C++中的文件file和流缓冲streambuf操作

1.引入头文件fstream
#include
fstream头文件定义了用于文件输入的类ifstream和文件输出的类ofstream
参考文档 http://www.cplusplus.com/reference/fstream/
ofstream: 写操作（输出）的文件类 (由ostream引申而来) 
ifstream: 读操作（输入）的文件类(由istream引申而来) 
fstream: 可同时读写操作的文件类 (由iostream引申而来)

stream主要类之间的集成关系
ios_base <|-- ios <|-- istream <|-- ifstream
ios_base <|-- ios <|-- ostream <|-- ofstream
ios_base <|-- ios <|-- istream <|-- iostream <|-- fstream
                  <|-- ostream

ios_base <|-- ios <|-- istream <|-- istringstream
ios_base <|-- ios <|-- ostream <|-- ostringstream
ios_base <|-- ios <|-- istream <|-- iostream <|-- stringstream
                  <|-- ostream

std::ifstream::ifstream
输入流ifstream对象构造
       default (1)    ifstream(); // 然后再用open()方法
initialization (2)    explicit ifstream (const char* filename, ios_base::openmode mode = ios_base::in);

std::ofstream::ofstream
输出流ofstream对象构造
       default (1)    ofstream(); // 然后再用open()方法
initialization (2)    explicit ofstream (const char* filename, ios_base::openmode mode = ios_base::out);

std::istringstream::istringstream
输入流istringstream对象构造
       default (1)    explicit istringstream (ios_base::openmode which = ios_base::in); // 然后再用str()方法
initialization (2)    explicit istringstream (const string& str, ios_base::openmode which = ios_base::in);

std::ostringstream::ostringstream
输出流ostringstream对象构造
       default (1)    explicit ostringstream (ios_base::openmode which = ios_base::out); // 然后再用str()方法
initialization (2)    explicit ostringstream (const string& str, ios_base::openmode which = ios_base::out);

重要方法原型
void open (const char* filename,  ios_base::openmode mode = ios_base::out); //C++98 C++11
void open (const string& filename,  ios_base::openmode mode = ios_base::out); // C++11
    Return Value: none
    If the function fails to open a file, the failbit state flag is set for the stream (which
    may throw ios_base::failure if that state flag was registered using member exceptions).

istream& read (char* s, streamsize n); //Extracts n characters from the stream and stores them in the array pointed to by s.
ostream& write (const char* s, streamsize n); //Inserts the first n characters of the array pointed by s into the stream.

std::istream::getline
Extracts characters from the stream as unformatted input and stores them into s as a c-string(public member function)
istream& getline (char* s, streamsize n );
istream& getline (char* s, streamsize n, char delim );
    Extracts characters from the stream as unformatted input and stores them into s as a c-string,
    until either the extracted character is the delimiting character, or n characters have been
    written to s (including the terminating null character).

std::getline
Get line from stream into string (function)
(1)    istream& getline (istream& is, string& str, char delim);
(2)    istream& getline (istream& is, string& str);
    Extracts characters from is and stores them into str until the delimitation character
    delim is found (or the newline character, '\n', for (2)).
注意getline()函数和getline()方法的不同，特别是输出参数，方法对应char*类型而函数对应string类型；
getline()方法的第一个参数是输出参数是char*类型；所以参数至少需要一个指针一个长度；
getline()函数的第二个参数是输出参数是string&类型；所以参数除了istream之外可以只有一个string参数；

////////////////////////////////////////////////////
std::ifstream
////////////////////////////////////////////////////
Public member functions
(constructor) Construct object (public member function )
open        Open file (public member function )
is_open        Check if file is open (public member function )
close        Close file (public member function )
rdbuf        Get stream buffer (public member function )
operator=     Move assignment (public member function )
swap         Swap internals (public member function )

Public member functions inherited from istream
operator>>    Extract formatted input (public member function )
gcount        Get character count (public member function )
get            Get characters (public member function )
getline        Get line (public member function )
ignore        Extract and discard characters (public member function )
peek        Peek next character (public member function )
read        Read block of data (public member function )
readsome    Read data available in buffer (public member function )
putback        Put character back (public member function )
unget        Unget character (public member function )
tellg        Get position in input sequence (public member function )
seekg        Set position in input sequence (public member function )
sync        Synchronize input buffer (public member function )

Public member functions inherited from ios
good        Check whether state of stream is good (public member function )
eof            Check whether eofbit is set (public member function )
fail        Check whether either failbit or badbit is set (public member function )
bad            Check whether badbit is set (public member function )
operator!    Evaluate stream (not) (public member function )
operator bool Evaluate stream (public member function )
rdstate        Get error state flags (public member function )
setstate    Set error state flag (public member function )
clear        Set error state flags (public member function )
copyfmt        Copy formatting information (public member function )
fill        Get/set fill character (public member function )
exceptions    Get/set exceptions mask (public member function )
imbue        Imbue locale (public member function )
tie            Get/set tied stream (public member function )
rdbuf        Get/set stream buffer (public member function )
narrow        Narrow character (public member function )
widen        Widen character (public member function )

Public member functions inherited from ios_base
flags        Get/set format flags (public member function )
setf        Set specific format flags (public member function )
unsetf        Clear specific format flags (public member function )
precision    Get/Set floating-point decimal precision (public member function )
width        Get/set field width (public member function )
imbue        Imbue locale (public member function )
getloc        Get current locale (public member function )
xalloc        Get new index for extensible array [static] (public static member function )
iword        Get integer element of extensible array (public member function )
pword        Get pointer element of extensible array (public member function )
register_callback    Register event callback function (public member function )
sync_with_stdio        Toggle synchronization with cstdio streams [static] (public static member function )

////////////////////////////////////////////////////
std::ofstream
////////////////////////////////////////////////////
Public member functions
(constructor)    Construct object (public member function )
open            Open file (public member function )
is_open            Check if file is open (public member function )
close            Close file (public member function )
rdbuf            Get stream buffer (public member function )
operator=         Move assignment (public member function )
swap             Swap internals (public member function )

Public member functions inherited from ostream
operator<<        Insert formatted output (public member function )
put                Put character (public member function )
write            Write block of data (public member function )
tellp            Get position in output sequence (public member function )
seekp            Set position in output sequence (public member function )
flush            Flush output stream buffer (public member function )

Public member functions inherited from ios
good            Check whether state of stream is good (public member function )
eof                Check whether eofbit is set (public member function )
fail            Check whether either failbit or badbit is set (public member function )
bad                Check whether badbit is set (public member function )
operator!        Evaluate stream (not) (public member function )
operator bool     Evaluate stream (public member function )
rdstate            Get error state flags (public member function )
setstate        Set error state flag (public member function )
clear            Set error state flags (public member function )
copyfmt            Copy formatting information (public member function )
fill            Get/set fill character (public member function )
exceptions        Get/set exceptions mask (public member function )
imbue            Imbue locale (public member function )
tie                Get/set tied stream (public member function )
rdbuf            Get/set stream buffer (public member function )
narrow            Narrow character (public member function )
widen            Widen character (public member function )

Public member functions inherited from ios_base
flags            Get/set format flags (public member function )
setf            Set specific format flags (public member function )
unsetf            Clear specific format flags (public member function )
precision        Get/Set floating-point decimal precision (public member function )
width            Get/set field width (public member function )
imbue            Imbue locale (public member function )
getloc            Get current locale (public member function )
xalloc            Get new index for extensible array [static] (public static member function )
iword            Get integer element of extensible array (public member function )
pword            Get pointer element of extensible array (public member function )
register_callback    Register event callback function (public member function )
sync_with_stdio        Toggle synchronization with cstdio streams [static] (public static member function )

// get size of file
  std::ifstream infile("test.txt", std::ifstream::binary);
  infile.seekg(0, infile.end);
  long size = infile.tellg();
  infile.seekg(0);

2.写文件
1)创建一个ofstream对象来管理输出流
2)将该对象与文件关联起来
3)使用cout方式使用该对象，唯一区别是输出将进入文件，而不是屏幕。
4)关闭到文件的连接
示例如下：
    ofstream fout;
    fout.open("test.txt");
    fout << "write something";
    fout.close();
创建对象和关联文件可使用构造函数合并到一步，以上代码等价于：
    ofstream fout("test.txt");
    fout << "write something";
    fout.close();

另一个例子：
  std::ifstream infile ("test.txt",std::ifstream::binary);
  std::ofstream outfile ("new.txt",std::ofstream::binary);

  // get size of file
  infile.seekg (0,infile.end);
  long size = infile.tellg();
  infile.seekg (0);

  // allocate memory for file content
  char* buffer = new char[size];

  // read content of infile
  infile.read(buffer,size);

  // write to outfile
  outfile.write(buffer,size);

  // release dynamically-allocated memory
  delete[] buffer;

  outfile.close();
  infile.close();
    
3.读文件
1)创建一个ifstream对象来管理输入流
2)将该对象与文件关联起来
3)使用ifstream的方法读文件
4)关闭到文件的连接
示例如下：
    ifstream fin("test.txt");
    char ch;
    fin >> ch;//从文件中读取一个字符
    char buf[80];
    fin >> buf;//从文件中读取一个单词
    /*
     * getline()方法的第一个参数是输出参数是char*类型；所以参数至少需要一个指针一个长度；
     * getline()函数的第二个参数是输出参数是string&类型；所以参数除了istream之外可以只有一个string参数；
     */
    fin.getline(buf, 80);//从文件中读取一行
    string line;
    getline(fin, line);//读取一行转换成字符串
    fin.close();

另一个例子：
  std::ifstream is ("test.txt", std::ifstream::binary);
  if (is) {
    // get length of file:
    is.seekg (0, is.end);
    int length = is.tellg();
    is.seekg (0, is.beg);

    char * buffer = new char[length];

    std::cout << "Reading " << length << " characters... ";
    // read data as a block:
    is.read(buffer, length);

    if (is)
      std::cout << "all characters read successfully.";
    else
      std::cout << "error: only " << is.gcount() << " could be read";
    is.close();

    // ...buffer contains the entire file...

    delete[] buffer;
  }

4.读写缓冲区
fstream流管理对象在使用cout或cin方式读或写时，是通过缓冲区和文件进行交互的，而不是逐字节读取或写入文件的。

关于close()函数，当流对象过期(如程序终止)时，到文件的连接自动关闭，也可使用close()显式关闭该连接。
连接关闭后，将刷新缓冲区。

与文件的连接关闭后，并不会删除流，fstream流对象和它管理的缓冲区仍然存在，可以将流重新连接到文件。

5.一个简单的读写文件示例
#include
#include
#include
using namespace std;
const string filename="test.txt";
int main()
{
    ofstream fout(filename.c_str());
    if(!fout.is_open())
    {
        cerr << "无法打开文件 " << filename << endl;
        exit(0);
    }
    cout << "输入密码：";
    float secret;
    cin >> secret;
    fout << secret << endl;
    fout.close();

    ifstream fin(filename.c_str());
    if(!fin.is_open())
    {
        cerr << "无法打开文件 " << filename << endl;
        exit(0);
    }
    cout << "输入的密码为：\n";
    char ch;
    while(fin.get(ch))
        cout << ch;
    fin.close();
    return 0;
}
程序中is_open()函数，是用来检查文件是否被打开
流对象的状态包括：
    一切顺利，已达到文件尾，I/O操作失败等。如果一切顺利，流状态设置为0，否则设置为1。

6.打开多个文件
1)需要同时打开多个文件，则需要为每个文件创建一个流。
2)需要依次打开一组文件，可以只打开一个流，将它依次关联到各个文件。
如下例所示：
    ifstream fin;
    fin.open("test.txt");
    fin.close();
    fin.clear();
    fin.open("test2.txt");
    fin.close();

7.文件模式
    常量    含义
    ios_base::in    打开文件，以便读取
    ios_base::out    打开文件，以便写入
    ios_base::ate    打开文件，并移到文件尾，和ios_base::app的区别是，后者只能在文件尾写入，前者只是将写入指针初始化在文件尾
    ios_base::app    追加到文件尾
    ios_base::trunc    如果文件存在，则清空文件内容
    ios_base::binary    二进制文件
    对于ifstream open()，默认的模式为ios_base::in
    对于ofstream open()，默认的模式为ios_base::out|ios_base::trunc，打开并清空文件

8.二进制模式 ios_base::binary 

文件的存储类型有两种，文本格式或二进制格式。
文本格式便于读取，而二进制格式更加精确，占用空间小，读取速度快。
写操作：fout.write((char*)&T, sizeof T);
读操作：fin.read((char*)&T, sizeof T);
注意需要将类型实例地址，强制转换为char*类型;

9.随机存取

随机存取是指，读写指针直接移动到文件的任何位置。
跳转函数：
istream & seekg(streamoff,ios_base::seekdir);//相对地址
istream & seekg(streampos);//绝对地址
第一个方法的意思是，离seekdir参数指定文件位置，距离streamoff的位置。
其中，streamoff单位的是字节，seekdir表示文件的三个位置(顶端ios_base::beg，底端ios_base::end，当前位置ios_base::cur)

第二个方法的意思是，离文件开头streampos的位置
streampos表示文件中的绝对位置，单位是字节

seekg() 移到文件开头
tellg() 返回读写指针当前的位置

下面，我们用二进制文件模式，写一个简单的随机存取示例：
#include
#include
#include
const int LIM=20;
struct planet
{
    char name[LIM];
    double population;
    double g;
};
const char *file = "planets.dat";
inline void eatline(){while(std::cin.get()!='\n') continue;}
int main()
{
    using namespace std;
    planet pl;
    cout<

    fstream finout;
    finout.open(file,ios_base::in|ios_base::out|ios_base::binary);
    int ct = 0;
    if(finout.is_open())
    {
        finout.seekg(0);
        cout<<"contents of file "<         while(finout.read((char *)&pl,sizeof pl))
        {
            cout<                 <                 <         }
        if(finout.eof())
            finout.clear();
        else{
            cerr<<"无法打开文件 "<             exit(0);
        }
    }
    else{
        cerr<<"无法打开文件 "<         exit(0);
    }
    cout<<"enter record number to change:";
    long rec;
    cin>>rec;
    eatline();
    if(rec<0||rec>=ct)
    {
        cerr<<"无效 index number"<         exit(0);
    }
    streampos place = rec*sizeof pl;
    finout.seekg(place);
    if(finout.fail())
    {
        cerr<<"无法找到 index number"<         exit(0);
    }
    finout.read((char *)&pl,sizeof pl);
    cout<<"找到的 index number"<     cout<                 <                 <     if(finout.eof())
        finout.clear();
    cout<<"输入name:";
    cin.get(pl.name,LIM);
    eatline();
    cout<<"population:";
    cin>>pl.population;
    cout<<"g:";
    cin>>pl.g;
    finout.seekp(place);
    finout.write((char*)&pl,sizeof pl)<     if(finout.fail())
    {
        cerr<<"写失败 index number"<         exit(0);
    }

    ct = 0;
    finout.seekg(0);
    cout<<"contents of file "<     while(finout.read((char *)&pl,sizeof pl))
    {
        cout<             <             <     }
    finout.close();
    cout<<"Done.\n";
    return 0;
}
程序中，我们使用了一个特别的流管理对象fstream，
fstream继承自iostream,而iostream继承自istream和ostream
因此fstream继承了两个缓冲区，一个用于输入，一个用于输出
并能同步化两个缓冲区的处理。即输入指针和输出指针的位置始终相同。
这样用一个流管理对象，就可以同时进行读和写。

/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
////////////////////////经典示例/////////////////////////////
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
stringstream 例子
std::stringbuf::str
    string str() const;                // get
    void str(const string& str);    // set

// istringstream::str // Assignment from str
#include       // std::string
#include     // std::cout
#include      // std::istringstream

int main () {
  std::istringstream iss;
  std::string strvalues = "32 240 2 1450";

  iss.str(strvalues); // Assignment from strstrvalues

  for (int n=0; n<4; n++)
  {
    int val;
    iss >> val;
    std::cout << val << '\n';
  }
  std::cout << "Finished writing the numbers in: ";
  std::cout << iss.str() << '\n';
  return 0;
}

// ostringstream::str // return a string
#include       // std::string
#include     // std::cout
#include      // std::ostringstream

int main () {
  std::ostringstream oss;
  oss << "One hundred and one: " << 101;
  std::string s = oss.str(); // return a string
  std::cout << s << '\n';
  return 0;
}

10. std::streambuf 流缓冲
streambuf <|-- filebuf 文件缓冲
streambuf <|-- stringbuf 字符串缓冲

// std::ifstream::rdbuf
filebuf* rdbuf() const;
Get stream buffer, Returns a pointer to the internal filebuf object.
返回指向内部filebuf对象的指针

// read file data using associated buffer's members
#include     // std::cout
#include      // std::filebuf, std::ifstream

int main () {
  std::ifstream ifs ("test.txt", std::ifstream::binary);
  std::filebuf *pbuf = ifs.rdbuf(); // get pointer to associated buffer object

  // get file size using buffer's members
  std::size_t size = pbuf->pubseekoff(0, ifs.end, ifs.in);
  pbuf->pubseekpos(0, ifs.in);

  // allocate memory to contain file data
  char *buffer = new char[size];

  pbuf->sgetn(buffer, size); // get file data: visit filebuf and copy to buffer

  ifs.close(); //close filebuf

  // write content to stdout
  std::cout.write(buffer, size);
  delete[] buffer;

  return 0;
}

接口解读
std::streambuf::pubseekoff
streampos pubseekoff(streamoff off, ios_base::seekdir way,
                      ios_base::openmode which = ios_base::in | ios_base::out);
Set internal position pointer to relative position
param way is Object of type ios_base::seekdir. It may take any of the following constant values: 
value    offset is relative to...
ios_base::beg    beginning of the stream buffer
ios_base::cur    current position in the stream buffer
ios_base::end    end of the stream buffer
param which is Generally used to determine the position on which of the controlled sequences shall be modified: the input sequence, the output sequence, or both.
It is an object of type ios_base::openmode that, for this function, may take any combination of the following significant constant values:
value    position pointer affected
ios_base::in    Modify current position in controlled input sequence
ios_base::out    Modify current position in controlled output sequence

Return The new position value of the modified position pointer.

std::streambuf::pubseekpos
streampos pubseekpos(streampos pos, ios_base::openmode which = ios_base::in | ios_base::out);
Set internal position pointer to absolute position
Return The new position value of the modified position pointer.

// std::ofstream::rdbuf
filebuf* rdbuf() const;
Get stream buffer, Returns a pointer to the internal filebuf object.
// copy a file using file stream buffers
#include      // std::filebuf, std::ifstream, std::ofstream
#include       // EOF

int main () {
  std::ifstream ifs ("test.txt");
  std::ofstream ofs ("copy.txt");

  std::filebuf *inbuf  = ifs.rdbuf();
  std::filebuf *outbuf = ofs.rdbuf();

  /*
   * int sbumpc();          Get current character and advance to next position
   * int sputc (char c);    Store character at current put position and increase put pointer
   */
  char c = inbuf->sbumpc();
  while (c != EOF)
  {
    outbuf->sputc(c);
    c = inbuf->sbumpc();
  }

  ofs.close();
  ifs.close();

  return 0;
}

// std::istringstream::rdbuf
stringbuf* rdbuf() const;
Get stream buffer, Returns a pointer to the internal stringbuf object, with which the object was associated on construction.
返回指向内部stringbuf对象的指针，该对象在构造时与之关联。
// istringstream::rdbuf
#include       // std::string
#include     // std::cout
#include      // std::istringstream, std::stringbuf

int main () {
  std::istringstream iss;
  std::stringbuf *pbuf = iss.rdbuf();

  // using stringbuf directly:
  pbuf->str("Example string");

  int size = pbuf->in_avail();

  while(pbuf->in_avail()>0)
    std::cout << static_cast(pbuf->sbumpc());

  return 0;
}

//std::ostringstream::rdbuf
stringbuf* rdbuf() const;
Get stream buffer
Returns a pointer to the internal stringbuf object, with which the object was associated on construction.
返回指向内部stringbuf对象的指针，该对象在构造时与之关联。
// ostringstream::rdbuf
#include       // std::string
#include     // std::cout
#include      // std::ostringstream, std::stringbuf

int main () {
  std::ostringstream oss;
  std::stringbuf *pbuf = oss.rdbuf();

  // using stringbuf directly
  pbuf->sputn("Sample string", 13);
  std::cout << pbuf->str();

  return 0;
}

使用的例子如下
/*****************************************
 * File name   : streambuf.cpp
 *******************************************/
#include
#include

#include
#include

using namespace std;

void ostringstreambuf()
{
   cout << __func__ << "#################" << endl;
   std::ostringstream oss;
   std::stringbuf *pbuf = oss.rdbuf();

   // using stringbuf directly
   pbuf->sputn("Sample string", 13);
   std::cout << "pbuf: " << pbuf->str();
   std::cout << endl;
   std::cout << "oss: " << oss.str();
   std::cout << endl;
   cout << __func__ << "#################" << endl;
}

void istringstreambuf()
{
   cout << __func__ << "#################" << endl;
   std::istringstream iss;
   std::stringbuf *pbuf = iss.rdbuf();

   // using stringbuf directly:
   pbuf->str("Example string");

   int size = pbuf->in_avail();

   while (pbuf->in_avail()>0)
      std::cout << static_cast(pbuf->sbumpc());
   std::cout << endl;
   std::cout << "pbuf: " << pbuf->str();
   std::cout << endl;
   std::cout << "iss: " << iss.str();
   std::cout << endl;
   cout << __func__ << "#################" << endl;
}

void ifilebuf()
{
   cout << __func__ << "#################" << endl;
   std::ifstream ifs("config.txt", std::ifstream::binary);
   std::filebuf *pbuf = ifs.rdbuf(); // get pointer to associated buffer object

   // get file size using buffer's members
   std::size_t size = pbuf->pubseekoff(0, ifs.end, ifs.in);
   pbuf->pubseekpos(0, ifs.in);

   // allocate memory to contain file data
   char *buffer = new char[size];

   pbuf->sgetn(buffer, size); // get file data: visit filebuf and copy to buffer
   ifs.close(); //close filebuf

   // write content to stdout
   std::cout.write(buffer, size);
   delete[] buffer; 
   cout << __func__ << "#################" << endl;
}

void ofilebuf()
{
   cout << __func__ << "#################" << endl;
   std::ifstream ifs ("config.txt");
   std::ofstream ofs ("copy.txt");

   std::filebuf *inbuf  = ifs.rdbuf();
   std::filebuf *outbuf = ofs.rdbuf();

   /*
    * int sbumpc();       Get current character and advance to next position
    * int sputc (char c); Store character at current put position and increase put pointer
    */

   char c = inbuf->sbumpc();
   while (c != EOF)
   {
      cout << "write to outbuf: " << c << endl;
      outbuf->sputc(c);
      c = inbuf->sbumpc();
   }

   ofs.close();
   ifs.close();
   cout << __func__ << "#################" << endl;
}

int main() {
   ostringstreambuf();
   istringstreambuf();
   ifilebuf();
   ofilebuf();
   return 0;
}

std::filebuf
Public member functions
(constructor)    Construct object (public member function )
(destructor)    Destruct object (public member function )
operator=         Move-assignment (public member function )
swap             Swap file buffers (public member function )

File association
open        Open file (public member function )
is_open        Check if a file is open (public member function )
close        Close file (public member function )

Public member functions inherited from streambuf
Locales:
pubimbue    Imbue locale (public member function )
getloc        Get current locale (public member function )
Buffer management and positioning:
pubsetbuf    Set buffer array (public member function )
pubseekoff    Set internal position pointer to relative position (public member function )
pubseekpos    Set internal position pointer to absolute position (public member function )
pubsync        Synchronize stream buffer (public member function )
Input functions (get):
in_avail    Get number of characters available to read (public member function )
snextc        Advance to next position and get character (public member function )
sbumpc        Get current character and advance to next position (public member function )
sgetc        Get current character (public member function )
sgetn        Get sequence of characters (public member function )
sputbackc    Put character back (public member function )
sungetc        Decrease current position (public member function )
Output functions (put):
sputc        Store character at current put position and increase put pointer (public member function )
sputn        Put sequence of characters (public member function )

// 构造
std::filebuf::filebuf
filebuf();
Constructs a filebuf object

std::stringbuf
Public member functions
(constructor)        Construct a string stream buffer object (public member function )
str                    Get/set the string content (public member function )

Public member functions inherited from streambuf
Locales:
pubimbue        Imbue locale (public member function )
getloc            Get current locale (public member function )
Buffer management and positioning:
pubsetbuf        Set buffer array (public member function )
pubseekoff        Set internal position pointer to relative position (public member function )
pubseekpos        Set internal position pointer to absolute position (public member function )
pubsync            Synchronize stream buffer (public member function )
Input functions (get):
in_avail        Get number of characters available to read (public member function )
snextc            Advance to next position and get character (public member function )
sbumpc            Get current character and advance to next position (public member function )
sgetc            Get current character (public member function )
sgetn            Get sequence of characters (public member function )
sputbackc        Put character back (public member function )
sungetc            Decrease current position (public member function )
Output functions (put):
sputc            Store character at current put position and increase put pointer (public member function )
sputn            Put sequence of characters (public member function )

// 构造
std::stringbuf::stringbuf
       default (1)    explicit stringbuf (ios_base::openmode which = ios_base::in | ios_base::out);
initialization (2)    explicit stringbuf (const string& str, ios_base::openmode which = ios_base::in | ios_base::out);
Construct a string stream buffer object

// stringbuf example
#include     // std::cout, std::ostream, std::hex
#include      // std::stringbuf
#include       // std::string

int main ()
{
  std::stringbuf buffer;      // empty stringbuf
  std::ostream os (&buffer);  // associate stream buffer to stream

  // mixing output to buffer with inserting to associated stream:
  buffer.sputn ("255 in hexadecimal: ",20);
  os << std::hex << 255;

  std::cout << buffer.str();

  return 0;
}

考虑一个生产者，消费者的问题，线程A 生成的数据，线程B读取，可以解决的方案如下：
1. 设立全局变量，缓冲数据，A,B都可以访问（在这种情况下，A 生产的时候要考虑缓冲区是否够用，B读取的时候要判断当前是否有有效数据可读，而且很难设计一个合理分配内存的缓冲区（想象A生产的数据有时很大，有时很小））

2. 网络通信（TCP,UDP）

3. 有了streambuf配合stream，A就像正常操作流一样往流对象里塞数据，而B就像正常操作流一样从流里面读数据，不用关心其他问题，
只要这两个流的sterambuf 是同一个对象

#include
#include
#include
#include
#include
#include
#include
#include
using namespace std;

stringbuf buf;
istream in(&buf);
ostream out(&buf);
bool flag = false;

void threadb() {
    char data;
    while (true) {
        if (flag) {
            // 缓冲buf既是输出缓冲又对应输入缓冲，从缓冲来的数据给data
            in >> data;
            cout << "thread B recv:" << data << endl;
            flag = false;
        }
    }
}
int main() {
    thread consumer(threadb);
    char data;
    while (true) {
        // 从标准输入赋值给缓冲buf
        cin >> data;
        out << data;
        flag = true;
    }
    return 0;
}

11. 总结
输出流对象的主要方法：<< put write
输出流对象的主要方法：>> get read getline peek putback
文件流对象和string流对象都可以使用缓冲（文件是filebuf，string流对象对应stringbuf）,使用rdbuf()方法返回内部的buf对象指针并进行操作，内容会自动同步，派生类可以调用pubsync()方法进行同步（如果有输出缓冲没有同步）；
流缓冲可以使用的主要方法：
    位置管理：pubsetbuf pubseekoff pubseekpos
    输入方法：in_avail sbumpc
    输出方法：sputc sputn