为什么这个流缓冲区不工作? [英] Why is this stream buffer not working?
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问题描述
我有一个非常简单的流缓冲区设置,将流从字节流转发到缓冲区。当我打印我得到一个奇怪的输出,即HellHelllo,
而不是Hello
。也许我的眼睛累了,但我找不到这里的问题。为什么要获取输出?
#include< iostream&
#include< vector>
#include< string>
#include< algorithm>
#include< sstream>
class io_buffer:public std :: streambuf
{
public:
io_buffer(std :: istream& is,int buf_size = 4)
: size(buf_size),is_(is),buffer(std :: max(buf_size,1))
{
char * end =& buffer.front()+ buffer.size
setg(end,end,end);
}
int_type underflow()
{
if(gptr()< egptr())
return traits_type :: to_int_type(* gptr ));
if(is_)
{
std :: copy_n(std :: istreambuf_iterator< char>(is_),
size,std :: back_inserter );
char * beg =& buffer.front();
setg(beg,beg,beg + buffer.size());
return traits_type :: to_int_type(* gptr());
}
return traits_type :: eof();
}
private:
int size;
std :: istream& is_;
std :: vector< char>缓冲;
};
int main()
{
std :: istringstream oss(Hello,World);
io_buffer buf(oss);
std :: istream is(& buf);
std :: string str;
是>> str;
std :: cout<< str<< std :: endl; //HellHelllo,
}
解决方案
您可以用(可能4)零初始化缓冲区
buffer(std :: max(buf_size,1))
$ b 您可以将其更改为: int_type underflow()
{
if(gptr < egptr())
return traits_type :: to_int_type(* gptr());
if(is_)
{
char * beg = buffer.data();
char * end = std :: copy_n(
std :: istreambuf_iterator< char>(is_),
size,beg);
setg(beg,beg,end);
return traits_type :: to_int_type(* gptr());
}
return traits_type :: eof();
}
这里还有一个错误:std :: copy_n可能要求太多字符。
创建:
int_type underflow $ b {
if(gptr()< egptr())
return traits_type :: to_int_type(* gptr());
if(is_)
{
char * beg = buffer.data();
char * end = beg;
char * limit = beg + buffer.size();
std :: istreambuf_iterator< Char> in(is_);
std :: istreambuf_iterator< Char> eos;
for(; end< limit&& in!= eos; ++ end,++ in)
* end = * in;
setg(beg,beg,end);
if(beg< end)
return traits_type :: to_int_type(* gptr());
}
return traits_type :: eof();
}
并消除会员人数。
I have a very simple stream buffer setup that forwards the bytes from a stream to the buffer. When I print I get a weird output, namely "HellHelllo,"
instead of "Hello"
. Maybe my eyes are tired, but I can't find the problem here. Why am I getting is output?
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <sstream>
class io_buffer : public std::streambuf
{
public:
io_buffer(std::istream& is, int buf_size = 4)
: size(buf_size) , is_(is) , buffer(std::max(buf_size, 1))
{
char* end = &buffer.front() + buffer.size();
setg(end, end, end);
}
int_type underflow()
{
if (gptr() < egptr())
return traits_type::to_int_type(*gptr());
if (is_)
{
std::copy_n(std::istreambuf_iterator<char>(is_),
size, std::back_inserter(buffer));
char* beg = &buffer.front();
setg(beg, beg, beg + buffer.size());
return traits_type::to_int_type(*gptr());
}
return traits_type::eof();
}
private:
int size;
std::istream& is_;
std::vector<char> buffer;
};
int main()
{
std::istringstream oss("Hello, World");
io_buffer buf(oss);
std::istream is(&buf);
std::string str;
is >> str;
std::cout << str << std::endl; // "HellHelllo,"
}
解决方案 You initialize the buffer with (maybe 4) zeros
buffer(std::max(buf_size, 1))
and append the content to the buffer without clearing the buffer.
You may change it to:
int_type underflow()
{
if (gptr() < egptr())
return traits_type::to_int_type(*gptr());
if (is_)
{
char* beg = buffer.data();
char* end = std::copy_n(
std::istreambuf_iterator<char>(is_),
size, beg);
setg(beg, beg, end);
return traits_type::to_int_type(*gptr());
}
return traits_type::eof();
}
You still have a bug here: std::copy_n might ask for too many characters.
Make it:
int_type underflow()
{
if (gptr() < egptr())
return traits_type::to_int_type(*gptr());
if (is_)
{
char* beg = buffer.data();
char* end = beg;
char* limit = beg + buffer.size();
std::istreambuf_iterator<char> in(is_);
std::istreambuf_iterator<char> eos;
for( ; end < limit && in != eos; ++end, ++in)
*end = *in;
setg(beg, beg, end);
if(beg < end)
return traits_type::to_int_type(*gptr());
}
return traits_type::eof();
}
And eliminate the member size.
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