvt

    locale::id codecvt< std::uint8_t, char, std::mbstate_t >::id;

    codecvt_base::result codecvt< std::uint8_t, char, std::mbstate_t >::do_out(state_type& state, const internal_type* from, const internal_type* from_end, const internal_type*& from_next, external_type* to, external_type* to_end, external_type*& to_next) const
    {
        (void) state; (void) from_end; (void) to_end; // Unused parameters
        from_next = from;
        to_next = to;
       return codecvt_base::noconv;
    }

    codecvt_base::result codecvt< std::uint8_t, char, std::mbstate_t >::do_in(state_type& state, const external_type* from, const external_type* from_end, const external_type*& from_next, internal_type* to, internal_type* to_end, internal_type*& to_next) const
    {
        (void) state; (void) from_end; (void) to_end; // Unused parameters
        from_next = from;
        to_next = to;
        return std::codecvt_base::noconv;
    }

    codecvt_base::result codecvt< std::uint8_t, char, std::mbstate_t >::do_unshift(state_type& state, external_type* to, external_type* to_end, external_type*& to_next) const
    {
        (void) state; (void) to_end; // Unused perameters
        to_next = to;
        return std::codecvt_base::noconv;
    }

    int codecvt< std::uint8_t, char, std::mbstate_t >::do_length(state_type& state, const external_type* from, const external_type* from_end, std::size_t max) const
    {
        (void) state; // Unused parameter
        return static_cast<int>(std::min< std::size_t >(max, static_cast<std::size_t>(from_end - from)));
    }

    int codecvt< std::uint8_t, char, std::mbstate_t >::do_max_length() const NOEXCEPT
    {
        return 1;
    }

    int codecvt< std::uint8_t, char, std::mbstate_t >::do_encoding() const NOEXCEPT
    {
        return 1;
    }

    bool codecvt< std::uint8_t, char, std::mbstate_t >::do_always_noconv() const NOEXCEPT
    {
        return true;
    }
} // namespace std


int main(int argc, char *argv [])
{
    if (argc < 2)
    {
        std::cerr << argv[0] << " {file to read}" << std::endl;
        return EXIT_FAILURE;
    }

    using stream_type = std::basic_ifstream< byte_type, std::char_traits<byte_type> >;

    stream_type stream(argv[1], std::ifstream::in | std::ifstream::binary);
    if (stream.is_open() == false)
    {
        std::cerr << "file not found" << std::endl;
        return EXIT_FAILURE;
    }
    stream.exceptions(std::ifstream::badbit);

    static const auto read_size = 4;
    stream_type::char_type buffer[read_size];

    stream.read(buffer, read_size);

    std::cout << "Got:" << stream.gcount() << std::endl;

    return EXIT_SUCCESS;
}

使用Visual Studio Express RC 2013:

$ g++ -std=c++11 -Wall -Wextra -pedantic stream.cpp -o stream && ./stream /dev/random 
terminate called after throwing an instance of 'std::bad_cast'
  what():  std::bad_cast
Aborted (core dumped)

First-chance exception at 0x76A6C41F in traits test.exe: Microsoft C++ exception: std::bad_cast at memory location 0x0038F978. Unhandled exception at 0x76A6C41F in traits test.exe: Microsoft C++ exception: std::bad_cast at memory location 0x0038F978. 更改为byte_type可获得预期的输出:

char

为什么要抛出std :: bad_cast以及如何解决?

1 个答案:

答案 0 :(得分:6)

我能够在我的gcc上重现bad_cast(AIX上的4.7.2)。

你得到它的原因是gcc库实现者优化basic_filebuf::xsgetn(称为from basic_istream::read)来调用普通的C fread来读取你的流的语言环境是非文件转换(也就是说,你不是试图将UTF-8或GB18030文件读入UTF-32字符串或其他东西),这绝对是正确的做法。要确定它是否为非转换,它会检查您的流中所包含的语言环境的codecvt方面的codecvt::always_noconv ...这是不存在的。

您可以通过执行

来重现异常
std::cout << std::use_facet<
                     std::codecvt<std::uint8_t, char, std::mbstate_t>
             >(stream.getloc()).always_noconv() << '\n';

我无权访问Visual Studio以查看它在那里工作的原因(他们只是为basic_filebuf::sgetc()中的每个字符调用basic_fstream::read()吗?)但是在任何情况下都要使用basic_filestream,你需要为您的内部和外部类型组合提供codecvt方面(在本例中为uint8_tchar)。

编辑:你几乎就在那里,最后一个缺失的部分是行

stream.imbue(std::locale(stream.getloc(), 
             new std::codecvt<uint8_t, char, std::mbstate_t>));

stream.read之前的任何地方,或者,在构建std::locale::global(std::locale(std::locale(), new std::codecvt<uint8_t, char, std::mbstate_t>));

之前,将全局:basic_ifstream灌输到任何地方