Be careful with EOF

Be careful with EOF

The standard library of the C programming language offers a functiongetc to read a single character from a file or stdin:

      char c = getc(stdin);

This function is useful if you really want to process allcharacters, because unlike scanf, it does not skip spaces,line breaks, tabs, unprintable characters, and the like. Thisfunction is also incorporated in the C++ standard:

      c = cin.get();  // A substitute for "c = getc(stdin);"

To process a file, you need a loop which terminates when the end ofthe file is seen. How do you formulate the break condition?You might know that an end-of-file indicator, EOF, isdefined in stdio.h. So, do you think the following loop is correct?

      while (true)      {        char c = cin.get();        if ( c == EOF ) break;        // Otherwise, process c.      }

Nope. If you switched on all compiler warnings, the line in whichcin.get() was invoked might cause a warning. The reason isthat cin.get() returns an int, not a char,and assigning an int value to a char variableis not a safe operation. In particular, the test for equalityis not safe, because EOF is also an int,and its value does not fit into a char!

Usually, EOF==(int)-1, which on most machines means that all bitsare ones. Hence, the test c==EOF fails even if EOF wasreturned by cin.get(), because the leading ones ofEOF were truncated in the assignment to c. In theif-statement, c is converted into an int,but this means that the leading bits are set to zero.

You are alwayson the safe side when you forget about a C feature such as EOF and use thesubstituting C++ feature:

      if ( cin.eof() ) break;
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