将窄 (char) 输入流重新解释为宽 (wchar_t) 流

Question

我得到一个包含 UTF-16 编码字符串的 std::istream。想象一下这样打开的 UTF-16 编码文本文件：

std::ifstream file( "mytext_utf16.txt", std::ios::binary );

我想将此流传递给采用 std::wistream& 参数的函数。我无法将文件流类型更改为 std::wifstream.

问题：标准库或增强库中是否有任何工具可以让我将 istream“重新解释”为 wistream？

我在想象一个类似于 std::wbuffer_convert 的适配器 class，只是它不应该进行任何编码转换。基本上，对于从适配器 class 读取的每个 wchar_t，它应该只从关联的 istream 读取两个字节并将它们 reinterpret_cast 到 wchar_t.

我已经使用 boost::iostreams 创建了一个实现，可以像这样使用并且非常有效：

std::ifstream file( "mytext_utf16.txt", std::ios::binary );

// Create an instance of my adapter class.
reinterpret_as_wide_stream< std::ifstream > wfile( &file );

// Read a wstring from file, using the adapter.
std::wstring str;
std::get_line( wfile, str );

那我为什么要问呢？因为我喜欢重用已有的代码，而不是重新发明轮子。

Answer 1

由于还没有其他答案，我发布了使用 Boost.Iostreams 库的解决方案。虽然它非常简单，但我仍然认为应该有一个更简单的解决方案。

首先，我们创建一个模板 class，它模拟 Boost.Iostreams device 概念，并用作相关窄设备的适配器。它将 read、write 和 seek 操作转发到相关设备，但调整流位置和大小值以适应窄字符类型和宽字符类型之间的大小差异。

"basic_reinterpret_device.h"

#pragma once
#include <boost/iostreams/traits.hpp>
#include <boost/iostreams/read.hpp>
#include <boost/iostreams/write.hpp>
#include <boost/iostreams/seek.hpp>

// CategoryT: boost.iostreams device category tag
// DeviceT  : type of associated narrow device
// CharT    : (wide) character type of this device adapter 
template< typename CategoryT, typename DeviceT, typename CharT >
class basic_reinterpret_device
{
public:
    using category = CategoryT;               // required by boost::iostreams device concept
    using char_type = CharT;                  // required by boost::iostreams device concept
    using associated_device = DeviceT;
    using associated_char_type = typename boost::iostreams::char_type_of< DeviceT >::type;
    static_assert( sizeof( associated_char_type ) == 1, "Associated device must have a byte-sized char_type" );

    // Default constructor.
    basic_reinterpret_device() = default;

    // Construct from a narrow device
    explicit basic_reinterpret_device( DeviceT* pDevice ) :
        m_pDevice( pDevice ) {}

    // Get the asociated device.
    DeviceT* get_device() const { return m_pDevice; }

    // Read up to n characters from the underlying data source into the buffer s, 
    // returning the number of characters read; return -1 to indicate EOF
    std::streamsize read( char_type* s, std::streamsize n )
    {
        ThrowIfDeviceNull();

        std::streamsize bytesRead = boost::iostreams::read( 
            *m_pDevice, 
            reinterpret_cast<associated_char_type*>( s ), 
            n * sizeof( char_type ) );

        if( bytesRead == static_cast<std::streamsize>( -1 ) )  // EOF
            return bytesRead;
        return bytesRead / sizeof( char_type );
    }

    // Write up to n characters from the buffer s to the output sequence, returning the 
    // number of characters written.
    std::streamsize write( const char_type* s, std::streamsize n )
    {
        ThrowIfDeviceNull();

        std::streamsize bytesWritten = boost::iostreams::write(
            *m_pDevice, 
            reinterpret_cast<const associated_char_type*>( s ), 
            n * sizeof( char_type ) );

        return bytesWritten / sizeof( char_type );
    }

    // Advances the read/write head by off characters, returning the new position, 
    // where the offset is calculated from:
    //  - the start of the sequence if way == ios_base::beg
    //  - the current position if way == ios_base::cur
    //  - the end of the sequence if way == ios_base::end
    std::streampos seek( std::streamoff off, std::ios_base::seekdir way ) 
    {
        ThrowIfDeviceNull();

        std::streampos newPos = boost::iostreams::seek( *m_pDevice, off * sizeof( char_type ), way );
        return newPos / sizeof( char_type );
    }

protected:
    void ThrowIfDeviceNull()
    {
        if( ! m_pDevice )
            throw std::runtime_error( "basic_reinterpret_device - no associated device" );
    }

private:
    DeviceT* m_pDevice = nullptr;
};

为了简化此模板的使用，我们为最常见的 Boost.Iostreams 设备标签创建了一些别名模板。基于这些我们创建别名模板来构建标准兼容的流缓冲区和流。

"reinterpret_stream.h"

#pragma once
#include "basic_reinterpret_device.h"

#include <boost/iostreams/categories.hpp>
#include <boost/iostreams/traits.hpp>
#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/stream_buffer.hpp>

struct reinterpret_device_tag : virtual boost::iostreams::source_tag, virtual boost::iostreams::sink_tag {};
struct reinterpret_source_seekable_tag : boost::iostreams::device_tag, boost::iostreams::input_seekable {};
struct reinterpret_sink_seekable_tag : boost::iostreams::device_tag, boost::iostreams::output_seekable {};


template< typename DeviceT, typename CharT >
using reinterpret_source = basic_reinterpret_device< boost::iostreams::source_tag, DeviceT, CharT >;

template< typename DeviceT, typename CharT >
using reinterpret_sink = basic_reinterpret_device< boost::iostreams::sink_tag, DeviceT, CharT >;

template< typename DeviceT, typename CharT >
using reinterpret_device = basic_reinterpret_device< reinterpret_device_tag, DeviceT, CharT >;

template< typename DeviceT, typename CharT >
using reinterpret_device_seekable = basic_reinterpret_device< boost::iostreams::seekable_device_tag, DeviceT, CharT >;

template< typename DeviceT, typename CharT >
using reinterpret_source_seekable = 
    basic_reinterpret_device< reinterpret_source_seekable_tag, DeviceT, CharT >;

template< typename DeviceT, typename CharT >
using reinterpret_sink_seekable = 
    basic_reinterpret_device< reinterpret_sink_seekable_tag, DeviceT, CharT >;


template< typename DeviceT >
using reinterpret_as_wistreambuf = boost::iostreams::stream_buffer< reinterpret_source_seekable< DeviceT, wchar_t > >;

template< typename DeviceT >
using reinterpret_as_wostreambuf = boost::iostreams::stream_buffer< reinterpret_sink_seekable< DeviceT, wchar_t > >;

template< typename DeviceT >
using reinterpret_as_wstreambuf = boost::iostreams::stream_buffer< reinterpret_device_seekable< DeviceT, wchar_t > >;


template< typename DeviceT >
using reinterpret_as_wistream = boost::iostreams::stream< reinterpret_source_seekable< DeviceT, wchar_t > >;

template< typename DeviceT >
using reinterpret_as_wostream = boost::iostreams::stream< reinterpret_sink_seekable< DeviceT, wchar_t > >;

template< typename DeviceT >
using reinterpret_as_wstream = boost::iostreams::stream< reinterpret_device_seekable< DeviceT, wchar_t > >;

用法示例：

#include "reinterpret_stream.h"

void read_something_as_utf16( std::istream& input )
{
    reinterpret_as_wistream< std::istream > winput( &input );
    std::wstring wstr;
    std::getline( winput, wstr );   
}

void write_something_as_utf16( std::ostream& output )
{
    reinterpret_as_wostream< std::ostream > woutput( &output );
    woutput << L"сайт вопросов и ответов для программистов";
}

Answer 2

这项工作正在进行中

这不是您应该使用的东西，但可能是您可以开始的提示，如果您还没有考虑过这样做的话。如果这没有帮助，或者当您可以找到更好的解决方案时，我很高兴删除或扩展此答案。

据我了解，您想读取 UTF-8 文件并将每个字符简单地转换为 wchar_t。

如果标准设施做的太多了，你能不能写你自己的方面。

#include <codecvt>
#include <locale>
#include <fstream>
#include <cwchar>
#include <iostream>
#include <fstream>

class MyConvert
{
 public:
  using state_type = std::mbstate_t;
  using result = std::codecvt_base::result;
  using From = char;
  using To = wchar_t;
  bool always_noconv() const throw() {
    return false;
  }
  result in(state_type& __state, const From* __from,
    const From* __from_end, const From*& __from_next,
    To* __to, To* __to_end, To*& __to_next) const
  {
    while (__from_next != __from_end) {
      *__to_next = static_cast<To>(*__from_next);
      ++__to_next;
      ++__from_next;
    }
    return result::ok;
  }
  result out(state_type& __state, const To* __from,
      const To* __from_end, const To*& __from_next,
      From* __to, From* __to_end, From*& __to_next) const
  {
    while (__from_next < __from_end) {
      std::cout << __from << " " << __from_next << " " << __from_end << " " << (void*)__to << 
        " " << (void*)__to_next << " " << (void*)__to_end << std::endl;
      if (__to_next >= __to_end) {
        std::cout << "partial" << std::endl;
        std::cout << "__from_next = " << __from_next << " to_next = " <<(void*) __to_next << std::endl;
        return result::partial;
      }
      To* tmp = reinterpret_cast<To*>(__to_next);
      *tmp = *__from_next;
      ++tmp;
      ++__from_next;
      __to_next = reinterpret_cast<From*>(tmp);
    }
    return result::ok;
  }
};

int main() {
  std::ofstream of2("test2.out");
  std::wbuffer_convert<MyConvert, wchar_t> conv(of2.rdbuf());
  std::wostream wof2(&conv);
  wof2 << L"сайт вопросов и ответов для программистов";
  wof2.flush();
  wof2.flush();
}

这不是您应该在代码中使用的内容。如果方向正确，您需要阅读文档，包括这方面需要什么，所有这些指针的含义，以及您需要如何写入它们。

如果你想使用这样的东西，你需要考虑你应该为 facet 使用哪些模板参数（如果有的话）。

更新我现在更新了我的代码。 out-function 现在更接近我想要的了。它并不漂亮，只是一个测试代码，我仍然不确定为什么 __from_next 没有更新（或保留）。

目前的问题是我们无法写入流。使用 gcc，我们只是失去了 wbuffer_convert 的同步，对于 clang，我们得到了一个 SIGILL。

将窄 (char) 输入流重新解释为宽 (wchar_t) 流

Reinterpret a narrow (char) input stream as a wide (wchar_t) stream

c++

boost

iostream

boost-iostreams