issue #60: add non-const if_contains() which can update the mapped_value.

parallel_hashmap/phmap.h

@@ -3431,12 +3431,27 @@ public:
 #endif
         typename Lockable::SharedLock m;
         auto it = const_cast<parallel_hash_map*>(this)->find(key, this->hash(key), m);
+        if (it == this->end())
+            return false;
+        std::forward<F>(f)((const mapped_type&)Policy::value(&*it));
+        return true;
+    }
+
+    template <class K = key_type, class F>
+    bool if_contains(const key_arg<K>& key, F&& f) {
+#if __cplusplus >= 201703L
+        static_assert(std::is_invocable<F, mapped_type&>::value);
+#endif
+        typename Lockable::UniqueLock m;
+        auto it = this->find(key, this->hash(key), m);
         if (it == this->end())
             return false;
         std::forward<F>(f)(Policy::value(&*it));
         return true;
     }
 
+    
+
     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

@@ -13,13 +13,18 @@ TEST(THIS_TEST_NAME, ThreadSafeContains) {
     // We can't test mutable keys, or non-copyable keys with ThisMap.
     // Test that the nodes have the proper API.
     ThisMap<int, int> m = { {1, 7}, {2, 9} };
-    auto val = 0;
-
-    auto func = [&val](int& v) { val = v; };
-    EXPECT_TRUE(m.if_contains(2, func));
+    const ThisMap<int, int>& const_m(m);
+    
+    auto val = 0; 
+    auto get_value = [&val](const int& v) { val = v; };
+    EXPECT_TRUE(const_m.if_contains(2, get_value));
     EXPECT_EQ(val, 9);
 
-    EXPECT_FALSE(m.if_contains(3, func));
+    EXPECT_FALSE(m.if_contains(3, get_value));
+
+    auto set_value = [&val](int& v) { v = 11; };
+    EXPECT_TRUE(m.if_contains(2, set_value));
+    EXPECT_EQ(m[2], 11);
 }
 
 }  // namespace