variant: add mutable unwrap and visitation

src/common/variant.h

@@ -62,7 +62,6 @@ namespace tools
 
 ////
 // variant: convenience wrapper around boost::variant with a cleaner interface
-// - the value may be assigned to but is otherwise read-only
 // - the variant is 'optional' - an empty variant will evaluate to 'false' and an initialized variant will be 'true'
 ///
 template <typename ResultT>
@@ -70,6 +69,7 @@ struct variant_static_visitor : public boost::static_visitor<ResultT>
 {
     /// provide visitation for empty variants
     /// - add this to your visitor with: using variant_static_visitor::operator();
+    [[noreturn]] ResultT operator()(const boost::blank)       { variant_static_visitor_blank_err(); }
     [[noreturn]] ResultT operator()(const boost::blank) const { variant_static_visitor_blank_err(); }
 };
 
@@ -107,14 +107,20 @@ public:
     template <typename T>
     bool is_type() const noexcept { return this->index() == this->type_index_of<T>(); }
 
-    /// try to get a read-only handle to the embedded value (return nullptr on failure)
+    /// try to get a handle to the embedded value (return nullptr on failure)
     template <typename T>
-    const T* try_unwrap() const
-    {
-        return boost::strict_get<T>(&m_value);
-    }
+          T* try_unwrap()       noexcept { return boost::strict_get<      T>(&m_value); }
+    template <typename T>
+    const T* try_unwrap() const noexcept { return boost::strict_get<const T>(&m_value); }
 
-    /// get a read-only handle to the embedded value
+    /// get a handle to the embedded value
+    template <typename T>
+    T& unwrap()
+    {
+        T *value_ptr{this->try_unwrap<T>()};
+        if (!value_ptr) variant_unwrap_err();
+        return *value_ptr;
+    }
     template <typename T>
     const T& unwrap() const
     {
@@ -142,6 +148,11 @@ public:
 
     /// apply a visitor to the variant
     template <typename VisitorT>
+    typename VisitorT::result_type visit(VisitorT &&visitor)
+    {
+        return boost::apply_visitor(std::forward<VisitorT>(visitor), m_value);
+    }
+    template <typename VisitorT>
     typename VisitorT::result_type visit(VisitorT &&visitor) const
     {
         return boost::apply_visitor(std::forward<VisitorT>(visitor), m_value);

tests/unit_tests/variant.cpp

@@ -232,8 +232,10 @@ struct test_stringify_visitor: public variant_static_visitor<std::string>
         return test_stringify_visitor::stringify(t);
     }
 };
-} // anonymous namespace
 //-------------------------------------------------------------------------------------------------------------------
+//-------------------------------------------------------------------------------------------------------------------
+} // anonymous namespace
+
 //-------------------------------------------------------------------------------------------------------------------
 TEST(variant, operatorbool)
 {
@@ -295,6 +297,25 @@ TEST(variant, unwrap)
     EXPECT_THROW(v.unwrap<std::string>(), std::runtime_error);
 }
 //-------------------------------------------------------------------------------------------------------------------
+TEST(variant, mutation)
+{
+    variant<uint8_t> v;
+    v = (uint8_t) 5;
+    EXPECT_EQ(5, v.unwrap<uint8_t>());
+    uint8_t &intref{v.unwrap<uint8_t>()};
+    intref = 10;
+    EXPECT_EQ(10, v.unwrap<uint8_t>());
+    EXPECT_TRUE(v.try_unwrap<uint8_t>());
+    uint8_t *intptr{v.try_unwrap<uint8_t>()};
+    *intptr = 15;
+    EXPECT_EQ(15, v.unwrap<uint8_t>());
+
+    const variant<uint8_t> &v_ref{v};
+    EXPECT_EQ(15, v_ref.unwrap<uint8_t>());
+    EXPECT_TRUE(v_ref.try_unwrap<uint8_t>());
+    EXPECT_EQ(15, *(v_ref.try_unwrap<uint8_t>()));
+}
+//-------------------------------------------------------------------------------------------------------------------
 TEST(variant, index)
 {
     variant<int8_t, uint8_t, int16_t, uint16_t, std::string> v;