diff --git a/src/AtomicFile.cpp b/src/AtomicFile.cpp
index 48d28814..92f722ee 100644
--- a/src/AtomicFile.cpp
+++ b/src/AtomicFile.cpp
@@ -51,7 +51,7 @@ struct AtomicFile::impl
   // the temp file until finalization.
   bool is_temp_active {false};
 
-  impl (const Path& path);
+  explicit impl (const Path& path);
   ~impl ();
 
   std::string name () const;
diff --git a/src/AtomicFile.h b/src/AtomicFile.h
index 53a4deea..412677a8 100644
--- a/src/AtomicFile.h
+++ b/src/AtomicFile.h
@@ -34,8 +34,8 @@ class Path;
 class AtomicFile
 {
 public:
-  AtomicFile (const Path& path);
-  AtomicFile (std::string path);
+  explicit AtomicFile (const Path& path);
+  explicit AtomicFile (std::string path);
   AtomicFile (const AtomicFile&) = delete;
   AtomicFile (AtomicFile&&);
   AtomicFile& operator= (const AtomicFile&) = delete;
diff --git a/src/CLI.cpp b/src/CLI.cpp
index 2695c05c..0f61768e 100644
--- a/src/CLI.cpp
+++ b/src/CLI.cpp
@@ -557,7 +557,7 @@ std::set<int> CLI::getIds() const
   for (auto& arg : _args)
   {
     if (arg.hasTag ("ID"))
-      ids.insert (strtol (arg.attribute ("value").c_str (), NULL, 10));
+      ids.insert (strtol (arg.attribute ("value").c_str (), nullptr, 10));
   }
 
   return ids;
@@ -666,16 +666,16 @@ Interval CLI::getFilter (const Range& default_range) const
         {
           if (range.is_empty ())
           {
-            args.emplace_back("<all>");
+            args.emplace_back ("<all>");
           }
           else
           {
             start = range.start.toISO ();
             end   = range.end.toISO ();
 
-            args.emplace_back("<date>");
-            args.emplace_back("-");
-            args.emplace_back("<date>");
+            args.emplace_back ("<date>");
+            args.emplace_back ("-");
+            args.emplace_back ("<date>");
           }
         }
 
@@ -688,14 +688,14 @@ Interval CLI::getFilter (const Range& default_range) const
         else if (end.empty ())
           end = raw;
 
-        args.emplace_back("<date>");
+        args.emplace_back ("<date>");
       }
       else if (arg._lextype == Lexer::Type::duration)
       {
         if (duration.empty ())
           duration = raw;
 
-        args.emplace_back("<duration>");
+        args.emplace_back ("<duration>");
       }
       else if (arg.hasTag ("KEYWORD"))
       {
diff --git a/src/Database.cpp b/src/Database.cpp
index 94728dae..7f7f5c17 100644
--- a/src/Database.cpp
+++ b/src/Database.cpp
@@ -547,8 +547,8 @@ void Database::initializeDatafiles ()
         file.find (".data") == file.length () - 5)
     {
       auto basename = Path (file).name ();
-      auto year  = strtol (basename.substr (0, 4).c_str (), NULL, 10);
-      auto month = strtol (basename.substr (5, 2).c_str (), NULL, 10);
+      auto year  = strtol (basename.substr (0, 4).c_str (), nullptr, 10);
+      auto month = strtol (basename.substr (5, 2).c_str (), nullptr, 10);
       getDatafile (year, month);
     }
   }
diff --git a/src/Datafile.cpp b/src/Datafile.cpp
index e1b23bc3..ee2f3328 100644
--- a/src/Datafile.cpp
+++ b/src/Datafile.cpp
@@ -41,8 +41,8 @@ void Datafile::initialize (const std::string& name)
 
   // From the name, which is of the form YYYY-MM.data, extract the YYYY and MM.
   auto basename = _file.name ();
-  auto year  = strtol (basename.substr (0, 4).c_str (), NULL, 10);
-  auto month = strtol (basename.substr (5, 2).c_str (), NULL, 10);
+  auto year  = strtol (basename.substr (0, 4).c_str (), nullptr, 10);
+  auto month = strtol (basename.substr (5, 2).c_str (), nullptr, 10);
 
   // The range is a month: [start, end).
   Datetime start (year, month, 1, 0, 0, 0);
@@ -151,7 +151,7 @@ void Datafile::commit ()
   if (_dirty)
   {
     AtomicFile file (_file);
-    if (_lines.size () > 0)
+    if (!_lines.empty ())
     {
       if (file.open ())
       {
diff --git a/src/Range.cpp b/src/Range.cpp
index cfc775e5..9da31df5 100644
--- a/src/Range.cpp
+++ b/src/Range.cpp
@@ -316,12 +316,12 @@ std::vector <Range> Range::subtract (const Range& other) const
   {
     if (start < other.start)
     {
-      results.emplace_back(start, other.start);
+      results.emplace_back (start, other.start);
 
       if (other.is_ended () &&
           (! is_ended () || end > other.end))
       {
-        results.emplace_back(other.end, end);
+        results.emplace_back (other.end, end);
       }
     }
     else
@@ -331,11 +331,11 @@ std::vector <Range> Range::subtract (const Range& other) const
         if (is_ended ())
         {
           if (end > other.end)
-            results.emplace_back(other.end, end);
+            results.emplace_back (other.end, end);
         }
         else
         {
-          results.emplace_back(other.end, end);
+          results.emplace_back (other.end, end);
         }
       }
     }
diff --git a/src/Rules.cpp b/src/Rules.cpp
index 3bca04c1..4c5ca6ad 100644
--- a/src/Rules.cpp
+++ b/src/Rules.cpp
@@ -605,7 +605,7 @@ bool Rules::setConfigVariable (
       {
         // Add blank line required by rules.
         if (lines.empty () || lines.back ().empty ())
-          lines.emplace_back("");
+          lines.emplace_back ("");
 
         // Add new line.
         lines.push_back (name + " = " + json::encode (value));
@@ -626,7 +626,7 @@ bool Rules::setConfigVariable (
 
       // Add blank line required by rules.
       if (lines.empty () || lines.back ().empty ())
-        lines.emplace_back("");
+        lines.emplace_back ("");
 
       // Add new line.
       lines.push_back (name + " = " + json::encode (value));
diff --git a/src/commands/CmdDiagnostics.cpp b/src/commands/CmdDiagnostics.cpp
index d7966fd7..b07f7881 100644
--- a/src/commands/CmdDiagnostics.cpp
+++ b/src/commands/CmdDiagnostics.cpp
@@ -179,9 +179,9 @@ int CmdDiagnostics (
 
   // Determine rc.editor/$EDITOR/$VISUAL.
   char* peditor;
-  if ((peditor = getenv ("VISUAL")) != NULL)
+  if ((peditor = getenv ("VISUAL")) != nullptr)
     out << "        $VISUAL: " << peditor << '\n';
-  else if ((peditor = getenv ("EDITOR")) != NULL)
+  else if ((peditor = getenv ("EDITOR")) != nullptr)
     out << "        $EDITOR: " << peditor << '\n';
 
   // Theme description, if present.
diff --git a/src/commands/CmdGaps.cpp b/src/commands/CmdGaps.cpp
index 479159fd..330a0099 100644
--- a/src/commands/CmdGaps.cpp
+++ b/src/commands/CmdGaps.cpp
@@ -84,7 +84,7 @@ int CmdGaps (
       {
         table.set (row, 0, format ("W{1}", day.week ()));
         table.set (row, 1, day.toString ("Y-M-D"));
-        table.set (row, 2, day.dayNameShort (day.dayOfWeek ()));
+        table.set (row, 2, Datetime::dayNameShort (day.dayOfWeek ()));
         previous = day;
       }
 
diff --git a/src/commands/CmdReport.cpp b/src/commands/CmdReport.cpp
index 4de24864..30003717 100644
--- a/src/commands/CmdReport.cpp
+++ b/src/commands/CmdReport.cpp
@@ -161,7 +161,7 @@ int CmdReport (
   // Run the extensions.
   std::vector <std::string> output;
   int rc = extensions.callExtension (script_path, split (input, '\n'), output);
-  if (rc != 0 && output.size () == 0)
+  if (rc != 0 && output.empty ())
   {
     throw format ("'{1}' returned {2} without producing output.", script_path, rc);
   }
diff --git a/src/data.cpp b/src/data.cpp
index 0904acfb..20f91e64 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -80,7 +80,7 @@ std::vector <Range> getAllExclusions (
   // Load all exclusions from configuration.
   std::vector <Exclusion> exclusions;
   for (auto& name : rules.all ("exclusions."))
-    exclusions.emplace_back(lowerCase (name), rules.get (name));
+    exclusions.emplace_back (lowerCase (name), rules.get (name));
   debug (format ("Found {1} exclusions", exclusions.size ()));
 
   // Find exclusions 'exc day on <date>' and remove from holidays.
diff --git a/test/test.h b/test/test.h
index d06acc1b..f5f1e67e 100644
--- a/test/test.h
+++ b/test/test.h
@@ -33,7 +33,7 @@ class UnitTest
 {
 public:
   UnitTest ();
-  UnitTest (int);
+  explicit UnitTest (int);
   ~UnitTest ();
 
   void plan (int);