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Overview of File Output

Basics:

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  • In C, we used global functions like fopen, fgets, fputs, fprintf, and fclose to open, read, write, and close files.
  • In C++, we will be using stream objects (instantiated from stream classes) to perform all of the file input and output.
  • This will give us a consistent interface for all I/O.
  • If you know how to use cin and cout, then you have 90% of the necessary knowledge. (cin and cout are stream objects themselves.)
  • As a class, the stream objects have many members that you can access. The most important ones are the overloaded operators: operator<< and operator>>
To read and write files, you must include the appropriate header file: The fstream header actually contains definitions for two types: ifstream and
ofstream.
  • ifstream is an input file stream (for reading input)
  • ofstream is an output file stream (for writing output)
  • Unlike the default input/output streams cin, cout and cerr, there are no default file streams. You must explicitly open them yourself.
Output example:
CodeOutput in foo.txt (showing the new lines)

For automatic objects (stack-based), we don't need to call close since it will be done in the destructor.

We can open the file in the constructor:

Streaming a user-defined type:
CodeOutput (in file1.txt)
Of course, this assumes an overloaded operator<< for both classes:
Student class
Hopefully, now you understand why it is important to use the ostream parameter passed into the function and NOT simply hard-code cout.
StopWatch class
Of course, it is important to check the status of the stream before using it:

Rebinding files:

CodeOutput (in files)
Aside:
  1. What is sizeof(fnames) ?
  2. What is sizeof(fnames[1]) ?
  3. What is sizeof(*fnames[1]) ?
Comparing the use of vector and string instead of an array:
vector<const char *>vector<string>
Aside:
  • What is sizeof(fnames)? In other words, what is sizeof(std::vector<const char *>) ?
  • Newer compilers (libraries) overload ofstream.open() to take std::string
Suppose the program used a list instead of a vector. What other changes would need to be made?


This code will work with any container:

This demonstrates again how iterators are very powerful and flexible and why using iterators instead of subscript operators can lead to more useful (reusable) code.

An even more C++-like implementation might look like this:

Or with C++11 uniform initializer syntax:

Overview of File Input

File input is very similar to the use of cin. Given the text in the file foo.txt from above and showing the spaces and newlines: We can read the words back into the program one at at time:
CodeOutput
Using FILE pointersUsing C++ streams

Reading in an entire line at a time:

Using FILE pointers w/C-style stringsUsing streams w/std::string
A corrected version:
Using FILE pointers w/C-style stringsUsing streams w/std::string
After Azrael Mac OS Refer to Input/output with files for information on state flags (search for Checking state flags).

File Modes

In C, we opened files with the fopen function. It had a signature like this: The mode parameter specified different attributes of the file: C++ streams use a slightly different approach to modes using flags. These flags are very similar to the ones used by the cin and cout objects and modify their behavior. Intro to I/O.
ModeMeaning
ios_base::inOpen file for input (default for ifstream)
ios_base::outOpen file for output (default for ofstream)
ios_base::appSeek to the end before each write (append)
ios_base::truncTruncate file (delete contents) after opening (default for ofstream)
ios_base::binaryOpen file in binary mode

Sample usage:

More information on C++ streams

In-Depth Example

The assignment is to create a program similar to the Unix (Cygwin) program wc. (Reference) Running the command: produces this output:
High-Level PlanDetailed Plan
  1. Read the filenames from the command line
  2. For each file given on the command line
    1. Process the file
  1. Read the filenames from the command line
  2. For each file given on the command line
    1. Open the file
    2. Initialize counters
    3. For each line in the file
      1. Read a line of text
      2. Increment the line count
      3. Count the characters in the line
        • Increment the character count
      4. Count the words in the line
        • Increment the word count
    4. Print out the counts and filename
    5. Close the file
Helper function to format and print the counts and filename: The function where the 'real' work is done: (Process the file) The main function:
Variation #1 on the main function (std::for_each

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):
Variation #2 on the main

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function (no intermediate vector): A closer look at

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for_each: These are the files that need to be included: Running the program: Output:
Cygwin/macOS/Linux (wc)Our program (mywc)

The complete program.

Exercise for students: Add the totals to the output.
Also, there is a caveat with this program: It reads files in one line-at-a-time and assumes that the files are text files. It also assumes that newlines are a single-character (LF) like Linux/Mac OS X, not two characters (CR/LF) like Windows. You would need to modify the file reading logic to handle both systems. Or, read in characters instead of lines and then account for whitespace to delimit words and newlines to delimit lines. This example chose to keep it simple to illustrate the file I/O.