C++读书笔记之 文件与流 文件读写操作 通过指针获取文件大小

在本例程中不仅概括了文件的基本读写操作，如何获取文件的大小，还牵扯到getline和cin.getline的区别，及string类字符串如何转换为c风格字符串的问题，这些问题将在下一篇博文中介绍。首先先看程序运行结果，运行结果如下：

/**********************************
程序运行结果如下：
Writing to the file
Enter your name: wangshihui
Enter your age: 22
Reading from the file
wangshihui
22
wangshihui

22

To recap, the three main objectives in the Mystery Method are:
            To attract a woman
            To establish comfort, trust, and connection
            To structure the opportunity to be seduced
A woman's number-one emotional priority is safety and security.

TheMisteryMethod.txt size is: 294 bytes.

Process returned 0 (0x0)   execution time : 13.100 s
Press any key to continue.

***********************************/

#include <fstream>
#include <iostream>
using namespace std;
void test_write_read_cin_getline()
{
    char data[100];
    // open a file in write mode.
    ofstream outfile;
    outfile.open("TheMisteryMethod.txt");
    cout << "Writing to the file" << endl;
    cout << "Enter your name: ";
    cin.getline(data, 100);
    // write inputted data into the file.
    outfile << data << endl;
    cout << "Enter your age: ";
    cin >> data;
    cin.ignore();
    // again write inputted data into the file.
    outfile << data << endl;
    // close the opened file.
    outfile.close();
    // open a file in read mode.
    ifstream infile;
    infile.open("TheMisteryMethod.txt");
    cout << "Reading from the file" << endl;
    infile >> data;
    // write the data at the screen.
    cout << data << endl;
    // again read the data from the file and display it.
    infile >> data;
    cout << data << endl;
    // close the opened file.
    infile.close();
}
void test_write()
{
  ofstream myfile;
  myfile.open ("TheMisteryMethod.txt",ios::app);
  if(myfile.is_open())
  {
     myfile <<
            "\nTo recap, the three main objectives in the Mystery Method are: \n\
            To attract a woman \n\
            To establish comfort, trust, and connection \n\
            To structure the opportunity to be seduced \n";
    myfile.close();
  }
  else
    cout<<"打开文件失败！\n";

}
void  test_write_read_getline()
{
  string str;

  //Creates an instance of ofstream, and opens TheMisteryMethod.txt
  ofstream a_file ( "TheMisteryMethod.txt",ios::app );//追加方式
  // Outputs to TheMisteryMethod.txt through a_file
  if(a_file.is_open())
  {
        a_file<<"A woman's number-one emotional priority is safety and security.";
        // Close the file stream explicitly
        a_file.close();
  }
   else
        cout << "Unable to open file\n";

  //Opens for reading the file  追加方式
  ifstream b_file ( "TheMisteryMethod.txt",ios::app );
  //Reads one string from the file
  b_file>> str; //只显示to 表示遇到空格停止接收字符
  cout<< str <<"\n";
  getline(b_file,str,'\0');//输出缓冲区剩余的字符
  cout<< str <<"\n";
  cin.get();    // wait for a keypress
  // b_file is closed implicitly here
}
void GetSizeOfFile(string filename)
{
  long begin,end;
  ifstream myfile (filename.c_str());//必须转换为c风格字符串
  begin = myfile.tellg();
  myfile.seekg (0, ios::end);
  end = myfile.tellg();
  myfile.close();
  cout <<filename<<" size is: " << (end-begin) << " bytes.\n";
}
int main ()
{
    test_write_read_cin_getline();
    test_write();
    test_write_read_getline();
    string file("TheMisteryMethod.txt");
    GetSizeOfFile(file);
    return 0;
}
/**********************************
程序运行结果如下：
Writing to the file
Enter your name: wangshihui
Enter your age: 22
Reading from the file
wangshihui
22
wangshihui

22

To recap, the three main objectives in the Mystery Method are:
            To attract a woman
            To establish comfort, trust, and connection
            To structure the opportunity to be seduced
A woman's number-one emotional priority is safety and security.

TheMisteryMethod.txt size is: 294 bytes.

Process returned 0 (0x0)   execution time : 13.100 s
Press any key to continue.

***********************************/

C++ has two basic classes to handle files, ifstream and ofstream. To use them, include the header file fstream. Ifstream handles file input (reading from files), and ofstream handles file output (writing to files). The way to declare an instance of the ifstream or ofstream class is:

ifstream a_file;

or

ifstream a_file ( "filename" );

 

The constructor for both classes will actually open the file if you pass the name as an argument. As well, both classes have an open command (a_file.open()) and a close command (a_file.close()). You aren't required to use the close command as it will automatically be called when the program terminates, but if you need to close the file long before the program ends, it is useful. 

The beauty of the C++ method of handling files rests in the simplicity of the actual functions used in basic input and output operations. Because C++ supports overloading operators, it is possible to use << and >> in front of the instance of the class as if it were cout or cin. In fact, file streams can be used exactly the same as cout and cin after they are opened.

Input/Output with files

C++ provides the following classes to perform output and input of characters to/from files:

• ofstream: Stream class to write on files
• ifstream: Stream class to read from files
• fstream: Stream class to both read and write from/to files.

Open a file

In order to open a file with a stream object we use its member function open():

open (filename, mode);

Where filename is a null-terminated character sequence of type const char * (the same type that string literals have) representing the name of the file to be opened, andmode is an optional parameter with a combination of the following flags:

ios::in	Open for input operations.
ios::out	Open for output operations.
ios::binary	Open in binary mode.
ios::ate	Set the initial position at the end of the file. If this flag is not set to any value, the initial position is the beginning of the file.
ios::app	All output operations are performed at the end of the file, appending the content to the current content of the file. This flag can only be used in streams open for output-only operations.
ios::trunc	If the file opened for output operations already existed before, its previous content is deleted and replaced by the new one.

All these flags can be combined using the bitwise operator OR (|). For example, if we want to open the fileexample.bin in binary mode to add data we could do it by the following call to member functionopen():

1 2	ofstream myfile; myfile.open ("example.bin", ios::out | ios::app | ios::binary);

Each one of the open() member functions of the classes ofstream,ifstream andfstream has a default mode that is used if the file is opened without a second argument:

class	default mode parameter
ofstream	ios::out
ifstream	ios::in
fstream	ios::in | ios::out

For ifstream and ofstream classes, ios::in and ios::out are automatically and respectively assumed, even if a mode that does not include them is passed as second argument to theopen() member function.

The default value is only applied if the function is called without specifying any value for the mode parameter. If the function is called with any value in that parameter the default mode is overridden, not combined.

To check if a file stream was successful opening a file, you can do it by calling to memberis_open() with no arguments. This member function returns a bool value of true in the case that indeed the stream object is associated with an open file, or false otherwise:

if (myfile.is_open()) { /* ok, proceed with output */ }

Closing a file

When we are finished with our input and output operations on a file we shall close it so that its resources become available again. In order to do that we have to call the stream's member functionclose(). This member function takes no parameters, and what it does is to flush the associated buffers and close the file:

myfile.close();

Once this member function is called, the stream object can be used to open another file, and the file is available again to be opened by other processes.

get and put stream pointers

All i/o streams objects have, at least, one internal stream pointer:

ifstream, like istream, has a pointer known as the get pointer that points to the element to be read in the next input operation.

ofstream, like ostream, has a pointer known as the put pointer that points to the location where the next element has to be written.

Finally, fstream, inherits both, the get and the put pointers, from iostream (which is itself derived from both istream and ostream).

These internal stream pointers that point to the reading or writing locations within a stream can be manipulated using the following member functions:

tellg() and tellp()

These two member functions have no parameters and return a value of the member typepos_type, which is an integer data type representing the current position of the get stream pointer (in the case oftellg) or the put stream pointer (in the case of tellp).

seekg() and seekp()

These functions allow us to change the position of the get and put stream pointers. Both functions are overloaded with two different prototypes. The first prototype is:

seekg ( position );
seekp ( position );

Using this prototype the stream pointer is changed to the absolute position position (counting from the beginning of the file). The type for this parameter is the same as the one returned by functionstellg andtellp: the member type pos_type, which is an integer value.

The other prototype for these functions is:

seekg ( offset, direction );
seekp ( offset, direction );

Using this prototype, the position of the get or put pointer is set to an offset value relative to some specific point determined by the parameterdirection.offset is of the member type off_type, which is also an integer type. Anddirection is of typeseekdir, which is an enumerated type (enum) that determines the point from where offset is counted from, and that can take any of the following values:

ios::beg	offset counted from the beginning of the stream
ios::cur	offset counted from the current position of the stream pointer
ios::end	offset counted from the end of the stream

==============================================================

The default mode for opening a file with ofstream's constructor is to create it if it does not exist, or delete everything in it if something does exist in it. If necessary, you can give a second argument that specifies how the file should be handled. They are listed below:

ios::app   -- Append to the file
ios::ate   -- Set the current position to the end
ios::trunc -- Delete everything in the file

For example:

ofstream a_file ( "test.txt", ios::app );

This will open the file without destroying the current contents and allow you to append new data. When opening files, be very careful not to use them if the file could not be opened. This can be tested for very easily:

ifstream a_file ( "example.txt" );

if ( !a_file.is_open() ) 
{
  // The file could not be opened
}
else 
{
  // Safely use the file stream
}