Further reduction of feature_compiler.py generated code

extensions/common/features/complex_feature.cc

Index: extensions/common/features/complex_feature.cc
diff --git a/extensions/common/features/complex_feature.cc b/extensions/common/features/complex_feature.cc
index 5f041dbbc982786192cf3db7d81f36cc041dcbc5..fa5325c7afcbd62f0782a0a2354cf6ad79c25b13 100644
--- a/extensions/common/features/complex_feature.cc
+++ b/extensions/common/features/complex_feature.cc
@@ -6,9 +6,10 @@
 
 namespace extensions {
 
-ComplexFeature::ComplexFeature(std::unique_ptr<FeatureList> features) {
-  DCHECK_GT(features->size(), 0UL);
-  features_.swap(*features);
+ComplexFeature::ComplexFeature(const std::vector<Feature*>& features) {
+  DCHECK_GT(features.size(), 0UL);
+  for (Feature* f : features)
+    features_.push_back(std::unique_ptr<Feature>(f));

[Devlin, 2016/07/29 20:13:49]

What if we did something in the generated code like:
std::vector<std::unique_ptr<Feature>> features;
{
  SimpleFeature* feature = new SimpleFeature();
  ...
  features.push_back(std::unique_ptr<Feature>(f));
}
new ComplexFeature(std::move(features));

(or ComplexFeature could take a std::vector<unique_ptr<Feature>>* and we could
call swap, or some other variation.)  That should avoid the wrap/unwrap, because
we're only wrapping them when they are placed in the vector, right?

Overall this is probably okay, but it seems intuitively like it would be faster
if we didn't have to construct two vectors.

[scottmg, 2016/07/29 20:24:53]

Sorry, I'm probably missing something, but the point is to avoid having N calls
to unique_ptr::unique_ptr() in the generated code, rather than only 1 in
ComplexFeature, so I don't see how that would help.

[Devlin, 2016/07/29 21:15:02]

Ah, right, sorry.  I misunderstood that the concern was the amount of moving
that had to happen (i.e., create a unique_ptr, then move it into the container
vs just construct it directly in the container) rather than the number of calls
to the ctor at all.

   no_parent_ = features_[0]->no_parent();
 
 #if !defined(NDEBUG) || defined(DCHECK_ALWAYS_ON)