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CS253 I/O

I/O Class Hierarchy

				       ┌─────┐
				       │ ios │
				       └─────┘
					  △
					  │
			 ┌────────────────┴────────────────┐
			 │				   │
		    ┌────┴────┐			      ┌────┴────┐
		    │ istream │			      │ ostream │
		    └─────────┘			      └─────────┘
			 △				   △
			 │				   │
		  ┌──────┴───────┐		    ┌──────┴───────┐
		  │		 │		    │		   │
	    ┌─────┴────┐ ┌───────┴───────┐  ┌───────┴───────┐ ┌────┴─────┐
	    │ ifstream │ │ istringstream │  │ ostringstream │ │ ofstream │
	    └──────────┘ └───────────────┘  └───────────────┘ └──────────┘

The △ indicates inheritance.

More

			   ┌─────┐
			   │ ios │
			   └─────┘
			      △
			      │
	     ┌────────────────┴────────────────┐
	     │				       │
	┌────┴────┐			  ┌────┴────┐
	│ istream │			  │ ostream │
	└─────────┘			  └─────────┘
	     △				       △
	     │				       │
      ┌──────┴───────┐			┌──────┴───────┐
      │		     │			│	       │
┌─────┴────┐ ┌───────┴───────┐	┌───────┴───────┐ ┌────┴─────┐
│ ifstream │ │ istringstream │  │ ostringstream │ │ ofstream │
└──────────┘ └───────────────┘  └───────────────┘ └──────────┘

• ios (input/output stream) is the base class
• istream and ostream are subclasses of ios
  • An istream is-a ios
  • An ostream is-a ios
• ifstream and istringstream are subclasses of istream
  • An ifstream is-a istream
  • An ifstream is-a ios
  • An istringstream is-a istream
• ofstream and ostringstream are subclasses of ostream
  • An ofstream is-a ostream
  • An ostringstream is-a ostream

Methods & Operator Overloading

			   ┌─────┐
			   │ ios │
			   └─────┘
			      △
			      │
	     ┌────────────────┴────────────────┐
	     │				       │
	┌────┴────┐			  ┌────┴────┐
	│ istream │			  │ ostream │
	└─────────┘			  └─────────┘
	     △				       △
	     │				       │
      ┌──────┴───────┐			┌──────┴───────┐
      │		     │			│	       │
┌─────┴────┐ ┌───────┴───────┐  ┌───────┴───────┐ ┌────┴─────┐
│ ifstream │ │ istringstream │  │ ostringstream │ │ ofstream │
└──────────┘ └───────────────┘  └───────────────┘ └──────────┘

• What class defines operator<<?
• What class defines operator>>?
• What class defines .open()?
• What class defines .get()?

What Header Files Define What

strange type?

Consider the error message from this bad code:

cout.zork();

c.cc:1: error: 'std::ostream' {aka 'class std::basic_ostream<char>'} has no 
   member named 'zork'

• What’s this “std::basic_ostream<char>” business?
• Isn’t cout an object of type ostream?

basic_ostream

• Isn’t cout an object of type ostream?
• Kind of:
  • cout is an object of type basic_ostream<char>
  • typedef basic_ostream<char> ostream;
  • Since ostream is an alias for basic_ostream<char>, then, sure.
• Similarly: typedef basic_istream<char> istream;
• And: typedef basic_string<char> string;

Why!?

w-streams

• This is not simply generality for the same of generality.
• A string is a sequence of char values;
  a wstring is a sequence of wchar_t values.
• An ostream is an output stream of char values;
  a wostream is an output stream of wchar_t values.
• An istream is an input stream of char values;
  a wistream is an input stream of wchar_t values.

wchar_t

• What’s a wchar_t? It’s a wide character.
• Despite its peculiar name, wchar_t is a fundamental type, like char or int.
• Its purpose is to hold a wide character, like ⻥ or 🐟, that doesn’t fit into a one-byte char.
• It’s usually identical to int, and so can hold a Unicode code point, an integer 0…143,000+.

char c = 'X';
wchar_t wc = L'⻥';
cout << sizeof(c) << '\n'
     << sizeof(wc) << '\n';

1
4

basic_whatever<type>

• How would my students have implemented all this? 💖 💖 💖
  • They’d have written ostream, then copied to ostream code to a new wostream class, changing char to wchar_t.
    • The code would then diverge as they fixed bugs in ostream but not in wostream.
• After learning about templates, however, they would implement the templated class basic_ostream, which takes either char or wchar_t as a template argument. Keep it DRY!
• Same for ostream and string.

Unformatted output

We’re familiar with formatted I/O using << or >>:

cout << "π ≈ " << 355/113.0 << endl;

π ≈ 3.14159

There’s also unformatted output:

cout.put('h');
cout.put('i');
char data[] = " there\nextra";
cout.write(data, 7);

hi there

Unformatted input

Similarly, there is unformatted input:

ifstream in("/etc/resolv.conf");
string s;
getline(in, s);
cout << "First: " << s << '\n';
char c;
while (in.get(c)) {
    if (c == '.') c = '*';
    cout << c;
}

First: search cs.colostate edu colostate.edu
nameserver 129*82*45*181
nameserver 129*82*103*78
nameserver 129*82*103*79

• istream::unget() undoes the most recent istream::get().
• istream::peek() is sort of like istream::get() followed by istream::unget().
• istream::ignore() reads until & including a delimiter.

seek/tell

Diogenes

• An ostream has a put pointer, an integer value that keeps track of how far along in the output we are. Similarly, an istream has a get pointer.
  • p = put, g = get
• You can query & change these:
  • ostream::tellp()/istream::tellg(): return put/get pointer
  • ostream::seekp()/istream::seekg(): change the put/get pointer

  ofstream out("xyz");
  out.write("Hxllo there\n", 11);
  out.seekp(1);
  out.put('e');
  out.close();
  cout << ifstream("xyz").rdbuf();
  

  Hello there

• Surprisingly, an fstream (ifstream/ofstream combination) has only a single shared put/get pointer, and not one for input and one for output.

seek/tell methods

• tell ≡ ask a question about location
• seek ≡ change location
• g ≡ discuss the input (get) pointer
• p ≡ discuss the output (put) pointer

• istream::tellg(): return location of input stream
• ostream::tellp(): return location of output stream
• istream::seekg(): change location of input stream
• ostream::seekp(): change location of output stream

Misuse of .open()

ifstream and ofstream both inherited fstream::open():

ifstream in;
const string home = getpwnam("cs253")->pw_dir;
in.open(home+"/pub/ducks");
char c;
while (in.get(c))
    cout << c;

Huey (red)
Dewey (blue)
Louie (green)

Why have that extra step? Just associate the filename at object construction:

const string home = getpwnam("cs253")->pw_dir;
ifstream in(home+"/pub/ducks");
char c;
while (in.get(c))
    cout << c;

Huey (red)
Dewey (blue)
Louie (green)

Misuse of .eof()

• People who program by guesswork are often wrong about istream::eof().
• They think that it means “Will the next read fail?”.
• They are wrong. It means “Did the previous read fail?”.

ifstream in("/etc/resolv.conf");
string line;

while (!in.eof()) {
    getline(in, line);
    cout << line << '\n';
}

search cs.colostate edu colostate.edu
nameserver 129.82.45.181
nameserver 129.82.103.78
nameserver 129.82.103.79

Where did that extra line come from?

The right way to detect end-of-file

Don’t inquire in advance—just go ahead and read. It will succeed or fail:

ifstream in("/etc/resolv.conf");
string line;
while (getline(in, line))
    cout << line << '\n';

search cs.colostate edu colostate.edu
nameserver 129.82.45.181
nameserver 129.82.103.78
nameserver 129.82.103.79

Or, with fewer source lines and a reduced scope for line:

ifstream in("/etc/resolv.conf");
for (string line; getline(in, line); )
    cout << line << '\n';

search cs.colostate edu colostate.edu
nameserver 129.82.45.181
nameserver 129.82.103.78
nameserver 129.82.103.79