c++输入文件流ifstream用法详解开发者社区

c++输入文件流ifstream用法详解

ios_base <- ios <- istream <- ifstream

default (1)	ifstream();
initialization (2)	
explicit ifstream (const char* filename, ios_base::openmode mode = ios_base::in);
explicit ifstream (const string& filename, ios_base::openmode mode = ios_base::in);

void open (const   char* filename,  ios_base::openmode mode = ios_base::in);
void open (const string& filename,  ios_base::openmode mode = ios_base::in);

member constant	stands for	access
in	input File	读的方式打开文件
out	output	写的方式打开文件
binary	binary	二进制方式打开
ate	at end	打开的时候定位到文件末尾
app	append	所有操作都定位到文件末尾
trunc	truncate	丢弃打开前文件存在的内容

bool is_open() const;

void close();   //关闭文件流

filebuf* rdbuf() const;

copy(1)  ifstream& operator= (const ifstream&) = delete;
move(2)  ifstream& operator= (ifstream&& rhs);

arithmetic types (1)	
istream& operator>> (bool& val);
istream& operator>> (short& val);
istream& operator>> (unsigned short& val);
istream& operator>> (int& val);
istream& operator>> (unsigned int& val);
istream& operator>> (long& val);
istream& operator>> (unsigned long& val);
istream& operator>> (long long& val);
istream& operator>> (unsigned long long& val);
istream& operator>> (float& val);
istream& operator>> (double& val);
istream& operator>> (long double& val);
istream& operator>> (void*& val);
stream buffers (2)	
istream& operator>> (streambuf* sb );
manipulators (3)	
istream& operator>> (istream& (*pf)(istream&));
istream& operator>> (ios& (*pf)(ios&));
istream& operator>> (ios_base& (*pf)(ios_base&));

streamsize gcount() const;

single character (1): //读取一个字符，遇到'\n',也从流中取出。
int get(); //字符按 int 返回
istream& get (char& c); // 读到c中
//读取n个 c 风格字符串到数组s中，遇到'\n'（或delim）停止读取，并把'\n'留在输入流中，若要读取多行，就要需要int get() 来取出'\n',才能读下一行。
c-string (2): 
istream& get (char* s, streamsize n);//默认delim是换行字符'\n'
istream& get (char* s, streamsize n, char delim) //指定读取停止字符 delim
stream buffer (3):  //内容读取到 streambuf 对象中。
istream& get (streambuf& sb);//默认delim是换行字符'\n'
istream& get (streambuf& sb, char delim);//指定读取停止字符 delim

#include <iostream>     // std::cout, std::streambuf, std::streamsize
#include <fstream>      // std::ifstream
using namespace std;
int main () {
    std::ifstream ifs ("test.txt");
    std::ofstream ofs ("out.txt");
    std::streambuf *pbuf = ofs.rdbuf();
    ifs.get(*pbuf);//默认读取截止字符是'\n', 所以读取一行停止，且没有读取'\n'。
    pbuf->sputc(ifs.get()); // '\n'并没有被读取到pbuf，所以需要get()来读取'\n',然后用函数sputc()加到 pbuf 中。
    ifs.get(*pbuf);  // 从流中取出了'\n' ，才能读取第二行
    pbuf->sputc(ifs.get());
    上面使用了函数 istream& get (streambuf& sb); 
    之后不能使用 istream& get (char* s, streamsize n);
    char s[20];       
    ifs.get(s,20);//虽然输入流有第三行，但是没法读取。
    cout<<"get:"<<s<<endl;  //内容为空
    ofs.close();
    ifs.close();
    return 0;
}

istream& getline (char* s, streamsize n );
//默认delim是换行字符'\n'，遇到后丢弃，第二次读取从delim后开始读。
istream& getline (char* s, streamsize n, char delim );
//自己定义停止符delim

(1)	用户定义截止字符
istream& getline (istream&  is, string& str, char delim);
istream& getline (istream&& is, string& str, char delim); //c++11 标准
(2)	截止字符默认'\n'
istream& getline (istream&  is, string& str);
istream& getline (istream&& is, string& str); // c++11 标准

#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main()
    string str;
    ifstream ifs("test.txt");
    if(!ifs){
        cout<<"open file fail!"<<endl;
        return 1;
    while( getline(ifs,str))
        cout<<str<<endl;
    return 0;
}

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

int peek();

istream& read (char* s, streamsize n);

// read a file into memory
#include <fstream>  // std::ifstream
#include <iostream> // std::cout
#define LEN 10
int main() {
  char buffer[LEN];
  buffer[LEN - 1] = '\0';
  std::ifstream is("test.txt", std::ifstream::binary);
  if (is) {
    while (is) {
      is.read(buffer, LEN - 1);
      //最后一次读取长度小于 LEN-1时候，会打印一些无效的字符
      std::cout << "Reading " << is.gcount() << " characters. is:" << buffer
                << std::endl;
    is.close();
  return 0;
}

istream& putback (char c);
// 用法，从输入流读取一个字符，再把它返回。
char c = std::cin.get(); 
std::cin.putback (c);

istream& unget();
// 返回最后一次读取的字符到输入流，类似putback()
char c = std::cin.get();
std::cin.unget();

streampos tellg();
// get length of file:
is.seekg (0, is.end);
int length = is.tellg();
is.seekg (0, is.beg);

(1)istream& seekg (streampos pos);
(2)istream& seekg (streamoff off, ios_base::seekdir way);

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

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

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

bool operator!() const;
//Returns true if either failbit or badbit is set, and false otherwise.
// 有错误状态返回 true
int main () {
  std::ifstream is;
  is.open ("test.txt");
  if (!is)
    std::cerr << "Error opening 'test.txt'\n";
  return 0;
}

//按行读文件，简洁的模板
#include<iostream>
#include<fstream>
#include<string>
using namespace std;
void print_state (const std::ios& stream) 
    cout << "good()=" << stream.good();
    cout << " eof()=" << stream.eof();
    cout << " fail()=" << stream.fail();
    cout << " bad()=" << stream.bad()<<endl;
int main()
    string str;
    ifstream ifs("test.txt");
	if(ifs)
    	//while( bool(getline(ifs,str)))// 等价
    	//while( getline(ifs,str).good())//等价
    	while( getline(ifs,str))
       		cout<<"line:"<<str<<endl;
	else{
		cout<<"open file fail!"<<endl;
		return 1;
    print_state(ifs);
    return 0;
}

iostate rdstate() const;
// Returns the current internal error state flags of the stream.
// 返回当前流中的内部错误状态，iostate二进制数，需要做位运算来获取其相应位置上的值。
//这个函数的功能可以被 good()，eof()，fail()，bad() 替换。
int main () {

c++输入文件流ifstream用法详解

c++输入文件流ifstream用法详解

输入流的继承关系：

成员函数

Public member functions

1， (constructor)

2，ifstream::open

3，ifstream:: is_open

4，ifstream:: close

5，ifstream:: rdbuf

6，ifstream:: operator =

Public member functions inherited from istream

7，std::istream::operator>>

8，istream::gcount

9，istream::get

10，istream::getline

11，istream::ignore

12，istream::peek

13，istream::read

14，istream::putback

15，istream::unget

16，istream::tellg

17，istream::seekg

Public member functions inherited from ios

18，ios::good

19，ios::operator!

20，ios::operator bool

21，ios::rdstate