Thinking In C++. Volume 2: Practical Programming

          mkdir(fullPath.c_str(), 0);  // Create subdir

#else

          mkdir(fullPath.c_str());  // Create subdir

#endif

        fullPath.append(line.substr(startOfFile,

         endOfFile - startOfFile));

        outf.open(fullPath.c_str());

        if(!outf) {

          cerr << "error opening " << fullPath

<< " for output\n";

          exit(EXIT_FAILURE);

        }

        inCode = true;

        cout << "Processing " << fullPath << endl;

        if(printDelims)

          outf << line << endl;

      }

      else if(inCode) {

        assert(outf);

        outf << line << endl;  // output middle code line

      }

    }

  }

  exit(EXIT_SUCCESS);

} ///:~

First, you’ll notice some conditional compilation directives. The mkdir( ) function, which creates a directory in the file system, is defined by the POSIX[37] standard in the header . Unfortunately, many compilers still use a different header (). The respective signatures for mkdir( ) also differ: POSIX specifies two arguments, the older versions just one. For this reason, there is more conditional compilation later in the program to choose the right call to mkdir( ). We normally don’t use conditional compilation in the examples in this book, but this particular program is too useful not to put a little extra work into, since you can use it to extract all the code with it^.

The exists( ) function in ExtractCode.cpp tests whether a directory exists by opening a temporary file in it. If the open fails, the directory doesn’t exist. You remove a file by sending its name as a char* to std::remove( )^.

The main program validates the command-line arguments and then reads the input file a line at a time, looking for the special source code delimiters. The Boolean flag inCode indicates that the program is in the middle of a source file, so lines should be output. The printDelims flag will be true if the opening token is not followed by an exclamation point; otherwise the first and last lines are not written. It is important to check for the closing delimiter first, because the start token is a subset of it, and searching for the start token first would return a successful find for both cases. If we encounter the closing token, we verify that we are in the middle of processing a source file; otherwise, something is wrong with the way the delimiters are laid out in the text file. If inCode is true, all is well, and we (optionally) write the last line and close the file. When the opening token is found, we parse the directory and file name components and open the file. The following string-related functions were used in this example: length( ), append( ), getline( ), find( ) (two versions), find_first_not_of( ), substr( ), find_first_of( ), c_str( ), and, of course, operator<<( )^.

We also use a standard C technique for reporting program status to the calling context by returning different values from main( ). It is portable to use the statement return 0; to indicate success, but there is no portable value to indicate failure. For this reason we use the macro declared for this very purpose in : EXIT_FAILURE. As a matter of consistency, whenever we use EXIT_FAILURE we also use EXIT_SUCCESS, even though the latter is always defined as zero^.

Summary

C++ string objects provide developers with a number of great advantages over their C counterparts. For the most part, the string class makes referring to strings through the use of character pointers unnecessary. This eliminates an entire class of software defects that arise from the use of uninitialized and incorrectly valued pointers. C++ strings dynamically and transparently grow their internal data storage space to accommodate increases in the size of the string data. This means that when the data in a string grows beyond the limits of the memory initially allocated to it, the string object will make the memory management calls that take space from and return space to the heap. Consistent allocation schemes prevent memory leaks and have the potential to be much more efficient than "roll your own" memory management^.

The string class member functions provide a fairly comprehensive set of tools for creating, modifying, and searching in strings. String comparisons are always case sensitive, but you can work around this by copying string data to C-style null-terminated strings and using case-insensitive string comparison functions, temporarily converting the data held in string objects to a single case, or by creating a case-insensitive string class that overrides the character traits used to create the basic_string object^.

Exercises

             1.             Write a program that reverses the order of the characters in a string.

         18.             A palindrome is a word or group of words that read the same forward and backward. For example "madam" or "wow." Write a program that takes a string argument from the command line and prints whether the string was a palindrome or not.