这是一个streambuf“过滤器”的示例,它从指定位置开始读取包含的streambuf 并读取指定大小。你创造substreambuf
,传递你原来的streambuf
in and substreambuf
然后转换访问权限,以便从底层的所需位置读取所有内容streambuf
.
大部分开销涉及调用sgetc
and snextc
from underflow
and uflow
应该优化掉。许多提取运算符逐字节工作,因此除了维护小节内的读取位置和检查小节末尾之外,不应有额外的开销。当然,使用此类读取大块数据的效率会较低(尽管这可以修复)。
这仍然需要改进,例如测试请求的位置是否在底层中streambuf
.
class substreambuf : public std::streambuf
{
public:
substreambuf(std::streambuf *sbuf, std::size_t start, std::size_t len) : m_sbuf(sbuf), m_start(start), m_len(len), m_pos(0)
{
std::streampos p = m_sbuf->pubseekpos(start);
assert(p != std::streampos(-1));
setbuf(NULL, 0);
}
protected:
int underflow()
{
if (m_pos + std::streamsize(1) >= m_len)
return traits_type::eof();
return m_sbuf->sgetc();
}
int uflow()
{
if (m_pos + std::streamsize(1) > m_len)
return traits_type::eof();
m_pos += std::streamsize(1);
return m_sbuf->sbumpc();
}
std::streampos seekoff(std::streamoff off, std::ios_base::seekdir way, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
{
std::streampos cursor;
if (way == std::ios_base::beg)
cursor = off;
else if (way == std::ios_base::cur)
cursor = m_pos + off;
else if (way == std::ios_base::end)
cursor = m_len - off;
if (cursor < 0 || cursor >= m_len)
return std::streampos(-1);
m_pos = cursor;
if (m_sbuf->pubseekpos(m_start + m_pos, std::ios_base::beg) == std::streampos(-1))
return std::streampos(-1);
return m_pos;
}
std::streampos seekpos(std::streampos sp, std::ios_base::openmode which = std::ios_base::in | std::ios_base::out)
{
if (sp < 0 || sp >= m_len)
return std::streampos(-1);
m_pos = sp;
if (m_sbuf->pubseekpos(m_start + m_pos, std::ios_base::beg) == std::streampos(-1))
return std::streampos(-1);
return m_pos;
}
private:
std::streambuf *m_sbuf;
std::streampos m_start;
std::streamsize m_len;
std::streampos m_pos;
};
可以这样使用
using namespace std;
void somefunc(ifstream &bigifs)
{
substreambuf sbuf(bigifs.rdbuf(),100,100);
//new istream with the substreambuf as its streambuf
istream isub(&sbuf);
//use isub normally
}
这是受到启发的过滤流缓冲区 http://lists.boost.org/Archives/boost/att-49459/fltrsbf1.htm