Animation: make it possible to easily create empty track(view)s.

src/Magnum/Animation/Test/TrackTest.cpp

@@ -34,6 +34,7 @@ namespace Magnum { namespace Animation { namespace Test {
 struct TrackTest: TestSuite::Tester {
     explicit TrackTest();
 
+    void constructEmpty();
     void constructArray();
     void constructArrayDefaults();
     void constructInitializerList();
@@ -87,7 +88,8 @@ const struct {
 }
 
 TrackTest::TrackTest() {
-    addTests({&TrackTest::constructArray,
+    addTests({&TrackTest::constructEmpty,
+              &TrackTest::constructArray,
               &TrackTest::constructArrayDefaults,
               &TrackTest::constructInitializerList,
               &TrackTest::constructInitializerListDefaults,
@@ -103,6 +105,15 @@ TrackTest::TrackTest() {
 
 using namespace Math::Literals;
 
+void TrackTest::constructEmpty() {
+    const Track<Float, Vector3> a;
+
+    CORRADE_VERIFY(!a.interpolator());
+    CORRADE_VERIFY(a.keys().empty());
+    CORRADE_VERIFY(a.values().empty());
+    CORRADE_COMPARE(a.at(42.0f), Vector3{});
+}
+
 void TrackTest::constructArray() {
     const Track<Float, Vector3> a{
         Containers::Array<std::pair<Float, Vector3>>{Containers::InPlaceInit,

src/Magnum/Animation/Test/TrackViewTest.cpp

@@ -34,6 +34,7 @@ namespace Magnum { namespace Animation { namespace Test {
 struct TrackViewTest: TestSuite::Tester {
     explicit TrackViewTest();
 
+    void constructEmpty();
     void construct();
     void constructDefaults();
     void constructSingleArray();
@@ -87,7 +88,8 @@ const struct {
 }
 
 TrackViewTest::TrackViewTest() {
-    addTests({&TrackViewTest::construct,
+    addTests({&TrackViewTest::constructEmpty,
+              &TrackViewTest::construct,
               &TrackViewTest::constructDefaults,
               &TrackViewTest::constructSingleArray,
               &TrackViewTest::constructSingleArrayDefaults,
@@ -103,6 +105,15 @@ TrackViewTest::TrackViewTest() {
 
 using namespace Math::Literals;
 
+void TrackViewTest::constructEmpty() {
+    const TrackView<Float, Vector3> a;
+
+    CORRADE_VERIFY(!a.interpolator());
+    CORRADE_VERIFY(a.keys().empty());
+    CORRADE_VERIFY(a.values().empty());
+    CORRADE_COMPARE(a.at(42.0f), Vector3{});
+}
+
 void TrackViewTest::construct() {
     constexpr Float keys[]{0.0f, 5.0f};
     constexpr Vector3 values[]{{3.0f, 1.0f, 0.1f}, {0.3f, 0.6f, 1.0f}};

src/Magnum/Animation/Track.h

@@ -125,6 +125,15 @@ template<class K, class V, class R
         /** @brief Interpolation function */
         typedef ResultType(*Interpolator)(const ValueType&, const ValueType&, Float);
 
+        /**
+         * @brief Construct an empty track
+         *
+         * The @ref data(), @ref keys(), @ref values() and @ref interpolator()
+         * functions return @cpp nullptr @ce, @ref at() always returns a
+         * default-constructed value.
+         */
+        explicit Track() noexcept: _data{}, _interpolator{}, _before{}, _after{} {}
+
         /**
          * @brief Constructor
          * @param data          Keyframe data
@@ -259,6 +268,9 @@ Cast to @ref TrackView of correct type to use.
 @experimental
 */
 class TrackViewStorage {
+    public:
+        constexpr /*implicit*/ TrackViewStorage() noexcept: _keys{}, _values{}, _interpolator{}, _before{}, _after{} {}
+
     private:
         template<class, class, class> friend class TrackView;
 
@@ -298,6 +310,15 @@ template<class K, class V, class R
         /** @brief Interpolation function */
         typedef ResultType(*Interpolator)(const ValueType&, const ValueType&, Float);
 
+        /**
+         * @brief Construct an empty track
+         *
+         * The @ref keys(), @ref values() and @ref interpolator() functions
+         * return @cpp nullptr @ce, @ref at() always returns a
+         * default-constructed value.
+         */
+        explicit TrackView() noexcept {}
+
         /**
          * @brief Constructor
          * @param keys          Frame keys