1.引入头文件fstream
#include <fstream>
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<iostream>
#include<fstream>
#include<string>
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<iostream>
#include<fstream>
#include<iomanip>
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<<fixed;
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 "<<file<<endl;
while(finout.read((char *)&pl,sizeof pl))
{
cout<<ct++<<":"<<setw(20)<<pl.name<<":"
<<setprecision(0)<<setw(12)<<pl.population
<<setprecision(2)<<setw(6)<<pl.g<<endl;
}
if(finout.eof())
finout.clear();
else{
cerr<<"无法打开文件 "<<file<<endl;
exit(0);
}
}
else{
cerr<<"无法打开文件 "<<file<<endl;
exit(0);
}
cout<<"enter record number to change:";
long rec;
cin>>rec;
eatline();
if(rec<0||rec>=ct)
{
cerr<<"无效 index number"<<endl;
exit(0);
}
streampos place = rec*sizeof pl;
finout.seekg(place);
if(finout.fail())
{
cerr<<"无法找到 index number"<<endl;
exit(0);
}
finout.read((char *)&pl,sizeof pl);
cout<<"找到的 index number"<<endl;
cout<<rec<<":"<<setw(20)<<pl.name<<":"
<<setprecision(0)<<setw(12)<<pl.population
<<setprecision(2)<<setw(6)<<pl.g<<endl;
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)<<flush;
if(finout.fail())
{
cerr<<"写失败 index number"<<endl;
exit(0);
}
ct = 0;
finout.seekg(0);
cout<<"contents of file "<<file<<endl;
while(finout.read((char *)&pl,sizeof pl))
{
cout<<ct++<<":"<<setw(20)<<pl.name<<":"
<<setprecision(0)<<setw(12)<<pl.population
<<setprecision(2)<<setw(6)<<pl.g<<endl;
}
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 <string> // std::string
#include <iostream> // std::cout
#include <sstream> // 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 <string> // std::string
#include <iostream> // std::cout
#include <sstream> // 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 <iostream> // std::cout
#include <fstream> // 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 <fstream> // std::filebuf, std::ifstream, std::ofstream
#include <cstdio> // 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 <string> // std::string
#include <iostream> // std::cout
#include <sstream> // 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<char>(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 <string> // std::string
#include <iostream> // std::cout
#include <sstream> // 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 <iostream>
#include <string>
#include <fstream>
#include <sstream>
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<char>(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 <iostream> // std::cout, std::ostream, std::hex
#include <sstream> // std::stringbuf
#include <string> // 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 <iostream>
#include <streambuf>
#include <sstream>
#include <fstream>
#include <string>
#include <cstring>
#include <memory>
#include <thread>
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