feat(furvm): introduce module, instruction and constant

furvm/include/furvm/constant.hpp

@@ -0,0 +1,112 @@
+#ifndef FURVM_CONSTANT_HPP
+#define FURVM_CONSTANT_HPP
+
+#include "furvm/fwd.hpp"
+
+#include <exception>
+#include <string_view>
+
+namespace furvm {
+
+/**
+ * @brief Bad constant access exception.
+ */
+class bad_constant_access : public std::exception {
+public:
+    bad_constant_access()           = default;
+    ~bad_constant_access() override = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    bad_constant_access(bad_constant_access&&) noexcept = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    bad_constant_access& operator=(bad_constant_access&&) noexcept = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    bad_constant_access(const bad_constant_access&) = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    bad_constant_access& operator=(const bad_constant_access&) = default;
+public:
+    /**
+     * @brief Returns a C-style string describing the cause of the error.
+     *
+     * @return The cause of the error.
+     */
+    const char* what() const noexcept override { return "bad constant access"; }
+};
+
+enum class constant_t : std::uint8_t {
+    String, /**< String constant. */
+};
+
+class constant {
+public:
+    using string_type = std::string_view; /**< String constant type. */
+public:
+    /**
+     * @brief Construct a new string constant.
+     *
+     * @param string String.
+     */
+    constant(string_type string)
+      : m_type(constant_t::String), m_value(string) {}
+
+    ~constant() = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    constant(constant&&) = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    constant& operator=(constant&&) = default;
+
+    constant(const constant&)            = delete;
+    constant& operator=(const constant&) = delete;
+public:
+    /**
+     * @brief Returns this constant's type.
+     * @see constant_t
+     *
+     * @return The constant type.
+     */
+    constexpr constant_t type() const { return m_type; }
+
+    /**
+     * @brief Returns this constant's string value.
+     * @throws bad_constant_access if this constant's type is not constant_t::String.
+     *
+     * @return The string value.
+     */
+    constexpr string_type string() const {
+        require_type(constant_t::String);
+        return m_value.string;
+    }
+private:
+    void require_type(constant_t type) const {
+        if (m_type != type) throw bad_constant_access();
+    }
+private:
+    constant_t m_type{};
+    union value {
+        string_type string;
+
+        value(string_type sv)
+          : string(sv) {}
+    } m_value;
+};
+
+} // namespace furvm
+
+#endif // FURVM_CONSTANT_HPP