File I/O

​Using Files for Data Storage

• Can use files instead of keyboard, monitor screen for program input, output

• Allows data to be retained between program runs

• Steps:

  • Open the file

  • Use the file (read from, write to, or both)

  • Close the file

General Process

1. Open File

2. Check that file opened successfully

3. Use file (i.e. read/write)

4. Close File

Files: What is Needed

• Use fstream header file for file access

• File stream types:

  • ifstream for input from a file

  • ofstream for output to a file

  • fstream for input from or output to a file

• Define file stream objects:

  • ifstream infile;

  • ofstream outfile;

RAII

• Resource Acquisition is Initialization (RAII)

• Preferred way of interacting with objects in C++

  • More on this later when we create our own objects that use dynamic memory.

• Prefer to let objects get resources (e.g. open a file) during its initialization.

• Prefer to let objects dispose of resources (e.g. close a file) during its destruction.

  • Automatically happens for objects created in a function.

RAII with File Streams

• RAII approach

  • used in Stroustroup

  1. Declare/define file stream and let initialization open file stream.

  2. Check if file opened successfully.

  3. Use file stream. (i.e. read/write)

  4. Implicitly close file.

  • i.e. Do nothing and let the file close when the object is destroyed.

• zyBooks approach

  1. Declare then define: a. Declare/define file stream. b. Explicitly open file stream.

  2. Check if file opened successfully.

  3. Use file stream. (i.e. read/write)

  4. Explicitly close file stream.

Sometimes you still need old fashioned way…

• Maybe you are running a program that opens multiple files inside a loop

  • Before Loop begins, declare/define file stream.

  • Inside loop

    1. Open file

    2. Check if it opened successfully

    3. Use file

    4. Close file

• Note that here,

  • open/close happens multiple times for the same file stream object

Letting the User Specify a Filename

• The open member function requires that you pass the name of the file as a null-terminated string, which is also known as a C-string.

• String literals are stored in memory as null-terminated C-strings, but string objects are not.

• string objects have a member function named c_str

  • It returns the contents of the object formatted as a nullterminated C-string.

  • Here is the general format of how you call the c_str function: stringObject.c_str()