fstream

C++11 C++ STL

---

ifstream

typedef basic_ifstream<char> ifstream;

ifstream::ifstream

ifstream();
explicit ifstream (const char* filename, ios_base::openmode mode = ios_base::in);
explicit ifstream (const string& filename, ios_base::openmode mode = ios_base::in);
ifstream (const ifstream&) = delete;
ifstream (ifstream&& x);

ifstream::open

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

#include <iostream>     // std::cout
#include <fstream>      // std::ifstream
int main () {
  std::ifstream ifs;
  ifs.open ("test.txt", std::ifstream::in);
  //std::ifstream ifs("test.txt", std::ifstream::in) 等价于上面两句
  char c = ifs.get();
  while (ifs.good()) {
    std::cout << c;
    c = ifs.get();
  }
  ifs.close();
  return 0;
}

ifstream::is_open

bool is_open() const;

Check if a file is open.Returns whether the stream is currently associated to a file.

ifstream::operator=

ifstream& operator= (const ifstream&) = delete;
ifstream& operator= (ifstream&& rhs);

ifstream::operator >>

以空格回车符为分隔符读取数据,

#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
    ifstream ifs("a.txt");
    int n;
    while(ifs>>n) {
        cout<<n<<endl;
    }
    ifs.close();
    return 0;
}
//当ifs>>n左右数据类型不匹配时会跳出循环，如
//a.txt如果是
//1 2 a b c
//输出2之后就退出循环结束了。
//右侧是stringstream则是另一种情况

　　ifstream 流 判断文件是否结尾的函数eof()，我个人理解就是，如ifs>>n，当读入文件最后一个数值，文档的eof，good标志依然是正常的，直到流发现没有下一个数值时才设置eof、good标志。这时候流还是保存上次的值，所以最后可能会读入两次最后的值。如下代码就是读入了两次。

ifstream ifs("a.txt");
int n;
while(ifs.good()) { //while(ifs.eof())
    ifs>>n
    cout<<n<<endl;
}
ifs.close();

可以如下改写

ifstream ifs("a.txt");
int n;
ifs>>n
while(ifs.good()) { //while(ifs.eof())
    cout<<n<<endl;
    ifs>>n
}
ifs.close();

如此现读再判断就正常了。个人还是倾向于上面while(ifs>>n)的写法。while判断语句的真实判断对象是ifs，也就是判断当前是否存在有效的输入流。譬如读完最后一个数据后再读时，while(ifs>>n)就会判断错误，因为ifs>>n无法读取数据，是一个无效流，在operator>> API 中清楚看到返回值是流的引用，所以使用while(ifs>>n)判断是否结束读取简单易用。同理使用string中的getline时也可以使用while(getline(ifs,str))很容易就可以判断了。

ifstream::getline

Public member functions inherited from istream

istream& getline (char* s, streamsize n );
istream& getline (char* s, streamsize n, char delim );

getline

getline function in string

istream& getline (istream&  is, string& str, char delim);
istream& getline (istream&& is, string& str, char delim);
istream& getline (istream&  is, string& str);
istream& getline (istream&& is, string& str);

example

#include <iostream>
#include <string> //std::getline
main ()
{
  std::string name;
  std::cout << "Please, enter your full name: ";
  std::getline (std::cin,name);
  std::cout << "Hello, " << name << "!\n";
  return 0;
}

ofstream

ofstream::ofstream

ofstream();
explicit ofstream (const char* filename, ios_base::openmode mode = ios_base::out);
explicit ofstream (const string& filename, ios_base::openmode mode = ios_base::out);

mode = std::ofstream::in,out,binary,ate,app

ofstream::operator<<

输出很简单，想输出神马，就ofs<<data就行了。最后关闭输出流。done

#include <fstream>      // std::ofstream
int main () {
  std::ofstream ofs;
  ofs.open ("test.txt", std::ofstream::out | std::ofstream::app);
  ofs << " more lorem ipsum";
  ofs.close();
  return 0;
}