C ++装饰basic_stream

问题描述

| 我想通过使用装饰器模式来扩展basic_streambuf对象的行为。那就是我目前得到的：

template<typename char_type,class traits_type>
class forwarding_basic_streambuf
    : boost::noncopyable,public std::basic_streambuf<char_type,traits_type>
{
public:
    typedef std::basic_streambuf<char_type,traits_type> forwarded_type;

    forwarding_basic_streambuf(forwarded_type& fwd_buf)
        : m_fwd(&fwd_buf) { }
    virtual ~forwarding_basic_streambuf() { }

    // locales:
//  std::locale pubimbue(std::locale const& loc);
//      => Calls: imbue(loc) | Returns: PrevIoUs value of getloc();
//  std::locale getloc  () const;
//      => Returns: If pubimbue() has ever been called,then the last value of loc supplied,otherwise the
//                  current global locale,locale(),in effect at the time of construction. If called after
//                  pubimbue() has been called but before pubimbue has returned (i.e.,from within the call
//                  of imbue()) then it returns the prevIoUs value.

    // buffer management and positioning:
//  forwarded_type* pubsetbuf (char_type* s,std::streamsize n); => Returns: setbuf(s,n)
//  pos_type        pubseekoff(off_type off,std::ios_base::seekdir way,//      std::ios_base::openmode which = std::ios_base::in | std::ios_base::out);
//      => Returns seekoff(off,way,which)
//  pos_type        pubseekpos(pos_type sp,//      std::ios_base::openmode which = std::ios_base::in | std::ios_base::out);
//      => Returns: seekpos(sp,which)
//  int             pubsync   (); => Returns: sync()

    // get and put areas:
    // get area:
//  std::streamsize sgetn   (char_type* s,std::streamsize n); => Returns: xsgetn(s,n)

    // put area:
//  std::streamsize sputn(char_type const* s,std::streamsize n); => Returns: xsputn(s,n)

protected:
    // virtual functions:
    // locales:
    virtual void imbue(std::locale const& loc) { this->m_fwd->pubimbue(loc); }

    // buffer management and positioning:
    virtual forwarded_type*  setbuf (char_type* s,std::streamsize n)
        { return this->m_fwd->pubsetbuf(s,n); }
    virtual pos_type         seekoff(off_type off,std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
        { return this->m_fwd->pubseekoff(off,way); }
    virtual pos_type         seekpos(pos_type sp,std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
        { return this->m_fwd->pubseekpos(sp,which); }
    virtual int              sync   ()
        { return this->m_fwd->pubsync(); }

    // get and put areas:
    // get area:
    virtual std::streamsize xsgetn(char_type* s,std::streamsize n)
        { return this->m_fwd->sgetn(s,n); }
    virtual int_type        uflow()
        {
            if (traits_type::eq_int_type(this->underflow(),traits_type::eof()))
                return traits_type::eof();
            return this->m_fwd->sgetc();
        }

    // put area:
    virtual std::streamsize xsputn  (char_type const* s,std::streamsize n)
        { return this->m_fwd->sputn(s,n); }
    virtual int_type        overflow(int_type c = traits_type::eof())
        {
            if (traits_type::eq_int_type(c,traits_type::eof()))
                return traits_type::not_eof(c);
            return this->m_fwd->sputc(traits_type::to_char_type(c));
        }

private:
    forwarded_type* m_fwd;
};

基本目标（作为第一步）将是简单地将每个功能转发到装饰对象。因此，即使有指向它的基类的指针，也应该可以使用该装饰器。一切对于编写方法都可以正常工作，但是我不知道如何处理underflow（）函数，该函数从uflow（）和sgetc（）调用。

解决方法

我可能会丢失一些东西，但是您的设计对我来说没有多大意义。如我所见，扩展ѭ1是通过覆盖受保护的虚拟接口来完成的，如果您想使用装饰器模式，则装饰器基类将仅执行此操作。

template<typename char_type,class traits_type>
class forwarding_basic_streambuf
    : boost::noncopyable,public std::basic_streambuf<char_type,traits_type>
{
public:
    typedef std::basic_streambuf<char_type,traits_type> forwarded_type;

    forwarding_basic_streambuf(forwarded_type& fwd_buf)
        : m_fwd(&fwd_buf) { }
    virtual ~forwarding_basic_streambuf() { }

protected:
   virtual streamsize xsputn ( const char * s,streamsize n ) {
       m_fwd->xsputn(s,n);
   }
   virtual int overflow ( int c) {
       m_fwd->overflow(c);
   }
   // etc.
};

然后，您真正的实现会添加任何所需的“装饰”，例如

template<typename char_type,class traits_type>
class add_timestamp_decorator
    : public forwarding_basic_streambuf<char_type,traits_type>
{
public:
    typedef std::forwarding_basic_streambuf<char_type,traits_type> base_type;

    add_timestamp_decorator(base_type::forwarded_type& fwd_buf)
        : base_type(&fwd_buf) { }
    virtual ~add_timestamp_decorator() { }

protected:
   virtual streamsize xsputn ( const char * s,streamsize n ) {
       // detect and remember when a newline is written
       // before the next char is written output the timestamp
       base_type::xsputn(s,n);
   }
   // etc.
};

然后在输出流上使用它（伪代码，为清楚起见省略了模板定义）

ostream outputstream;
// ....
add_timestamp_decorator decorator = new add_timestamp_decorator(outputstream.rdbuf());
outputstream.rdbuf(decorator);

outputstream << \"some lines\\ntimestamps will be inserted\\n\\n\";

, 规范以下列方式描述“ 5”（27.6.3.4.3。[15-17]）：要求：约束与underflow（）相同，不同之处在于结果字符从挂起序列传输到备份序列，并且挂接序列在传输前不得为空。默认行为：调用underflow（）。如果underflow（）返回traits :: eof（），则返回traits :: eof（）。否则，返回traits :: to_int_type（* gptr（））的值，并为输入序列增加下一个指针的值。返回：traits :: eof（）表示失败。在C ++中，外观如下所示：

if (traits::eq_int_type(this->underflow(),traits::eof())
    return traits::eof();    
return *fetch_gptr_and_inc();

我们无法直接在装饰对象上调用“ 7”，因为它不可访问。另外，我们不想获取自己的gptr（），而是想要装饰的。但是，我们可以通过调用sgetc()和sbumpc间接实现此目的：

template<typename char_type,class traits_type>
typename forwarding_basic_streambuf<char_type,traits_type>::int_type
forwarding_basic_streambuf<char_type,traits_type>::uflow()
{   
    if (traits_type::eq_int_type(this->m_fwd->sgetc(),traits_type::eof()))
        return traits_type::eof();
    return this->m_fwd->sbumpc();
}

++basic underflow 装饰

C ++装饰basic_stream :: underflow

问题描述

解决方法