Re-land: "Use an explicit identifier in Config"

webrtc/common.h

Index: webrtc/common.h
diff --git a/webrtc/common.h b/webrtc/common.h
index dda045ee6c0ca3a64756a706f9a6417c774a1b35..d705d4b8ed12ea6efe357feec8e46bbd3ea8814b 100644
--- a/webrtc/common.h
+++ b/webrtc/common.h
@@ -17,6 +17,23 @@
 
 namespace webrtc {
 
+// Only add new values to the end of the enumeration and never remove (only
+// deprecate) to maintain binary compatibility.
+enum class ConfigOptionID {
+  kMyExperimentForTest,
+  kAlgo1CostFunctionForTest,
+  kTemporalLayersFactory,
+  kNetEqCapacityConfig,
+  kNetEqFastAccelerate,
+  kVoicePacing,
+  kExtendedFilter,
+  kDelayAgnostic,
+  kExperimentalAgc,
+  kExperimentalNs,
+  kBeamforming,
+  kIntelligibility
+};
+
 // Class Config is designed to ease passing a set of options across webrtc code.
 // Options are identified by typename in order to avoid incorrect casts.
 //
@@ -61,8 +78,6 @@ class Config {
   }
 
  private:
-  typedef void* OptionIdentifier;
-
   struct BaseOption {
     virtual ~BaseOption() {}
   };
@@ -76,11 +91,9 @@ class Config {
     T* value;
   };
 
-  // Own implementation of rtti-subset to avoid depending on rtti and its costs.
   template<typename T>
-  static OptionIdentifier identifier() {
-    static char id_placeholder;
-    return &id_placeholder;
+  static ConfigOptionID identifier() {
+    return T::identifier;
   }
 
   // Used to instantiate a default constructed object that doesn't needs to be
@@ -92,7 +105,7 @@ class Config {
     return def;
   }
 
-  typedef std::map<OptionIdentifier, BaseOption*> OptionMap;
+  typedef std::map<ConfigOptionID, BaseOption*> OptionMap;
   OptionMap options_;
 
   // RTC_DISALLOW_COPY_AND_ASSIGN