Fixed a Visual Studio 2017 build error. (#4488)

The current 15.6.x versions of Visual Studio 2017 contain a bug that
prevent them from compiling the following construct under certain
conditions:

std::unique_ptr<std::unique_ptr<Foo> []> foos;

This will fail to compile if Foo is an abstract class. To work-around
the problem the whole construct was change into:

std::vector<std::unique_ptr<Foo>> foos;

This not only fixes the compiler error but is also more readable than
previous version.

1 files changed, 2 insertions, 4 deletions

diff --git a/src/google/protobuf/compiler/objectivec/objectivec_field.cc b/src/google/protobuf/compiler/objectivec/objectivec_field.cc
index 67b026c3..f74599ba 100644
--- a/src/google/protobuf/compiler/objectivec/objectivec_field.cc
+++ b/src/google/protobuf/compiler/objectivec/objectivec_field.cc
@@ -410,10 +410,8 @@ bool RepeatedFieldGenerator::RuntimeUsesHasBit(void) const {
 FieldGeneratorMap::FieldGeneratorMap(const Descriptor* descriptor,
                                      const Options& options)
     : descriptor_(descriptor),
-      field_generators_(
-          new std::unique_ptr<FieldGenerator>[descriptor->field_count()]),
-      extension_generators_(
-          new std::unique_ptr<FieldGenerator>[descriptor->extension_count()]) {
+      field_generators_(descriptor->field_count()),
+      extension_generators_(descriptor->extension_count()) {
   // Construct all the FieldGenerators.
   for (int i = 0; i < descriptor->field_count(); i++) {
     field_generators_[i].reset(