Reading Specific Types of Input from a File in C++

Strategy Pattern

The strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

Here's some excellent examples of the strategy pattern

• http://phalanx.spartansoft.org/2009/11/06/implementing-strategy-design-pattern-in-cshar/
• http://www.dofactory.com/Patterns/PatternStrategy.aspx#_self2

As you can see from the 1st link above, you can detect the file type in the header in your main method, and then feed the correct version of the Strategy into the context and then call the context.

If you really want to you could write a Factory Method to automatically create your context.

• http://www.dofactory.com/Patterns/PatternFactory.aspx#_self2

So you could call something like the following

var context = FileFactory.CreateContext(myFile);
context.Parse();

A factory method might be a little overkill for this however, this would be a judgement call on your part. However using the strategy pattern as above gives you the flexibility to add any number of new file types to parse. Additionally the factory will give you the ability to abstract away all of the polymorphism from your logic code.

Edit

Here is what you would end up with using a strategy pattern combined with a Factory Method.

public interface IFileParserStrategy
{
    IList<Person>  Parse(String path);
}


public class FileFormatNo1 : IFileParserStrategy
{    
   private string _path; 
    public FileFormatNo1(String path){
      _path = path;
    }

    public IList<Person> Parse()
    {
        // Actual Logic for handling File Format #1 
        // and returning a IList<Person>   
    }
}

public class FileFormatNo2 : IFileParserStrategy
{     
    private string _path; 
    public FileFormatNo1(String path){
      _path = path;
    }

    public IList<Person> Parse(String path)
    {
        // Actual Logic for handling File Format 2
        // and returning a IList<Person>   
    }
}

public interface IFileParserContext 
{ 
    public IList<Person> Parse();
}

public class FileParserContext : IFileParserContext
{
    private IFileParserStrategy _strategy;

     public FileParserContext(IFileParserStrategy fileParserStrategy)
     {
         _strategy = fileParserStrategy;
     }

     public IList<Person> Parse()
     {
         return _strategy.Parse();
     }
}

public static class FileParserFactory
{
    public static IFileParserContext CreateContext(String path)
    {
           FileParserContext context = null;

           var checkFileFormat = File.ReadAllLines();

            if(checkFileFormat.Contains("FileFormatNo1"))
              context = new FileParserContext(new FileFormatNo1(path));

            if(checkFileFormat.Contains("FileFormatNo2"))
               context = new FileParserContext(new FileFormatNo2(path));

             if(context == null)
                throw new FileFormatException("File is not in a readable state");

             return context;            
    }       
}


public static void Main()
{    
    var filePath = "someFile.txt";
    var context = FileParserFactory.CreateContext(filePath);    
    var personList = context.Parse();
}

Update

You could modify the above strategy to pass around a FileStream if you only wanted to open the file once. You could modify FileParserFactory.CreateContext to accept a FileStream for its parameter, and refactor the strategies to accept the FileStream instead of the filePath.

public static void Main()
{    
    var filePath = "someFile.txt";
    List<Person> personList;

    using(var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read)) 
    {
        var context = FileParserFactory.CreateContext(fileStream);    
        personList = context.Parse();
    }        
}

From a code perspective, the cleanest method would be to switch to the Properties class. It's made for handling this, although you would either need to define all of your settings (even the unused ones) in the file it reads from or have predefined constants in a source file that you can use if the properties file returns null for a particular named property.

If you can't change the scheme, you could follow the general strategy of having a Hashtable, which is really what the Properties class does - it reads a file into a Hashtable conveniently. It would pretty much be rolling your own properties. Again, you would have to have preset values defined somehow to handle cases where a setting that you need might not exist in the file.

The way you would use either is to simply query the data structure any time you needed a value. You can either enforce that it exists in the data structure (throw an Exception and terminate it it doesn't) or define default values somehow (probably in a source file).