COMS W4995 C++ Deep Dive for C Programmers

I/O Streams

Lecture outline

io1

1. Code overview

   • istream& istream::operator>>(int& value)

   • ios::operator bool() const
     • same as !ios::fail()
   • I/O manipulator
     • out << setw(n) will invoke out.width(n)

2. I/O Stream Library class hierarchy

   • std::ios, std::istream, std::cin

3. ios_base::iostate flags and ios accessors

   • https://en.cppreference.com/w/cpp/io/ios_base/iostate

   • ios_base::iostate
     • failbit: formatting or parsing error
     • badbit: irrecoverable I/O error
     • eofbit: end-of-file (EOF) is reached
   • ios accessor functions:
     • fail(): failbit or badbit are set
     • bad(): badbit is set
     • eof(): eofbit is set
     • operator bool(): equivalent to !fail()

4. Examples runs:

   $ echo "10 2 3 4" | ./io1
   current sum:   10	 fail()==0  bad()==0  eof()==0
   current sum:   12	 fail()==0  bad()==0  eof()==0
   current sum:   15	 fail()==0  bad()==0  eof()==0
   current sum:   19	 fail()==0  bad()==0  eof()==0
     final sum:   19	 fail()==1  bad()==0  eof()==1
   

   • Note that echo “10 2 3 4” adds ‘\n’ at the end, so 4 was read without hitting EOF.

   $ echo -n "10 2 3 4" | ./io1
   current sum:   10	 fail()==0  bad()==0  eof()==0
   current sum:   12	 fail()==0  bad()==0  eof()==0
   current sum:   15	 fail()==0  bad()==0  eof()==0
   current sum:   19	 fail()==0  bad()==0  eof()==1
     final sum:   19	 fail()==1  bad()==0  eof()==1
   

   • echo -n does not add ‘\n’ at the end, so when 4 was read, we hit EOF.

   $ echo "10 abc 3 4" | ./io1 
   current sum:   10	 fail()==0  bad()==0  eof()==0
     final sum:   10	 fail()==1  bad()==0  eof()==0
   

   • “abc” failed to parse.

io2

1. sum_ints() rewritten to recover from error

   $ echo "10 abc 3 4" | ./io2
   current sum:   10	 fail()==0  bad()==0  eof()==0
     skipped: a
     skipped: b
     skipped: c
   current sum:   13	 fail()==0  bad()==0  eof()==0
   current sum:   17	 fail()==0  bad()==0  eof()==0
     final sum:   17	 fail()==1  bad()==0  eof()==1
   

2. sum_ints() code walk-through

   • careful coverage of every possible case
   • ios::clear(), istream::get()

3. C++ I/O streams vs. C standard I/O

   By default, std::cin, std::cout, and std::err are synchronized with stdin, stdout, and stderr. This means that the C++ streams are unbuffered, and each I/O operation on a C++ stream is immediately applied to the corresponding C stream’s buffer. This makes it possible to freely mix C++ and C I/O.

   You can turn off this behavior by detaching C++ streams from C streams:

   std::ios_base::sync_with_stdio(false);
   

   Possible benefits of detaching C++ streams:

   • The C++ streams are allowed to buffer their I/O independently, which may increase performance.

   But you lose the following benefits when you detach C++ streams:

   • You can no longer mix C & C++ I/O.
   • C++ streams will no longer be thread-safe.
   • '\n' may not flush standard output connected to terminal.

   Recommendation:

   • Keep the default behavior (i.e. C++ & C I/O synchronized) unless performance measurement reveals significant overhead.
   • Use '\n' instead of std::endl for terminal output.

io3

1. Code walk-through

   • Demo (f1() version)
   • std::pair class template
   • f1()
   • operator>>()

2. String stream

   • Revisit I/O Stream Hierarchy

   • f2():

     • getline() reads a line from an istream
     • istringstream is an istream whose character source is a string
     • f2() calls f1() not with cin, but with an istringstream
     • Error recovery is achieved without modifying f1()

3. File stream

   • f3():
     • ifstream is an istream whose character source is a file
     • f3() calls f2() not with cin, but with an ifstream
     • Reading from a file without redirection is achieved without modifying f2()
   • Also note:
     • ifstream’s RAII paradigm – the constructor opens the file and the destructor closes it
     • failbit is set if it fails to open the file
     • ""s denotes an std::string literal