Take a StringPiece when looking up CommandLine switches.

base/command_line.h

Index: base/command_line.h
diff --git a/base/command_line.h b/base/command_line.h
index 439921ed6e50ae08a7054740b4559ee0546d3e71..bfccb5e66d54ac634563f34d6074d09409e17c51 100644
--- a/base/command_line.h
+++ b/base/command_line.h
@@ -22,6 +22,7 @@
 
 #include "base/base_export.h"
 #include "base/strings/string16.h"
+#include "base/strings/string_piece.h"
 #include "build/build_config.h"
 
 namespace base {
@@ -40,6 +41,7 @@ class BASE_EXPORT CommandLine {
   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 };
@@ -52,6 +54,10 @@ class BASE_EXPORT CommandLine {
   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)
@@ -142,17 +148,18 @@ class BASE_EXPORT CommandLine {
   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_; }
@@ -223,6 +230,12 @@ class BASE_EXPORT CommandLine {
   // 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_;
 };