Allow passing a dynamic context to render().

include/mstch/mstch.hpp

@@ -16,6 +16,23 @@ struct config {
 
 namespace internal {
 
+template<class N>
+class dynamic_t {
+ public:
+  virtual ~dynamic_t() = default;
+  const N& at (const std::string& name) const {
+    cache = this->get_value(name);
+    return cache;
+  }
+  bool has(const std::string& name) const {
+    return this->has_value(name);
+  }
+ private:
+  virtual N get_value(const std::string& name) const = 0;
+  virtual bool has_value(const std::string& name) const = 0;
+  mutable N cache;
+};
+
 template<class N>
 class object_t {
  public:
@@ -97,9 +114,11 @@ using node = boost::make_recursive_variant<
     std::nullptr_t, std::string, int, double, bool,
     internal::lambda_t<boost::recursive_variant_>,
     std::shared_ptr<internal::object_t<boost::recursive_variant_>>,
+    std::shared_ptr<internal::dynamic_t<boost::recursive_variant_>>,
     std::map<const std::string, boost::recursive_variant_>,
     std::vector<boost::recursive_variant_>>::type;
 using object = internal::object_t<node>;
+using dynamic = internal::dynamic_t<node>;
 using lambda = internal::lambda_t<node>;
 using map = std::map<const std::string, node>;
 using array = std::vector<node>;

src/visitor/get_token.hpp

@@ -27,6 +27,10 @@ class get_token: public boost::static_visitor<const mstch::node&> {
     return object->at(m_token);
   }
 
+  const mstch::node& operator()(const std::shared_ptr<dynamic>& object) const {
+    return object->at(m_token);
+  }
+
  private:
   const std::string& m_token;
   const mstch::node& m_node;

src/visitor/has_token.hpp

@@ -24,6 +24,10 @@ class has_token: public boost::static_visitor<bool> {
     return object->has(m_token);
   }
 
+  bool operator()(const std::shared_ptr<dynamic>& object) const {
+    return object->has(m_token);
+  }
+
  private:
   const std::string& m_token;
 };