Using emplace_back replace push_back (#7568)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7568

Reviewed By: akankshamahajan15

Differential Revision: D24437383

Pulled By: jay-zhuang

fbshipit-source-id: 7c9b3c4944b959aa7796c53b410c2b1055dc5641

db/column_family_test.cc

@@ -187,8 +187,8 @@ class ColumnFamilyTestBase : public testing::Test {
     std::vector<ColumnFamilyDescriptor> column_families;
     names_.clear();
     for (size_t i = 0; i < cf.size(); ++i) {
-      column_families.push_back(ColumnFamilyDescriptor(
-          cf[i], options.size() == 0 ? column_family_options_ : options[i]));
+      column_families.emplace_back(
+          cf[i], options.size() == 0 ? column_family_options_ : options[i]);
       names_.push_back(cf[i]);
     }
     return DB::Open(db_options_, dbname_, column_families, &handles_, &db_);
@@ -199,8 +199,8 @@ class ColumnFamilyTestBase : public testing::Test {
     std::vector<ColumnFamilyDescriptor> column_families;
     names_.clear();
     for (size_t i = 0; i < cf.size(); ++i) {
-      column_families.push_back(ColumnFamilyDescriptor(
-          cf[i], options.size() == 0 ? column_family_options_ : options[i]));
+      column_families.emplace_back(
+          cf[i], options.size() == 0 ? column_family_options_ : options[i]);
       names_.push_back(cf[i]);
     }
     return DB::OpenForReadOnly(db_options_, dbname_, column_families, &handles_,