diff --git a/src/idlist.cpp b/src/idlist.cpp
index 341ec4a6f..d90ab0643 100644
--- a/src/idlist.cpp
+++ b/src/idlist.cpp
@@ -10,9 +10,18 @@
 #include "idlist.hpp"
 
 #include <algorithm>
+#include <cassert>
 #include <iterator>
 #include <utility>
 
+osmid_t idlist_t::pop_id()
+{
+    assert(!m_list.empty());
+    auto const id = m_list.back();
+    m_list.pop_back();
+    return id;
+}
+
 void idlist_t::sort_unique()
 {
     std::sort(m_list.begin(), m_list.end());
diff --git a/src/idlist.hpp b/src/idlist.hpp
index 72e98ba05..e719f01a9 100644
--- a/src/idlist.hpp
+++ b/src/idlist.hpp
@@ -18,9 +18,14 @@
 
 #include "osmtypes.hpp"
 
-#include <cassert>
 #include <vector>
 
+/**
+ * A list of OSM object ids. Internally this is a vector of ids.
+ *
+ * Some operations are only allowed when the list of ids is sorted and
+ * without duplicates. Call sort_unique() to achieve this.
+ */
 class idlist_t
 {
 public:
@@ -51,20 +56,20 @@ class idlist_t
 
     osmid_t operator[](std::size_t n) const noexcept { return m_list[n]; }
 
-    void clear() { m_list.clear(); }
+    void clear() noexcept { m_list.clear(); }
 
     void push_back(osmid_t id) { m_list.push_back(id); }
 
     void reserve(std::size_t size) { m_list.reserve(size); }
 
-    osmid_t pop_id()
-    {
-        assert(!m_list.empty());
-        auto const id = m_list.back();
-        m_list.pop_back();
-        return id;
-    }
+    /**
+     * Remove id at the end of the list and return it.
+     *
+     * \pre \code !m_list.empty()) \endcode
+     */
+    osmid_t pop_id();
 
+    /// List are equal if they contain the same ids in the same order.
     friend bool operator==(idlist_t const &lhs, idlist_t const &rhs) noexcept
     {
         return lhs.m_list == rhs.m_list;
@@ -75,13 +80,22 @@ class idlist_t
         return !(lhs == rhs);
     }
 
+    /**
+     * Sort this list and remove duplicates.
+     */
     void sort_unique();
 
+    /**
+     * Merge other list into this one.
+     *
+     * \pre Both lists must be sorted and without duplicates.
+     */
     void merge_sorted(idlist_t const &other);
 
     /**
-     * Remove all ids in this list that are also in the other list. Both
-     * lists must be sorted.
+     * Remove all ids in this list that are also in the other list.
+     *
+     * \pre Both lists must be sorted and without duplicates.
      */
     void remove_ids_if_in(idlist_t const &other);
 
diff --git a/src/middle.hpp b/src/middle.hpp
index 832550baf..2edacfe5c 100644
--- a/src/middle.hpp
+++ b/src/middle.hpp
@@ -15,10 +15,11 @@
 
 #include <memory>
 
-#include "idlist.hpp"
 #include "osmtypes.hpp"
 #include "thread-pool.hpp"
 
+class idlist_t;
+
 struct options_t;
 struct output_requirements;
 
diff --git a/src/pgsql-helper.cpp b/src/pgsql-helper.cpp
index 8d367a6a5..b438594e4 100644
--- a/src/pgsql-helper.cpp
+++ b/src/pgsql-helper.cpp
@@ -14,8 +14,9 @@
 #include <cassert>
 
 idlist_t get_ids_from_result(pg_result_t const &result) {
-    idlist_t ids;
     assert(result.num_tuples() >= 0);
+
+    idlist_t ids;
     ids.reserve(static_cast<std::size_t>(result.num_tuples()));
 
     for (int i = 0; i < result.num_tuples(); ++i) {