Merge pull request #234 from BenLeadbetter/feature/boolean-operators-for-cursors

Feature | Boolean operators for cursors

Author: arximboldi

lager/cursor.hpp

@@ -35,6 +35,7 @@ struct cursor_mixin
     using writer_mixin<DerivT>::zoom;
     using writer_mixin<DerivT>::xform;
     using reader_mixin<DerivT>::xform;
+    using reader_mixin<DerivT>::operator bool;
 
 protected:
     ~cursor_mixin() = default;

lager/reader.hpp

@@ -32,6 +32,11 @@ class cursor_base;
 template <typename DerivT>
 struct reader_mixin
 {
+    explicit operator bool() const
+    {
+        return maybe_node_() != nullptr;
+    };
+
     decltype(auto) get() const { return node_()->last(); }
     decltype(auto) operator*() const { return get(); }
     decltype(auto) operator->() const { return &get(); }
@@ -71,12 +76,16 @@ struct reader_mixin
 
     auto node_() const
     {
-        if (auto node =
-                detail::access::node(*static_cast<const DerivT*>(this))) {
+        if (auto node = maybe_node_()) {
             return node;
         }
         LAGER_THROW(std::runtime_error("Accessing uninitialized reader"));
     }
+
+    auto maybe_node_() const
+    {
+        return detail::access::node(*static_cast<const DerivT*>(this));
+    }
 };
 
 template <typename NodeT>

lager/writer.hpp

@@ -28,6 +28,11 @@ class cursor_base;
 template <typename DerivT>
 struct writer_mixin
 {
+    explicit operator bool() const
+    {
+        return maybe_node_() != nullptr;
+    };
+
     template <typename T>
     void set(T&& value)
     {
@@ -70,12 +75,16 @@ struct writer_mixin
 
     auto node_() const
     {
-        if (auto node =
-                detail::access::node(*static_cast<const DerivT*>(this))) {
+        if (auto node = maybe_node_()) {
             return node;
         }
         LAGER_THROW(std::runtime_error("Accessing uninitialized writer"));
     }
+
+    auto maybe_node_() const
+    {
+        return detail::access::node(*static_cast<const DerivT*>(this));
+    }
 };
 
 template <typename NodeT>

test/cursor.cpp

@@ -37,30 +37,36 @@ using namespace lager;
 TEST_CASE("in, construction and assignment from temporary")
 {
     reader<int> in1;
+    CHECK(!in1);
     // Access to uninitialized reader should throw an exception
     REQUIRE_THROWS(in1.get());
     in1 = make_state(0);
+    CHECK(in1);
     reader<int> in2{make_state(0)};
 }
 
 TEST_CASE("out, construction_and_assignment_from_temporary")
 {
     writer<int> out1;
+    CHECK(!out1);
     // Access to uninitialized writer should throw an exception
     REQUIRE_THROWS(out1.set(42));
     REQUIRE_THROWS(out1.update([](auto) { return 42; }));
     out1 = make_state(0);
+    CHECK(out1);
     writer<int> out2{make_state(0)};
 }
 
 TEST_CASE("inout, construction_and_assignment_from_temporary")
 {
     cursor<int> inout1;
+    CHECK(!inout1);
     // Access to uninitialized cursor should throw an exception
     REQUIRE_THROWS(inout1.get());
     REQUIRE_THROWS(inout1.set(42));
     REQUIRE_THROWS(inout1.update([](auto) { return 42; }));
     inout1 = make_state(0);
+    CHECK(inout1);
     cursor<int> inout2{make_state(0)};
 }