OpenRTMFP/Cumulus Primer(10)IO 管理之流缓冲区
2012-04-24 11:31
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OpenRTMFP/Cumulus Primer(10)IO 管理之流缓冲区
Author: 柳大·Poechant(钟超)Email: zhongchao.ustc#gmail.com (#->@)
Blog: Blog.CSDN.net/Poechant
Date: April 24th, 2012
本文主要分析 MemoryStream.h 文件中定义的类。
1 了解 std::streambuf
首先要了解 streambuf 内置了一个 get 指针和一个 put 指针。streambuf 的所有操作基本都是对这两个指针的操作。其一些成员函数的缩写中的 g 和 p 就分别表示 get pointer 和 put pointer。1.1 单步移动内置指针
Increase get pointer: Advances the get pointer by n positions. The get pointer is the internal pointer that points to the next location in the controlled input sequence.void gbump ( int n );
Increase put pointer: Advances the put pointer by n positions. The put pointer is the internal pointer that points to the next location of the controlled output sequence.
void pbump ( int n );
1.2 获取 get 指针和 put 指针的位置
Pointer to current position of input sequence: Returns a reference to the current element of the controlled input sequence (i.e., the “get pointer”).char * gptr ( ) const;
Pointer to current position of output sequence: Returns a reference to the current element of the output sequence (the put pointer).
char * pptr ( ) const;
1.3 设置 get 和 put 指针可达区域的上下界
Set input sequence pointers: Sets values for the pointers that define both the boundaries of the accessible part of the controlled input sequence and the get pointer itself.void setg ( char* gbeg, char* gnext, char* gend );
gbeg: New value for the pointer to the beginning of the accessible part of the controlled input sequence.
gnext: New value for the get pointer, which points to the next element within the controlled input sequence where the next input operation shall be performed.
gend: New value for the end pointer, just past the end of the accessible part of the controlled input sequence.
Set output sequence pointers: Sets the values that define the boundaries of the accessible part of the controlled output sequence.
void setp ( char* pbeg, char* pend );
pbeg: New value for the pointer to the beginning of the accessible part of the controlled output sequenceand put pointer.
pend: New value for the end pointer, just past the end of the accessible part of the controlled output sequence.
2 MemoryStreamBuf
类定义:class MemoryStreamBuf: public std::streambuf { friend class ScopedMemoryClip; public: MemoryStreamBuf(char* pBuffer,Poco::UInt32 bufferSize); MemoryStreamBuf(MemoryStreamBuf&); ~MemoryStreamBuf(); void next(Poco::UInt32 size); // Explaint below Poco::UInt32 written(); // Explaint below void written(Poco::UInt32 size); Poco::UInt32 size(); // Explaint below void resize(Poco::UInt32 newSize); // Explaint below char* begin(); // Explaint below void position(Poco::UInt32 pos=0); // Explaint below char* gCurrent(); // Explaint below char* pCurrent(); // Explaint below private: virtual int overflow(int_type c); virtual int underflow(); virtual int sync(); Poco::UInt32 _written; char* _pBuffer; Poco::UInt32 _bufferSize; MemoryStreamBuf(); MemoryStreamBuf& operator = (const MemoryStreamBuf&); };ScopedMemoryClip 是 MemoryStreamBuf 的友元,其内部有 MemoryStreamBuf 的成员,这里暂且不管。构造函数传入的参数是缓冲区的地址和缓冲区大小(字节数)。拷贝构造函数和析构函数不必赘述。
2.1 移动内置的 get 和 put 指针:
put 和 get 指针都移动:void MemoryStreamBuf::next(UInt32 size) { pbump(size); gbump(size); }
2.2 获取 get 和 put 指针当前位置:
封装 streambuf 的 gptr 和 pptr:inline char* MemoryStreamBuf::gCurrent() { return gptr(); } inline char* MemoryStreamBuf::pCurrent() { return pptr(); }
2.3 获取缓冲区的起始位置和大小:
依赖于内置成员变量 pBuffer 和 bufferSize:inline char* MemoryStreamBuf::begin() { return _pBuffer; } inline Poco::UInt32 MemoryStreamBuf::size() { return _bufferSize; }
2.4 缓冲区的已写字节数
读取(其中也可能发生设置操作):UInt32 MemoryStreamBuf::written() { int written = pCurrent() - begin(); // 已写字节数 if (written < 0) written = 0; if (written > _written) // 保存已写字节数 _written = (UInt32)written; return _written; }设置:
void MemoryStreamBuf::written(UInt32 size) { _written=size; }
2.5 显式设定 put 和 get 指针位置
设定 put 和 get 指针为以缓冲区首地址为开始偏移量为 pos 的位置:void MemoryStreamBuf::position(UInt32 pos) { // 保存已写字节数 written(); // Save nb char written // 移动 put 指针 setp(_pBuffer, _pBuffer + _bufferSize); if (pos > _bufferSize) pos = _bufferSize; pbump((int) pos); // 移动 get 指针 setg(_pBuffer, _pBuffer + pos, _pBuffer + _bufferSize); }
2.6 修改缓冲区大小
void MemoryStreamBuf::resize(UInt32 newSize) { // 大小标识 _bufferSize = newSize; // gptr 当前位置 int pos = gCurrent() - _pBuffer; if (pos > _bufferSize) pos = _bufferSize; // 设置 gptr 可达范围和当前位置 setg(_pBuffer, _pBuffer + pos, _pBuffer + _bufferSize); // pptr 当前位置 pos = pCurrent() - _pBuffer; if (pos > _bufferSize) pos = _bufferSize; // 设置 pptr 可达范围和当前位置 setp(_pBuffer,_pBuffer + _bufferSize); pbump(pos); }
2.7 构造函数、拷贝构造函数和析构函数
构造函数会设定 pptr 和 gptr,并初始化 pBuffer 和 bufferSizeMemoryStreamBuf::MemoryStreamBuf(char* pBuffer, UInt32 bufferSize): _pBuffer(pBuffer),_bufferSize(bufferSize),_written(0) { setg(_pBuffer, _pBuffer,_pBuffer + _bufferSize); setp(_pBuffer, _pBuffer + _bufferSize); }析构函数会拷贝对方的 pBuffer、bufferSizse、_written,并设定 gptr、pptr。注意设定 pptr 时,要分别调用 setp 和 pbump,因为 setp 仅将 pptr 设定为传入的首个参数值(与可达范围的首地址相同)。
MemoryStreamBuf::MemoryStreamBuf(MemoryStreamBuf& other): _pBuffer(other._pBuffer),_bufferSize(other._bufferSize),_written(other._written) { setg(_pBuffer, other.gCurrent(), _pBuffer + _bufferSize); setp(_pBuffer, _pBuffer + _bufferSize); pbump((int)(other.pCurrent()-_pBuffer)); }析构函数:
MemoryStreamBuf::~MemoryStreamBuf() { }
Reference
http://www.cplusplus.com/reference/iostream/streambuf/gbump/ http://www.cplusplus.com/reference/iostream/streambuf/pbump/ -转载请注明来自柳大的CSDN博客:Blog.CSDN.net/Poechant-相关文章推荐
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