jatoben · jatoben · Jun 15, 2015 · Jun 14, 2015 · Jun 15, 2015
diff --git a/CommandLine/CommandLine.swift b/CommandLine/CommandLine.swift
@@ -31,6 +31,14 @@ let ArgumentStopper = "--"
  */
 let ArgumentAttacher: Character = "="
 
+/* An output stream to stderr; used by CommandLine.printUsage(). */
+private struct StderrOutputStream: OutputStreamType {
+  static let stream = StderrOutputStream()
+  func write(s: String) {
+    fputs(s, stderr)
+  }
+}
+
 /**
  * The CommandLine class implements a command-line interface for your app.
  * 
@@ -242,30 +250,60 @@ public class CommandLine {
     }
   }
 
+  /* printUsage() is generic for OutputStreamType because the Swift compiler crashes
+   * on inout protocol function parameters in Xcode 7 beta 1 (rdar://21372694).
+   */
+
   /**
-   * Prints a usage message to stdout.
+   * Prints a usage message.
    * 
-   * - parameter error: An optional error thrown from `parse()`. A description of the error
-   *   (e.g. "Missing required option --extract") will be printed before the usage message.
+   * - parameter to: An OutputStreamType to write the error message to.
    */
-  public func printUsage(error: ErrorType? = nil) {
-    if let err = error as? ParseError {
-      print("\(err)\n")
-    }
-
+  public func printUsage<TargetStream: OutputStreamType>(inout to: TargetStream) {
     let name = _arguments[0]
 
     var flagWidth = 0
     for opt in _options {
       flagWidth = max(flagWidth,
         "  \(ShortOptionPrefix)\(opt.shortFlag), \(LongOptionPrefix)\(opt.longFlag):".characters.count)
     }
-    
-    print("Usage: \(name) [options]")
+
+    print("Usage: \(name) [options]", &to)
     for opt in _options {
       let flags = "  \(ShortOptionPrefix)\(opt.shortFlag), \(LongOptionPrefix)\(opt.longFlag):".paddedToWidth(flagWidth)
 
-      print("\(flags)\n      \(opt.helpMessage)")
+      print("\(flags)\n      \(opt.helpMessage)", &to)
     }
   }
+
+  /**
+   * Prints a usage message.
+   *
+   * - parameter error: An error thrown from `parse()`. A description of the error
+   *   (e.g. "Missing required option --extract") will be printed before the usage message.
+   * - parameter to: An OutputStreamType to write the error message to.
+   */
+  public func printUsage<TargetStream: OutputStreamType>(error: ErrorType, inout to: TargetStream) {
+    print("\(error)\n", &to)
+    printUsage(&to)
+  }
+
+  /**
+   * Prints a usage message.
+   *
+   * - parameter error: An error thrown from `parse()`. A description of the error
+   *   (e.g. "Missing required option --extract") will be printed before the usage message.
+   */
+  public func printUsage(error: ErrorType) {
+    var out = StderrOutputStream.stream
+    printUsage(error, to: &out)
+  }
+
+  /**
+   * Prints a usage message.
+   */
+  public func printUsage() {
+    var out = StderrOutputStream.stream
+    printUsage(&out)
+  }
 }
diff --git a/CommandLineTests/CommandLineTests.swift b/CommandLineTests/CommandLineTests.swift
@@ -16,7 +16,6 @@
  */
 
 import XCTest
-import CommandLine
 
 internal class CommandLineTests: XCTestCase {
 
@@ -512,7 +511,6 @@ internal class CommandLineTests: XCTestCase {
       "-f", "45", "-p", "0.05", "-x", "extra1", "extra2", "extra3" ])
 
     let boolOpt = BoolOption(shortFlag: "d", longFlag: "debug", helpMessage: "Enables debug mode.")
-
     let counterOpt = CounterOption(shortFlag: "v", longFlag: "verbose",
       helpMessage: "Enables verbose output. Specify multiple times for extra verbosity.")
     let stringOpt = StringOption(shortFlag: "n", longFlag: "name", required: true,
@@ -559,4 +557,72 @@ internal class CommandLineTests: XCTestCase {
       XCTFail("Unexpected parse error: \(error)")
     }
   }
+
+  func testPrintUsage() {
+    let cli = CommandLine(arguments: [ "CommandLineTests", "-dvvv", "--name", "John Q. Public",
+      "-f", "45", "-p", "0.05", "-x", "extra1", "extra2", "extra3" ])
+
+    let boolOpt = BoolOption(shortFlag: "d", longFlag: "debug", helpMessage: "Enables debug mode.")
+    let counterOpt = CounterOption(shortFlag: "v", longFlag: "verbose",
+      helpMessage: "Enables verbose output. Specify multiple times for extra verbosity.")
+    let stringOpt = StringOption(shortFlag: "n", longFlag: "name", required: true,
+      helpMessage: "Name a Cy Young winner.")
+    let intOpt = IntOption(shortFlag: "f", longFlag: "favorite", required: true,
+      helpMessage: "Your favorite number.")
+    let doubleOpt = DoubleOption(shortFlag: "p", longFlag: "p-value", required: true,
+      helpMessage: "P-value for test.")
+    let extraOpt = MultiStringOption(shortFlag: "x", longFlag: "Extra", required: true,
+      helpMessage: "X is for Extra.")
+
+    let opts = [boolOpt, counterOpt, stringOpt, intOpt, doubleOpt, extraOpt]
+    cli.addOptions(opts)
+
+    var out = ""
+    cli.printUsage(&out)
+    XCTAssertGreaterThan(out.characters.count, 0)
+
+    /* There should be at least 2 lines per option, plus the intro Usage statement */
+    XCTAssertGreaterThanOrEqual(out.splitByCharacter("\n").count, (opts.count * 2) + 1)
+  }
+
+  func testPrintUsageError() {
+    let cli = CommandLine(arguments: [ "CommandLineTests" ])
+    cli.addOption(StringOption(shortFlag: "n", longFlag: "name", required: true,
+      helpMessage: "Your name"))
+
+    do {
+      try cli.parse()
+      XCTFail("Didn't throw with missing required argument")
+    } catch {
+      var out = ""
+      cli.printUsage(error, to: &out)
+
+      let errorMessage = out.splitByCharacter("\n", maxSplits: 1)[0]
+      XCTAssertTrue(errorMessage.hasPrefix("Missing required"))
+    }
+  }
+
+  func testPrintUsageToStderr() {
+    let cli = CommandLine(arguments: [ "CommandLineTests" ])
+    cli.addOption(StringOption(shortFlag: "n", longFlag: "name", required: true,
+      helpMessage: "Your name"))
+
+    /* Toss stderr into /dev/null, so the printUsage() output doesn't pollute regular
+     * XCTest messages.
+     */
+    let origStdErr = dup(fileno(stderr))
+    let null = fopen("/dev/null", "w")
+    dup2(fileno(null), fileno(stderr))
+
+    defer {
+      dup2(origStdErr, fileno(stderr))
+      fclose(null)
+    }
+
+    let error = CommandLine.ParseError.InvalidArgument("ack")
+
+    /* Just make sure these doesn't crash or throw */
+    cli.printUsage()
+    cli.printUsage(error)
+  }
 }