I am Using cocoaLumberjack logging framework for iOS logging. For storing logs in a file I used this code.

DDFileLogger* fileLogger = [[DDFileLogger alloc] init];
fileLogger.rollingFrequency = 60 * 60 * 24; 
fileLogger.logFileManager.maximumNumberOfLogFiles = 7;

[DDLog addLogger:fileLogger];

DDLogVerbose(@"hello");
NSLog(@"hihihihihi");

I am unable to find where exactly the logfile generated by this code is stored. Can someone help me with this problem ?

You can download the log files from connected device, or you can send directly from app. Both approaches are described below.

Send log files from app through email, in Swift

Write this in the class where you have a reference to DDFileLogger. I would put this in a custom logger class e.g. MyLogger.swift

var ddFileLogger: DDFileLogger!

var logFileDataArray: [NSData] {
    get {
        let logFilePaths = ddFileLogger.logFileManager.sortedLogFilePaths() as! [String]
        var logFileDataArray = [NSData]()
        for logFilePath in logFilePaths {
            let fileURL = NSURL(fileURLWithPath: logFilePath)
            if let logFileData = try? NSData(contentsOfURL: fileURL, options: NSDataReadingOptions.DataReadingMappedIfSafe) {
                // Insert at front to reverse the order, so that oldest logs appear first.
                logFileDataArray.insert(logFileData, atIndex: 0)
            }
        }
        return logFileDataArray
    }
}

Then, when user taps on a button to indicate that they want to send the logs,

// Required by MFMailComposeViewController
import MessageUI

@IBAction func writeEmailTapped(sender: AnyObject) {
    if MFMailComposeViewController.canSendMail() {
        let composeVC = MFMailComposeViewController()
        composeVC.mailComposeDelegate = self

        // Configure the fields of the interface.
        composeVC.setToRecipients(["your-email@company.com"])
        composeVC.setSubject("Feedback for app")
        composeVC.setMessageBody("", isHTML: false)

        let attachmentData = NSMutableData()
        for logFileData in MyLogger.sharedInstance.logFileDataArray {
            attachmentData.appendData(logFileData)
        }
        composeVC.addAttachmentData(attachmentData, mimeType: "text/plain", fileName: "diagnostic.log")
        self.presentViewController(composeVC, animated: true, completion: nil)
    } else {
        // Tell user about not able to send email directly.
    }
}

This results in a compose email pop-up with an attachment file named diagnostic.log, which is all the log files concatenated together.

Special thanks - This is pretty much a Swift translation from the Objective-C version given by the other answer.

Get log file(s) from device directly, through USB cable

If you want to get the log files that your app created while running on device,

1. Connect your device to your mac
2. In Xcode, go to Window -> Devices
3. On top-left in the device list, click on the connected device.
4. In the main panel, under Installed Apps section, click on the application in which you ran CocoaLumberjack.
5. At the bottom of the Installed Apps list, click on the gear icon and then Download Container.
6. In Finder, right click (show menu) on the saved .xcappdata file and select Show Package Contents
7. Log files are saved in /AppData/Library/Caches/Logs/

Up-vote would be nice if this is helpful to you!