Add new API try_emplace_l (issue #60)

parallel_hashmap/phmap.h

@@ -3424,16 +3424,46 @@ public:
         return Policy::value(&*it);
     }
 
+    // ----------- phmap extensions --------------------------
+
+    // if map contains key, lambda is called with the mapped value (under read lock protection),
+    // and if_contains returns true. This is a const API and lambda should not modify the value
+    // -----------------------------------------------------------------------------------------
     template <class K = key_type, class F>
     bool if_contains(const key_arg<K>& key, F&& f) const {
-        return const_cast<parallel_hash_map*>(this)->template modify_if_impl<K, F, typename Lockable::SharedLock>(key, std::forward<F>(f));
+        return const_cast<parallel_hash_map*>(this)->template 
+            modify_if_impl<K, F, typename Lockable::SharedLock>(key, std::forward<F>(f));
     }
 
+    // if map contains key, lambda is called with the mapped value  (under write lock protection),
+    // and modify_if returns true. This is a non-const API and lambda is allowed to modify the mapped value
+    // ----------------------------------------------------------------------------------------------------
     template <class K = key_type, class F>
     bool modify_if(const key_arg<K>& key, F&& f) {
         return modify_if_impl<K, F, typename Lockable::UniqueLock>(key, std::forward<F>(f));
     }
 
+    // if map does not contains key, it is inserted and the mapped value is value-constructed 
+    // with the provided arguments (if any), as with try_emplace. 
+    // Then the lambda is called with the mapped value (under write lock protection) and can 
+    // update the mapped value.
+    // ---------------------------------------------------------------------------------------
+    template <class K = key_type, class F, class... Args>
+    bool try_emplace_l(K&& k, F&& f, Args&&... args) {
+        typename Lockable::UniqueLock m;
+        auto res = this->find_or_prepare_insert(k, m);
+        typename Base::Inner *inner = std::get<0>(res);
+        if (std::get<2>(res))
+            inner->set_.emplace_at(std::get<1>(res), std::piecewise_construct,
+                                   std::forward_as_tuple(std::forward<K>(k)),
+                                   std::forward_as_tuple(std::forward<Args>(args)...));
+        auto it = this->iterator_at(inner, inner->set_.iterator_at(std::get<1>(res)));
+        std::forward<F>(f)(Policy::value(&*it));
+        return std::get<2>(res);
+    }
+
+    // ----------- end of phmap extensions --------------------------
+
     template <class K = key_type, class P = Policy, K* = nullptr>
     MappedReference<P> operator[](key_arg<K>&& key) {
         return Policy::value(&*try_emplace(std::forward<K>(key)).first);

tests/parallel_hash_map_test.cc

@@ -22,11 +22,25 @@ TEST(THIS_TEST_NAME, ThreadSafeContains) {
 
     EXPECT_FALSE(m.if_contains(3, get_value));
 
-    auto set_value = [&val](int& v) { v = 11; };
-    EXPECT_TRUE(m. modify_if(2, set_value));
+    auto set_value = [](int& v) { v = 11; };
+    EXPECT_TRUE(m.modify_if(2, set_value));
     EXPECT_EQ(m[2], 11);
 
     EXPECT_FALSE(m.modify_if(3, set_value));
+
+    // overwrite an existing value
+    m.try_emplace_l(2, [](int& v) { v = 5; });
+    EXPECT_EQ(m[2], 5);
+
+    // insert a valye that is not already present
+    m.try_emplace_l(3, [](int& v) { assert(v == 0); v = 6; });
+    EXPECT_EQ(m[3], 6);
+    
+    // insert a valye that is not already present, provide argument to value-construct it
+    m.try_emplace_l(4, [](int& v) { assert(v == 999); v = 5; }, 999);
+    EXPECT_EQ(m[4], 5);
+    
+    
 }
 
 }  // namespace