Specialize F14Table default constructor to avoid reserve() instantiation

Summary: F14 class member fields are often in headers, and often the default constructor is implicit. The current implementation unconditionally instantiates `reserve()`, which has non-negligible cost. By specializing the default constructor we can avoid that in the common case.

Reviewed By: philippv, luciang

Differential Revision: D22166465

fbshipit-source-id: d31ebc7f6ab71392d49a6722545f8df9027d034b

folly/container/F14Map.h

@@ -105,8 +105,7 @@ class F14BasicMap {
  public:
   //// PUBLIC - Member functions
 
-  F14BasicMap() noexcept(Policy::kDefaultConstructIsNoexcept)
-      : F14BasicMap(0) {}
+  F14BasicMap() noexcept(Policy::kDefaultConstructIsNoexcept) : table_{} {}
 
   explicit F14BasicMap(
       std::size_t initialCapacity,

folly/container/F14Set.h

@@ -99,8 +99,7 @@ class F14BasicSet {
  public:
   //// PUBLIC - Member functions
 
-  F14BasicSet() noexcept(Policy::kDefaultConstructIsNoexcept)
-      : F14BasicSet(0) {}
+  F14BasicSet() noexcept(Policy::kDefaultConstructIsNoexcept) : table_{} {}
 
   explicit F14BasicSet(
       std::size_t initialCapacity,

folly/container/detail/F14Table.h

@@ -938,6 +938,11 @@ class F14Table : public Policy {
   }
 
  public:
+  // Equivalent to F14Table(0, ...), but implemented separately to avoid forcing
+  // a reserve() instantiation in the common case.
+  F14Table() noexcept(Policy::kDefaultConstructIsNoexcept)
+      : Policy{Hasher{}, KeyEqual{}, Alloc{}} {}
+
   F14Table(
       std::size_t initialCapacity,
       Hasher const& hasher,