Change casting to value_t as explicit and update tests and concepts

include/autoppl/expression/variable/constant.hpp

@@ -10,7 +10,7 @@ struct Constant
     Constant(value_t c)
         : c_{c}
     {}
-    operator value_t() const { return get_value(); }
+    explicit operator value_t() const { return get_value(); }
     value_t get_value() const { return c_; }
 
 private:

include/autoppl/expression/variable/variable_viewer.hpp

@@ -21,7 +21,7 @@ struct VariableViewer
         : var_ref_{var}
     {}
 
-    operator value_t() const { return get_value(); }
+    explicit operator value_t() const { return get_value(); }
 
     value_t get_value() const 
     { return static_cast<value_t>(var_ref_.get()); }

include/autoppl/util/type_traits.hpp

@@ -0,0 +1,15 @@
+#pragma once
+#include <type_traits>
+
+namespace ppl {
+
+/*
+ * Checks if type From can be explicitly converted to type To.
+ */
+template <class From, class To>
+inline constexpr bool is_explicitly_convertible_v =
+    std::is_constructible_v<To, From> &&
+    !std::is_convertible_v<From, To>
+    ;
+
+} // namespace ppl

include/autoppl/util/var_expr_traits.hpp

@@ -1,4 +1,5 @@
 #pragma once
+#include <autoppl/util/type_traits.hpp>
 #include <autoppl/util/concept.hpp>
 #include <autoppl/util/var_traits.hpp>
 
@@ -27,14 +28,14 @@ struct var_expr_traits<double>
  * A variable expression is any class that the following:
  * - is_var_v<T> must be false
  * - var_expr_traits must be well-defined for T
- * - T must be convertible to its value_t
+ * - T must be explicitly convertible to its value_t
  */
 template <class T>
 inline constexpr bool is_var_expr_v = 
     !is_var_v<T> &&
     has_type_value_t_v<T> &&
     has_func_get_value_v<const T> &&
-    std::is_convertible_v<const T, get_type_value_t_t<T>>
+    is_explicitly_convertible_v<const T, get_type_value_t_t<T>>
     ;
 
 } // namespace util

include/autoppl/util/var_traits.hpp

@@ -1,5 +1,5 @@
 #pragma once
-#include <type_traits>
+#include <autoppl/util/type_traits.hpp>
 #include <autoppl/util/concept.hpp>
 
 namespace ppl {
@@ -22,7 +22,7 @@ struct var_traits
 /*
  * C++17 version of concepts to check var properties.
  * - var_traits must be well-defined under type T
- * - T must be convertible to its value_t
+ * - T must be explicitly convertible to its value_t
  * - not possible to get overloads
  */
 template <class T>
@@ -36,7 +36,7 @@ inline constexpr bool is_var_v =
     has_func_set_storage_v<T> &&
     has_func_set_state_v<T> &&
     has_func_get_state_v<const T> &&
-    std::is_convertible_v<const T, get_type_value_t_t<T>>
+    is_explicitly_convertible_v<const T, get_type_value_t_t<T>>
     ;
 
 } // namespace util

include/autoppl/variable.hpp

@@ -57,7 +57,7 @@ struct Variable
     void set_state(state_t state) { state_ = state; }
     state_t get_state() const { return state_; }
 
-    operator value_t () const { return get_value(); }
+    explicit operator value_t () const { return get_value(); }
 
     /*
      * Sets underlying value to "value".

test/expression/model/model_unittest.cpp

@@ -75,9 +75,10 @@ TEST_F(var_dist_fixture, log_pdf_valid)
 struct many_var_dist_fixture : ::testing::Test
 {
 protected:
-    MockVar x, y, z, w;
-    double xv, yv, zv, wv;
+    using value_t = double;
     using eq_t = EqNode<MockVar, MockDistExpr>;
+    MockVar x, y, z, w;
+    value_t xv, yv, zv, wv;
 
     using model_two_t = GlueNode<eq_t, eq_t>;
     model_two_t model_two = {
@@ -153,7 +154,7 @@ TEST_F(many_var_dist_fixture, four_vars_traverse_pdf)
     model_four.traverse([&](auto& model) {
             auto& var = model.get_variable();
             auto& dist = model.get_distribution();
-            actual *= dist.pdf(var);
+            actual *= dist.pdf(static_cast<value_t>(var));
         });
     EXPECT_EQ(actual, model_four.pdf());
 }

test/testutil/mock_types.hpp

@@ -23,7 +23,7 @@ struct MockVar
     using state_t = MockState;
 
     value_t get_value() const {return x_;}
-    operator value_t() const { return get_value(); }
+    explicit operator value_t() const { return get_value(); }
 
     void set_value(value_t x) {x_ = x;}
 
@@ -57,7 +57,7 @@ struct MockVarExpr
 {
     using value_t = double;
     value_t get_value() const { return x_; }
-    operator value_t() const { return get_value(); }
+    explicit operator value_t() const { return get_value(); }
 
     /* not part of API */
     MockVarExpr(value_t x)