C++ 为什么会有这种特殊的特征<;uint8_t>;和编解码器vt<;uint8_t>;用于基本\u ifstream模板throw std::bad \u cast?
这里是Stackoverflow,询问为什么C++ 为什么会有这种特殊的特征<;uint8_t>;和编解码器vt<;uint8_t>;用于基本\u ifstream模板throw std::bad \u cast?,c++,visual-studio,c++11,g++,char-traits,C++,Visual Studio,C++11,G++,Char Traits,这里是Stackoverflow,询问为什么
不起作用。答案是,char\u traits
只专门用于char
和wchar\u t
(加上char16\u t
,char32\u t
在C++11中),你应该坚持使用basic\u fstream
读取二进制数据并在需要时强制转换
好吧,该死,这还不够好!:)
没有一个答案(我能找到)说如何专门化char\u traits
并将其与basic\u fstream
模板一起使用,或者如果可能的话。所以我想我应该试着自己实现它
在Windows 7 64位上使用Visual Studio Express 2013 RC时,以及在Kubuntu GNU/Linux 13.04 64位上使用g++-4.7时,以下代码编译无误。但是,它在运行时抛出std::bad_cast异常。我没有访问带有libc++的clang++来测试这种组合
#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:
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.
将byte_type
更改为char
将提供预期的输出:
$ g++ -std=c++11 -Wall -Wextra -pedantic stream.cpp -o stream && ./stream /dev/random
Got:4
为什么会抛出std::bad_cast?我如何修复它?我能够在我的gcc(AIX上的4.7.2)上复制一个bad_cast 得到它的原因是,gcc库实现人员优化了
basic_filebuf::xsgetn
(从basic_istream::read中称为),以便在流的区域设置未转换时调用普通Cfread
从文件中读取(也就是说,您没有试图将UTF-8或GB18030文件读入UTF-32字符串或其他内容),这绝对是正确的做法。要确定它是否未转换,它会检查流中嵌入的区域设置的codevt方面……它不存在
您可以通过执行
std::cout << std::use_facet<
std::codecvt<std::uint8_t, char, std::mbstate_t>
>(stream.getloc()).always_noconv() << '\n';
在stream.read
之前的任何位置,或者插入全局:std::locale::global(std::locale(std::locale(),new std::codecvt))
在构建基本IFU流之前的任何地方
谢谢您的建议。我已经更新了代码,以包含一个专门的编解码器VT,但现在它在g++和Visual Studio上都会抛出std::bad_强制转换。@DrTwox在添加imbue后适用于我的gcc(请参见编辑)我面临的问题和你一样,这篇文章的答案是@Cubbi,非常有用!只是一个旁注:在我看来,即使没有std::char_traits
的专门化,也能完成整个工作,即std::codevt
的专门化和imbue
技巧就足够了。我错了吗?
std::cout << std::use_facet<
std::codecvt<std::uint8_t, char, std::mbstate_t>
>(stream.getloc()).always_noconv() << '\n';
stream.imbue(std::locale(stream.getloc(),
new std::codecvt<uint8_t, char, std::mbstate_t>));