输入输出流类iostream常用函数解析

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一.成员类型

1. ios::fmtflags：

格式标志，常用来设置输出的格式，用于函数flags、setf、unsetf作为其参数或返回类型。

field	member constant	effect when set
independent flags	boolalpha	read/write bool elements as alphabetic strings (true and false).
	showbase	write integral values preceded by their corresponding numeric base prefix.
	showpoint	write floating-point values including always the decimal point.
	showpos	write non-negative numerical values preceded by a plus sign (+).
	skipws	skip leading whitespaces on certain input operations.
	unitbuf	flush output after each inserting operation.
	uppercase	write uppercase letters replacing lowercase letters in certain insertion operations.
numerical base  (basefield)	dec	read/write integral values using decimal base format.
	hex	read/write integral values using hexadecimal base format.
	oct	read/write integral values using octal base format.
float format  (floatfield)	fixed	write floating point values in fixed-point notation.
	scientific	write floating-point values in scientific notation.
adjustment  (adjustfield)	internal	the output is padded to the field width by inserting fill characters at a specified internal point.
	left	the output is padded to the field width appending fill characters at the end.
	right	the output is padded to the field width by inserting fill characters at the beginning.

2. ios_base::iostate

flag value	indicates
eofbit	End-Of-File reached while performing an extracting operation on an input stream.
failbit	The last input operation failed because of an error related to the internal logic of the operation itself.
badbit	Error due to the failure of an input/output operation on the stream buffer.
goodbit	No error. Represents the absence of all the above (the value zero).

3. 其他：sentry、event、event_callback、failure、Init、openmode、seekdir

二.从输入流类istream继承的函数

1. gcount：streamsize gcount() const;

返回从最后次非格式化输入操作中提取的读取的字符个数

  char str[20];
  std::cout << "Please, enter a word: ";
  std::cin.getline(str,20);
  std::cout << std::cin.gcount() << " characters read: " << str << '\n';
输出为：

Please, enter a word: simplify
9 characters read: simplify

2. get：int get();读取一个字符，通常用来排除换行符的影响

istream& get (char& c);读取一个字符到c中

istream& get (char* s, streamsize n);读取一个字符串，直到遇到换行符或者读取了n-1个字符，会自动在末尾添加字符'\0'

istream& get (char* s, streamsize n, char delim);读取一个字符串，直到遇到字符delim或者读取了n-1个字符，会自动在末尾添加字符'\0'

换行符会留在输入队列中

3. getline：istream& getline (char* s, streamsize n );读取一个字符串，直到遇到换行符或者读取了n个字符（换行符也会读取），并将换行符替换为'\0'

istream& getline (char* s, streamsize n, char delim );读取一个字符串，直到遇到字符delim或者读取了n个字符，并将换行符替换为'\0'

3. ignore：istream& ignore (streamsize n = 1, int delim = EOF);

从输入队列中提取字符，并忽略它们，直到提取完前面n个字符或者遇到字符delim为止（delim也提取并忽略）。完成后输入队列中的第一个字符为第n+1个字符或者delim后面个字符

  char first, last;
  std::cout << "Please, enter your first name followed by your surname: ";
  first = std::cin.get();     // get one character
  std::cin.ignore(256,' ');   // ignore until space
  last = std::cin.get();      // get one character
  std::cout << "Your initials are " << first << last << '\n';

输出为：

Please, enter your first name followed by your surname: John Smith
Your initials are JS

4. 其他：peek、read、readsome、putback、unget

5. tellg、seekg

6. sync：int sync();

输入缓冲区同步

三、从输出流中继承的函数

1. put：ostream& put (char c);

输出字符c

2. 其他：write、tellp、seekp、flush

四、保护成员函数（C++11）

=操作、swap

五、从ios类继承来的函数

1. ios::good：bool good() const;

如果错误状态标识（eofbit、failbit、badbit）被设置了返回false，如果没有返回ture

2. ios::eof：bool eof() const;

如果eofbit被设置了返回ture,说明到达了输入末尾了

3. fail：bool fail() const;

如果failbit或者badbit被设置了返回true

4. bad：bool bad() const;

如果badbit被设置了返回true

iostate value (member constants)	indicates	functions to check state flags
		good()	eof()	fail()	bad()	rdstate()
goodbit	No errors (zero value iostate)	true	false	false	false	goodbit
eofbit	End-of-File reached on input operation	false	true	false	false	eofbit
failbit	Logical error on i/o operation	false	false	true	false	failbit
badbit	Read/writing error on i/o operation	false	false	true	true	badbit

5. ios::!操作：

如果failbit或者badbit被设置了返回true

6. ios::clear：void clear (iostate state = goodbit);

为错误状态标识设置新的值

7. ios::fill：char fill() const;返回填充字符

char fill (char fillch);用字符fillch进行填充，返回填充字符

填充输出的空闲空间

  char prev;
  std::cout.width (10);
  std::cout << 40 << '\n';
  prev = std::cout.fill ('x');
  std::cout.width (10);
  std::cout << 40 << '\n';
  std::cout.fill(prev);

输出：

40
xxxxxxxx40

8. 其他：rdstate、setstate、copyfmt、exceptions、imbue、tie、rdbuf、narrow、widen

六、从ios_base类中继承的函数

1. flags：fmtflags flags() const;返回当前格式

fmtflags flags (fmtflags fmtfl);设置新的格式，返回之前的格式

  std::cout.flags ( std::ios::right | std::ios::hex | std::ios::showbase );
  std::cout.width (10);
  std::cout << 100 << '\n';

输出：      0x64

2. setf：fmtflags setf (fmtflags fmtfl);

fmtflags setf (fmtflags fmtfl, fmtflags mask);

设置格式，返回设置之前的格式

  std::cout.setf ( std::ios::hex, std::ios::basefield );  // set hex as the basefield
  std::cout.setf ( std::ios::showbase );                  // activate showbase
  std::cout << 100 << '\n';
  std::cout.unsetf ( std::ios::showbase );                // deactivate showbase
  std::cout << 100 << '\n';

输出：

0x64
64

3. unset：void unsetf (fmtflags mask);

清空mask选择的格式

4. precision：streamsize precision() const;返回当前的浮点精度（即小数点位数）

streamsize precision (streamsize prec);设置浮点精度，返回之前的浮点精度

5. width：streamsize width() const;返回当前域宽度

streamsize width (streamsize wide);设置域宽度，返回之前的域宽度

6. 其他：imbue、getloc、xalloc、iword、pword、register_callback、sync_with_stdio

参考：http://www.cplusplus.com/reference/istream/iostream/