Reduce size of generated extension FeatureProviders.

extensions/common/features/base_feature_provider.h

 // Copyright 2014 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.
 
 #ifndef EXTENSIONS_COMMON_FEATURES_BASE_FEATURE_PROVIDER_H_
 #define EXTENSIONS_COMMON_FEATURES_BASE_FEATURE_PROVIDER_H_
 
 #include <map>
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
+#include "base/strings/string_piece.h"
 #include "extensions/common/features/feature_provider.h"
 
 namespace extensions {
 class Feature;
 
 // A FeatureProvider contains the mapping of all feature names specified in the
 // _*_features.json files to the Feature classes. Look up a Feature by its name
 // to determine whether or not it is available in a certain context.
 // Subclasses implement the specific logic for how the features are populated;
 // this class handles vending the features given the query.
 // TODO(devlin): We could probably combine this and FeatureProvider, since both
 // contain common functionality and neither are designed to be full
 // implementations.
 class BaseFeatureProvider : public FeatureProvider {
  public:
   ~BaseFeatureProvider() override;
 
   // Gets the feature |feature_name|, if it exists.
   Feature* GetFeature(const std::string& feature_name) const override;
   Feature* GetParent(Feature* feature) const override;
   std::vector<Feature*> GetChildren(const Feature& parent) const override;
 
   const FeatureMap& GetAllFeatures() const override;
 
  protected:
   BaseFeatureProvider();
+
+  void AddFeature(base::StringPiece name, std::unique_ptr<Feature> feature);

[Devlin, 2016/07/27 16:46:23]

For my own edification, what's the motivation behind this one?

[dcheng, 2016/07/27 17:02:36]

1. map::operator[] generates a bunch of inline code
2. the string literal to std::string conversion also generates a bunch of inline
code

Previously, we paid the cost of this every time we called feature_["literal"] =
std::move(feature);

Now, we only pay the cost once (in the definition of AddFeature). We still have
to pay a few instructions to set up this actual function call, but it still
works out to a pretty significant savings.

[Devlin, 2016/07/27 17:04:59]

Ah, makes sense.  Thanks!

+
+ private:
   std::map<std::string, std::unique_ptr<Feature>> features_;
 
- private:
   DISALLOW_COPY_AND_ASSIGN(BaseFeatureProvider);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_COMMON_FEATURES_BASE_FEATURE_PROVIDER_H_