feat(furvm): add stack underflow exception

furvm/include/furvm/exceptions.hpp

@@ -0,0 +1,43 @@
+#ifndef FURVM_EXCEPTIONS_HPP
+#define FURVM_EXCEPTIONS_HPP
+
+#include <exception>
+
+namespace furvm {
+
+class stack_underflow : public std::exception {
+public:
+    stack_underflow()           = default;
+    ~stack_underflow() override = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    stack_underflow(stack_underflow&&) noexcept = default;
+
+    /**
+     * @brief Move constructor.
+     */
+    stack_underflow& operator=(stack_underflow&&) noexcept = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    stack_underflow(const stack_underflow&) = default;
+
+    /**
+     * @brief Copy constructor.
+     */
+    stack_underflow& operator=(const stack_underflow&) = 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 "stack underflow"; }
+};
+
+} // namespace furvm
+
+#endif // FURVM_EXCEPTIONS_HPP

furvm/include/furvm/fwd.hpp

@@ -140,6 +140,13 @@ class context;
  */
 using context_p = std::shared_ptr<context>;
 
+// exceptions.hpp:
+
+/**
+ * @brief Stack underflow exception.
+ */
+class stack_underflow;
+
 } // namespace furvm
 
 #endif // FURVM_FWD_HPP