ofstream 文件流用法详解c++

头文件 <fstream>包含的多个文件流类，这里列出常用的4个：

• ifstream Input file stream class (class )链接
• ofstream Output file stream (class )链接
• fstream Input/output file stream class (class )链接
• filebuf File stream buffer (class )链接

一. 输入流 ofstream 用法

输入流的继承关系：

ios_base <- ios <- ostream <- ofstream

Public member functions (1-6)

1， (constructor)

default (1) ofstream(); // 只定义不关联

initialization (2)  //关联文件filename，默认模式 ios_base::out
explicit ofstream (const char* filename, ios_base::openmode mode = ios_base::out);
explicit ofstream (const string& filename, ios_base::openmode mode = ios_base::out);

copy (3)    //禁止拷贝赋值
ofstream (const ofstream&) = delete;

move (4)    //可以右值引用临时变量赋值
ofstream (ofstream&& x);

2， ofstream::open

//和第二种构造函数一样，绑定文件
void open (const   char* filename,  ios_base::openmode mode = ios_base::out);
void open (const string& filename,  ios_base::openmode mode = ios_base::out);

3， ofstream::is_open

bool is_open() const;
// 文件打开返回 true ，否则 false

4， ofstream::close

void close();
// Closes the file currently associated with the object, disassociating it from the stream.

5， ofstream::rdbuf

filebuf* rdbuf() const;
// 返回一个指针，指向 filebuf 对象，filebuf 要么通过open()和文件绑定，要么通过rdbuf（）与fstream对象绑定，绑定后才能使用。

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

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

  char c = inbuf->sbumpc();
  while (c != EOF)
  {
    outbuf->sputc (c);
    c = inbuf->sbumpc();
  }
  ofs.close();
  ifs.close();
  return 0;
}

6，ofstream::operator=

copy (1) //禁止copy赋值
ofstream& operator= (const ofstream&) = delete;
move (2） // 可以右值引用创建对象。
ofstream& operator= (ofstream&& rhs);

Public member functions inherited from ostream (7-11)

7，std::ostream::operator<<
用法和 cout<< 一样，是写数据到文件最方便的函数，重载了常用的数据类型。

 arithmetic types (1)   
ostream& operator<< (bool val);
ostream& operator<< (short val);
ostream& operator<< (unsigned short val);
ostream& operator<< (int val);
ostream& operator<< (unsigned int val);
ostream& operator<< (long val);
ostream& operator<< (unsigned long val);
ostream& operator<< (long long val);
ostream& operator<< (unsigned long long val);
ostream& operator<< (float val);
ostream& operator<< (double val);
ostream& operator<< (long double val);
ostream& operator<< (void* val);

stream buffers (2)  
ostream& operator<< (streambuf* sb );

manipulators (3)    
ostream& operator<< (ostream& (*pf)(ostream&));
ostream& operator<< (ios& (*pf)(ios&));
ostream& operator<< (ios_base& (*pf)(ios_base&));

用string数据类型，作为参数写入到ofstream，在头文件<string>
中对输出运算符重载。
std::operator<< (string)

ostream& operator<< (ostream& os, const string& str);

8，ostream::put

ostream& put (char c);
//插入字符 c 到流中

9，ostream::write

ostream& write (const char* s, streamsize n);
//从数组s中取n 个字符插入到流中

10，ostream::tellp

streampos tellp();
返回文件指针的位置， streampos 可以转为int

11，ostream::seekp

(1) 
ostream& seekp (streampos pos);
(2) 
ostream& seekp (streamoff off, ios_base::seekdir way);

//Sets the position where the next character is to be inserted into the output stream.

参数 pos 是流中的绝对位置可以转化为 int
参数 off 是偏移量，与way相关，类型是 int
参数 way 可以选下表中的任意一个常量。

value	offset is relative to…
ios_base::beg	beginning of the stream
ios_base::cur	current position in the stream
ios_base::end	end of the stream

Public member functions inherited from ios(12-14)

12，ios::good

bool good() const;
bool eof() const;
bool fail() const;
bool bad() const;

检测流的状态是否正常。当错误的状态flags (eofbit, failbit and badbit) 都没被设置的时候返回true
特定的错误状态可以用下面的函数（eof, fail, and bad）来检测。

iostate value (member constant)	indicates	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

13，ios::operator!

bool operator!() const;
//Returns true if either failbit or badbit is set, and false otherwise.
// 有错误状态返回 true

// evaluating a stream (not)
#include <iostream>     // std::cout
#include <fstream>      // std::ifstream

int main () {
  std::ifstream is;
  is.open ("test.txt");
  if (!is)
    std::cerr << "Error opening 'test.txt'\n";
  return 0;
}

14，ios::operator bool

explicit operator bool() const;
C++11: Return true if none of failbit or badbit is set. false otherwise.
// 在条件语句中，无错误返回真，有错返回假。
int main () {
  std::ifstream is;
  is.open ("test.txt");
  if (is) {
    // read file
  }
  else {
    std::cerr << "Error opening 'test.txt'\n";
  }
  return 0;
}