diff --git a/tests/common-cleanup.hpp b/tests/common-cleanup.hpp
index 171a97263..46a1dbe5d 100644
--- a/tests/common-cleanup.hpp
+++ b/tests/common-cleanup.hpp
@@ -26,7 +26,8 @@ namespace testing::cleanup {
 class file_t
 {
 public:
-    file_t(std::string const &filename, bool remove_on_construct = true)
+    explicit file_t(std::string const &filename,
+                    bool remove_on_construct = true)
     : m_filename(filename)
     {
         if (remove_on_construct) {
diff --git a/tests/common-import.hpp b/tests/common-import.hpp
index 532d84a7c..901df7e9c 100644
--- a/tests/common-import.hpp
+++ b/tests/common-import.hpp
@@ -111,7 +111,7 @@ class data_t
     static std::pair<osmium::item_type, osmium::object_id_type>
     get_type_id(std::string const &str)
     {
-        std::string ti(str, 0, str.find(' '));
+        std::string const ti(str, 0, str.find(' '));
         return osmium::string_to_object_id(ti.c_str(),
                                            osmium::osm_entity_bits::nwr);
     }
diff --git a/tests/common-pg.hpp b/tests/common-pg.hpp
index 999c686dd..d48628499 100644
--- a/tests/common-pg.hpp
+++ b/tests/common-pg.hpp
@@ -37,7 +37,7 @@ namespace testing::pg {
 class conn_t : public pg_conn_t
 {
 public:
-    conn_t(connection_params_t const &connection_params)
+    explicit conn_t(connection_params_t const &connection_params)
     : pg_conn_t(connection_params, "test")
     {
     }
@@ -105,7 +105,7 @@ class tempdb_t
         try {
             connection_params_t connection_params;
             connection_params.set("dbname", "postgres");
-            conn_t conn{connection_params};
+            conn_t const conn{connection_params};
 
             m_db_name =
                 fmt::format("osm2pgsql-test-{}-{}", getpid(), time(nullptr));
@@ -113,7 +113,7 @@ class tempdb_t
             conn.exec(R"(CREATE DATABASE "{}" WITH ENCODING 'UTF8')",
                       m_db_name);
 
-            conn_t local = connect();
+            conn_t const local = connect();
             local.exec("CREATE EXTENSION postgis");
             local.exec("CREATE EXTENSION hstore");
             init_database_capabilities(local);
@@ -145,7 +145,7 @@ class tempdb_t
             try {
                 connection_params_t connection_params;
                 connection_params.set("dbname", "postgres");
-                conn_t conn{connection_params};
+                conn_t const conn{connection_params};
                 conn.exec(R"(DROP DATABASE IF EXISTS "{}")", m_db_name);
             } catch (...) {
                 fprintf(stderr, "DROP DATABASE failed. Ignored.\n");