[英]C++ Decorate basic_stream::underflow()
我想通過使用裝飾器模式來擴展basic_streambuf對象的行為。 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::seekdir way,
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;
};
基本目標(作為第一步)將是簡單地將每個功能轉發到裝飾對象。 因此,即使有指向其基類的指針,也應該可以使用此修飾器。
一切對於編寫方法都可以正常工作,但是我不知道如何處理underflow()函數,該函數從uflow()和sgetc()調用。
我可能會丟失一些東西,但是您的設計對我來說沒有多大意義。
正如我所看到的,擴展streambuf
是通過重寫受保護的虛擬接口來完成的,如果您想使用裝飾器模式,則裝飾器基類可以做到這一點。
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,您真正的實現將添加任何需要的“裝飾”,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.
};
然后在輸出流上使用它(偽代碼,為清楚起見省略了模板定義)
ostream outputstream;
// ....
add_timestamp_decorator decorator = new add_timestamp_decorator(outputstream.rdbuf());
outputstream.rdbuf(decorator);
outputstream << "some lines\ntimestamps will be inserted\n\n";
規范以以下方式(27.6.3.4.3。[15-17])描述uflow()
):
在C ++中,外觀如下所示:
if (traits::eq_int_type(this->underflow(), traits::eof())
return traits::eof();
return *fetch_gptr_and_inc();
我們無法直接在裝飾對象上調用underflow()
,因為它不可訪問。 另外,我們不想獲取自己的gptr(),而是想要裝飾的。 但是,我們可以通過調用sgetc()
和sbumpc
間接實現此sbumpc
:
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();
}
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