C++之IO

Input and 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.

These classes are derived directly or indirectly from the classes istream and ostream. We have already used objects whose types were these classes: cin is an object of class istream and cout is an object of class ostream. Therefore, we have already been using classes that are related to our file streams. And in fact, we can use our file streams the same way we are already used to use cin and cout, with the only difference that we have to associate these streams with physical files. Let's see an example:

// basic file operations
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile;
  myfile.open ("example.txt");
  myfile << "Writing this to a file.
";
  myfile.close();
  return 0;
}
//[file example.txt]
//Writing this to a file.

This code creates a file called example.txt and inserts a sentence into it in the same way we are used to do with cout, but using the file stream myfile instead.

But let's go step by step:

Open a file

The first operation generally performed on an object of one of these classes is to associate it to a real file. This procedure is known as to open a file. An open file is represented within a program by a stream (i.e., an object of one of these classes; in the previous example, this was myfile) and any input or output operation performed on this stream object will be applied to the physical file associated to it.

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

open (filename, mode);

Where filename is a string representing the name of the file to be opened, and mode 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, 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.
ios::trunc	If the file is opened for output operations and it already existed, 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 file example.bin in binary mode to add data we could do it by the following call to member function open:

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

Each of the open member functions of classes ofstream, ifstream and fstream 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 `

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 the open member function (the flags are combined).

For fstream, 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.

File streams opened in binary mode perform input and output operations independently of any format considerations. Non-binary files are known as text files, and some translations may occur due to formatting of some special characters (like newline and carriage return characters).

Since the first task that is performed on a file stream is generally to open a file, these three classes include a constructor that automatically calls the open member function and has the exact same parameters as this member. Therefore, we could also have declared the previous myfile object and conduct the same opening operation in our previous example by writing:

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

Combining object construction and stream opening in a single statement. Both forms to open a file are valid and equivalent.

To check if a file stream was successful opening a file, you can do it by calling to member is_open. 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 the operating system is notified and its resources become available again. For that, we call the stream's member function close. This member function takes flushes the associated buffers and closes the file:

myfile.close();

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

In case that an object is destroyed while still associated with an open file, the destructor automatically calls the member function close.

Text files

Text file streams are those where the ios::binary flag is not included in their opening mode. These files are designed to store text and thus all values that are input or output from/to them can suffer some formatting transformations, which do not necessarily correspond to their literal binary value.

Writing operations on text files are performed in the same way we operated with cout:

// writing on a text file
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile ("example.txt");
  if (myfile.is_open())
  {
    myfile << "This is a line.
";
    myfile << "This is another line.
";
    myfile.close();
  }
  else cout << "Unable to open file";
  return 0;
}
//[file example.txt]
//This is a line.
//This is another line.

Reading from a file can also be performed in the same way that we did with cin:

// reading a text file
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main () {
  string line;
  ifstream myfile ("example.txt");
  if (myfile.is_open())
  {
    while ( getline (myfile,line) )
    {
      cout << line << '
';
    }
    myfile.close();
  }

  else cout << "Unable to open file"; 

  return 0;
}
//This is a line.
//This is another line.  

This last example reads a text file and prints out its content on the screen. We have created a while loop that reads the file line by line, using getline. The value returned by getline is a reference to the stream object itself, which when evaluated as a boolean expression (as in this while-loop) is true if the stream is ready for more operations, and false if either the end of the file has been reached or if some other error occurred.

Checking state flags

The following member functions exist to check for specific states of a stream (all of them return a bool value):

• bad()
  Returns true if a reading or writing operation fails. For example, in the case that we try to write to a file that is not open for writing or if the device where we try to write has no space left.
• fail()
  Returns true in the same cases as bad(), but also in the case that a format error happens, like when an alphabetical character is extracted when we are trying to read an integer number.
• eof()
  Returns true if a file open for reading has reached the end.
• good()
  It is the most generic state flag: it returns false in the same cases in which calling any of the previous functions would return true. Note that good and bad are not exact opposites (good checks more state flags at once).

The member function clear() can be used to reset the state flags.

get and put stream positioning

All i/o streams objects keep internally -at least- one internal position:

ifstream, like istream, keeps an internal get position with the location of the element to be read in the next input operation.

ofstream, like ostream, keeps an internal put position with the location where the next element has to be written.

Finally, fstream, keeps both, the get and the put position, like iostream.

These internal stream positions point to the locations within the stream where the next reading or writing operation is performed. These positions can be observed and modified using the following member functions:

• tellg() and tellp()
  These two member functions with no parameters return a value of the member type streampos, which is a type representing the current get position (in the case of tellg) or the put position (in the case of tellp).

• seekg() and seekp()
  These functions allow to change the location of the get and put positions. Both functions are overloaded with two different prototypes. The first form 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 streampos, which is the same type as returned by functions tellg and tellp.

The other form for these functions is:

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

Using this prototype, the get or put position is set to an offset value relative to some specific point determined by the parameter direction. offset is of type streamoff. And direction is of type seekdir, which is an enumerated type 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
ios::end	offset counted from the end of the stream

The following example uses the member functions we have just seen to obtain the size of a file:

// obtaining file size
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  streampos begin,end;
  ifstream myfile ("example.bin", ios::binary);
  begin = myfile.tellg();
  myfile.seekg (0, ios::end);
  end = myfile.tellg();
  myfile.close();
  cout << "size is: " << (end-begin) << " bytes.
";
  return 0;
}
//size is: 40 bytes.

Notice the type we have used for variables begin and end:

streampos size;

streampos is a specific type used for buffer and file positioning and is the type returned by file.tellg(). Values of this type can safely be subtracted from other values of the same type, and can also be converted to an integer type large enough to contain the size of the file.

These stream positioning functions use two particular types: streampos and streamoff. These types are also defined as member types of the stream class:

Type	Member type	Description
streampos	ios::pos_type	Defined as fpos<mbstate_t>. It can be converted to/from streamoff and can be added or subtracted values of these types.
streamoff	ios::off_type	It is an alias of one of the fundamental integral types (such as int or long long).

Each of the member types above is an alias of its non-member equivalent (they are the exact same type). It does not matter which one is used. The member types are more generic, because they are the same on all stream objects (even on streams using exotic types of characters), but the non-member types are widely used in existing code for historical reasons.

Binary files

For binary files, reading and writing data with the extraction and insertion operators (<< and >>) and functions like getline is not efficient, since we do not need to format any data and data is likely not formatted in lines.

File streams include two member functions specifically designed to read and write binary data sequentially: write and read. The first one (write) is a member function of ostream (inherited by ofstream). And read is a member function of istream (inherited by ifstream). Objects of class fstream have both. Their prototypes are:

write ( memory_block, size );
read ( memory_block, size );

Where memory_block is of type char* (pointer to char), and represents the address of an array of bytes where the read data elements are stored or from where the data elements to be written are taken. The size parameter is an integer value that specifies the number of characters to be read or written from/to the memory block.

// reading an entire binary file
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  streampos size;
  char * memblock;

  ifstream file ("example.bin", ios::in|ios::binary|ios::ate);
  if (file.is_open())
  {
    size = file.tellg();
    memblock = new char [size];
    file.seekg (0, ios::beg);
    file.read (memblock, size);
    file.close();

    cout << "the entire file content is in memory";

    delete[] memblock;
  }
  else cout << "Unable to open file";
  return 0;
}
//he entire file content is in memory

In this example, the entire file is read and stored in a memory block. Let's examine how this is done:

First, the file is open with the ios::ate flag, which means that the get pointer will be positioned at the end of the file. This way, when we call to member tellg(), we will directly obtain the size of the file.

Once we have obtained the size of the file, we request the allocation of a memory block large enough to hold the entire file:

memblock = new char[size];

Right after that, we proceed to set the get position at the beginning of the file (remember that we opened the file with this pointer at the end), then we read the entire file, and finally close it:

file.seekg (0, ios::beg);
file.read (memblock, size);
file.close();

At this point we could operate with the data obtained from the file. But our program simply announces that the content of the file is in memory and then finishes.

Buffers and Synchronization

When we operate with file streams, these are associated to an internal buffer object of type streambuf. This buffer object may represent a memory block that acts as an intermediary between the stream and the physical file. For example, with an ofstream, each time the member function put (which writes a single character) is called, the character may be inserted in this intermediate buffer instead of being written directly to the physical file with which the stream is associated.

The operating system may also define other layers of buffering for reading and writing to files.

When the buffer is flushed, all the data contained in it is written to the physical medium (if it is an output stream). This process is called synchronization and takes place under any of the following circumstances:

• When the file is closed: before closing a file, all buffers that have not yet been flushed are synchronized and all pending data is written or read to the physical medium.
• When the buffer is full: Buffers have a certain size. When the buffer is full it is automatically synchronized.
• Explicitly, with manipulators: When certain manipulators are used on streams, an explicit synchronization takes place. These manipulators are: flush and endl.
• Explicitly, with member function sync(): Calling the stream's member function sync() causes an immediate synchronization. This function returns an int value equal to -1 if the stream has no associated buffer or in case of failure. Otherwise (if the stream buffer was successfully synchronized) it returns 0.

---

Parse command line parameters

argv and argc are how command line arguments are passed to main() in C and C++.

argc will be the number of strings pointed to by argv. This will (in practice) be 1 plus the number of arguments, as virtually all implementations will prepend the name of the program to the array.

The variables are named argc (argument count) and argv (argument vector) by convention, but they can be given any valid identifier: To pass command line arguments, we typically define main() with two arguments : first argument is the number of command line arguments and second is list of command-line arguments.

int main(int argc, char *argv[]) { /* ... */ }

or

int main(int argc, char **argv) { /* ... */ }

• argc (ARGument Count) is int and stores number of command-line arguments passed by the user including the name of the program. So if we pass a value to a program, value of argc would be 2 (one for argument and one for program name)
• The value of argc should be non negative.
• argv(ARGument Vector) is array of character pointers listing all the arguments.
• If argc is greater than zero,the array elements from argv[0] to argv[argc-1] will contain pointers to strings.
• Argv[0] is the name of the program , After that till argv[argc-1] every element is command -line arguments.

They can also be omitted entirely, yielding int main(), if you do not intend to process command line arguments.

Try the following program:

#include <iostream>

int main(int argc, char** argv) {
    std::cout << "Have " << argc << " arguments:" << std::endl;
    for (int i = 0; i < argc; ++i) {
        std::cout << argv[i] << std::endl;
    }
}
//Running it with ./test a1 b2 c3 will output

//Have 4 arguments:
//./test
//a1
//b2
//c3

Introduction

Command-line parameters are passed to a program at runt-time by the operating system when the program is requested by another program, such as a command interpreter ("shell") like cmd.exe on Windows or bash on Linux and OS X. The user types a command and the shell calls the operating system to run the program. Exactly how this is done is beyond the scope of this article (on Windows, look up CreateProcess; on UNIX and UNIX-like systems look up fork(3) and exec(3) in the manual).

The uses for command-line parameters are various, but the main two are:

• 1.Modifying program behaviour - command-line parameters can be used to tell a program how you expect it to behave; for example, some programs have a -q (quiet) option to tell them not to output as much text.
• 2.Having a program run without user interaction - this is especially useful for programs that are called from scripts or other programs.

The command-line

Adding the ability to parse command-line parameters to a program is very easy. Every C and C++ program has a main function. In a program without the capability to parse its command-line, main is usually defined like this:

int main()

To see the command-line we must add two parameters to main which are, by convention, named argc (argument count) and argv (argument vector [here, vector refers to an array, not a C++ or Euclidean vector]). argc has the type int and argv usually has the type char** or char* [] (see below). main now looks like this:

int main(int argc, char* argv[]) // or char** argv 

argc tells you how many command-line arguments there were. It is always at least 1, because the first string in argv (argv[0]) is the command used to invoke the program. argv contains the actual command-line arguments as an array of strings, the first of which (as we have already discovered) is the program's name. Try this example:

#include <iostream>

int main(int argc, char* argv[])
{
    std::cout << argv[0] << std::endl;
    return 0;
}

This program will print the name of the command you used to run it: if you called the executable "a.exe" (Windows) or "a.out" (UNIX) it would likely print "a.exe" or "./a.out" (if you ran it from the shell) respectively.

Earlier it was mentioned that argc contains the number of arguments passed to the program. This is useful as it can tell us when the user hasn't passed the correct number of arguments, and we can then inform the user of how to run our program:

#include <iostream>

int main(int argc, char* argv[])
{
    // Check the number of parameters
    if (argc < 2) {
        // Tell the user how to run the program
        std::cerr << "Usage: " << argv[0] << " NAME" << std::endl;
        /* "Usage messages" are a conventional way of telling the user
         * how to run a program if they enter the command incorrectly.
         */
        return 1;
    }
    // Print the user's name:
    std::cout << argv[0] << "says hello, " << argv[1] << "!" << std::endl;
    return 0;
}

Example output (no arguments passed):
Usage: a.exe <NAME>
Example output (one argument passed):
a.exe says hello, Chris!

Arguments and Parameters

Arguments and parameters are strings passed to your program to give it information. A program for moving files, for example, may be invoked with two arguments - the source file and the destination: move /path/to/source /path/to/destination (note: on Windows these paths would use backslashes instead [and would probably have a drive prefix, like C:], however as Windows supports backwards and forwards slashes in paths while UNIX systems only support forward slashes, forward slashes will be used throughout this article).

In this example, the program would look something like this:

#include <iostream>

int main(int argc, char* argv[])
{
    if (argc < 3) { // We expect 3 arguments: the program name, the source path and the destination path
        std::cerr << "Usage: " << argv[0] << "SOURCE DESTINATION" << std::endl;
        return 1;
    }
    return move(argv[1], argv[2]);  // Implementation of the move function is platform dependent
                    // and beyond the scope of this article, so it is left out.
}

If we wanted to allow the use of multiple source paths we could use a loop and a std::vector:

#include <iostream>
#include <string>
#include <vector>

int main(int argc, char* argv[])
{
    if (argc < 3) { // We expect 3 arguments: the program name, the source path and the destination path
        std::cerr << "Usage: " << argv[0] << "SOURCE DESTINATION" << std::endl;
        return 1;
    }
    std::vector <std::string> sources;
    std::string destination;
    for (int i = 1; i < argc; ++i) { // Remember argv[0] is the path to the program, we want from argv[1] onwards
        if (i + 1 < argc)
            sources.push_back(argv[i]); // Add all but the last argument to the vector.
        else
            destination = argv[i];
    }
    return move(sources, destination);
}

Arguments may be passed as values to options. An option usually starts with a single hyphen (-) for a "short option" or a double hyphen (--) for a "long option" on UNIX, or a forward slash on Windows. Hyphens (single and double) will be used in this article. Continuing the example of the move program, the program could use a -d/--destination option to tell it which path is the source and which is the destination, as in move -d /path/to/destination /path/to/source and move --destination /path/to/destination /path/to/source. Options are always right-associative, meaning that the argument to an option is always the text directly to the right of it.

Let's extend the previous example to use the destination option.

#include <iostream>
#include <string>
#include <vector>

int main(int argc, char* argv[])
{
    if (argc < 3) {
        std::cerr << "Usage: " << argv[0] << "--destination DESTINATION SOURCE" << std::endl;
        return 1;
    }
    std::vector <std::string> sources;
    std::string destination;
    for (int i = 1; i < argc; ++i) {
        if (std::string(argv[i]) == "--destination") {
            if (i + 1 < argc) { // Make sure we aren't at the end of argv!
                destination = argv[i++]; // Increment 'i' so we don't get the argument as the next argv[i].
            } else { // Uh-oh, there was no argument to the destination option.
                  std::cerr << "--destination option requires one argument." << std::endl;
                return 1;
            }  
        } else {
            sources.push_back(argv[i]);
        }
    }
    return move(sources, destination);
}

Now the parameters can be in any order as long as the destination path is immediately to the right of "--destination".

More on usage messages

Our usage message is helpful, but if we need to print it from more than one place, we have to copy the code. Obviously the way around this is to use a function.

#include <iostream>
#include <string>
#include <vector>

static void show_usage(std::string name)
{
    std::cerr << "Usage: " << argv[0] << " <option(s)> SOURCES"
              << "Options:
"
              << "	-h,--help		Show this help message
"
              << "	-d,--destination DESTINATION	Specify the destination path"
              << std::endl;
}

int main(int argc, char* argv[])
{
    if (argc < 3) {
        show_usage(argv[0]);
        return 1;
    }
    std::vector <std::string> sources;
    std::string destination;
    for (int i = 1; i < argc; ++i) {
        std::string arg = argv[i];
        if ((arg == "-h") || (arg == "--help")) {
            show_usage(argv[0]);
            return 0;
        } else if ((arg == "-d") || (arg == "--destination")) {
            if (i + 1 < argc) { // Make sure we aren't at the end of argv!
                destination = argv[i++]; // Increment 'i' so we don't get the argument as the next argv[i].
            } else { // Uh-oh, there was no argument to the destination option.
                  std::cerr << "--destination option requires one argument." << std::endl;
                return 1;
            }  
        } else {
            sources.push_back(argv[i]);
        }
    }
    return move(sources, destination);
}

Now, rather than having to guess, the user can call our program with the -h or --help options to find out how to run the command.

Getopt

These methods of finding command-line arguments are simple and not very robust. The best way of finding options is using the getopt family of functions:

#include <unistd.h>

int getopt(int argc, char * const argv[],
       const char *optstring);

extern char *optarg;
extern int optind, opterr, optopt;

#include <getopt.h>

int getopt_long(int argc, char * const argv[],
                const char *optstring,
                const struct option *longopts, int *longindex);

int getopt_long_only(int argc, char * const argv[],
                     const char *optstring,
                     const struct option *longopts, int *longindex);

(from the manual page)

The manual page has examples of how to use them.

References

[1] Input/output with files
[2] How to parse command line parameters?
[3] Command line arguments in C/C++
[4] What does int argc, char *argv[] mean?

• 修改记录

时间	地点	修改人	备注
2020-10-06	Foshan	PatrickLee	The first version of this note