Take a StringPiece when looking up CommandLine switches.

base/command_line.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This class works with command lines: building and parsing.
 // Arguments with prefixes ('--', '-', and on Windows, '/') are switches.
 // Switches will precede all other arguments without switch prefixes.
 // Switches can optionally have values, delimited by '=', e.g., "-switch=value".
 // An argument of "--" will terminate switch parsing during initialization,
 // interpreting subsequent tokens as non-switch arguments, regardless of prefix.
 
 // There is a singleton read-only CommandLine that represents the command line
 // that the current process was started with.  It must be initialized in main().
 
 #ifndef BASE_COMMAND_LINE_H_
 #define BASE_COMMAND_LINE_H_
 
 #include <stddef.h>
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/base_export.h"
 #include "base/strings/string16.h"
+#include "base/strings/string_piece.h"
 #include "build/build_config.h"
 
 namespace base {
 
 class FilePath;
 
 class BASE_EXPORT CommandLine {
  public:
 #if defined(OS_WIN)
   // The native command line string type.
   typedef base::string16 StringType;
 #elif defined(OS_POSIX)
   typedef std::string StringType;
 #endif
 
   typedef StringType::value_type CharType;
   typedef std::vector<StringType> StringVector;
   typedef std::map<std::string, StringType> SwitchMap;
+  typedef std::map<base::StringPiece, const StringType*> StringPieceSwitchMap;
 
   // A constructor for CommandLines that only carry switches and arguments.
   enum NoProgram { NO_PROGRAM };
   explicit CommandLine(NoProgram no_program);
 
   // Construct a new command line with |program| as argv[0].
   explicit CommandLine(const FilePath& program);
 
   // Construct a new command line from an argument list.
   CommandLine(int argc, const CharType* const* argv);
   explicit CommandLine(const StringVector& argv);
 
+  // Override copy and assign to ensure |switches_by_stringpiece_| is valid.
+  CommandLine(const CommandLine& other);
+  CommandLine& operator=(const CommandLine& other);
+
   ~CommandLine();
 
 #if defined(OS_WIN)
   // By default this class will treat command-line arguments beginning with
   // slashes as switches on Windows, but not other platforms.
   //
   // If this behavior is inappropriate for your application, you can call this
   // function BEFORE initializing the current process' global command line
   // object and the behavior will be the same as Posix systems (only hyphens
   // begin switches, everything else will be an arg).
@@ skipping 70 matching lines @@
 #endif
 
   // Returns the original command line string as a vector of strings.
   const StringVector& argv() const { return argv_; }
 
   // Get and Set the program part of the command line string (the first item).
   FilePath GetProgram() const;
   void SetProgram(const FilePath& program);
 
   // Returns true if this command line contains the given switch.
-  // Switch names should only be lowercase.
+  // Switch names must be lowercase.
   // The second override provides an optimized version to avoid inlining the
-  // codegen for the string allocation.
-  bool HasSwitch(const std::string& switch_string) const;
+  // codegen to find the length of the constant and construct a StringPiece.
+  bool HasSwitch(const base::StringPiece& switch_string) const;
   bool HasSwitch(const char switch_constant[]) const;

[brettw, 2015/04/24 19:50:50]

I think this second version is no longer necessary, and the comment above is
wrong (you construct a string piece in the implementation so it doesn't save
anything).

[jackhou1, 2015/05/11 13:51:42]

IIUC having the second version saves an appreciable amount on binary size
because the codegen for constructing a StringPiece from a char* exists once in
the implementation, rather than at every callsite.

tapted@'s measurements for when HasSwitch took a std::string are here:
https://chromium.googlesource.com/chromium/src/+/009a1dc860942184fc6abdb4bedf...

Made the comment a bit clearer.

 
   // Returns the value associated with the given switch. If the switch has no
   // value or isn't present, this method returns the empty string.
-  std::string GetSwitchValueASCII(const std::string& switch_string) const;
-  FilePath GetSwitchValuePath(const std::string& switch_string) const;
-  StringType GetSwitchValueNative(const std::string& switch_string) const;
+  // Switch names must be lowercase.
+  std::string GetSwitchValueASCII(const base::StringPiece& switch_string) const;
+  FilePath GetSwitchValuePath(const base::StringPiece& switch_string) const;
+  StringType GetSwitchValueNative(const base::StringPiece& switch_string) const;
 
   // Get a copy of all switches, along with their values.
   const SwitchMap& GetSwitches() const { return switches_; }
 
   // Append a switch [with optional value] to the command line.
   // Note: Switches will precede arguments regardless of appending order.
   void AppendSwitch(const std::string& switch_string);
   void AppendSwitchPath(const std::string& switch_string,
                         const FilePath& path);
   void AppendSwitchNative(const std::string& switch_string,
@@ skipping 50 matching lines @@
 
   // The singleton CommandLine representing the current process's command line.
   static CommandLine* current_process_commandline_;
 
   // The argv array: { program, [(--|-|/)switch[=value]]*, [--], [argument]* }
   StringVector argv_;
 
   // Parsed-out switch keys and values.
   SwitchMap switches_;
 
+  // A mirror of |switches_| with only references to the actual strings.
+  // The StringPiece internally holds a pointer to a key in |switches_| while
+  // the mapped_type points to a value in |switches_|.
+  // Used for allocation-free lookups.
+  StringPieceSwitchMap switches_by_stringpiece_;
+
   // The index after the program and switches, any arguments start here.
   size_t begin_args_;
 };
 
 }  // namespace base
 
 #endif  // BASE_COMMAND_LINE_H_