First drop the game's core mechanic that compiles.

dbc.h

@@ -0,0 +1,49 @@
+#include <string>
+#include <fmt/core.h>
+
+using namespace std;
+
+namespace dbc {
+  class Error {
+    public:
+      const string message;
+      Error(string m) : message{m} {}
+      Error(const char *m) : message{m} {}
+  };
+
+  class CheckError : public Error {};
+
+  class SentinelError : public Error {};
+  class PreCondError : public Error {};
+  class PostCondError : public Error {};
+
+  void log(const string &message) {
+    fmt::print("{}\n", message);
+  }
+
+  void sentinel(const string &message) {
+    string err = fmt::format("[SENTINEL!] {}\n", message);
+    throw SentinelError{err};
+  }
+
+  void pre(const string &message, std::function<bool()> tester) {
+    if(!tester()) {
+      string err = fmt::format("[PRE!] {}\n", message);
+      throw PreCondError{err};
+    }
+  }
+
+  void post(const string &message, std::function<bool()> tester) {
+    if(!tester()) {
+      string err = fmt::format("[POST!] {}\n", message);
+      throw PostCondError{err};
+    }
+  }
+
+  void check(bool test, const string &message) {
+    if(!test) {
+      string err = fmt::format("[CHECK!] {}\n", message);
+      throw CheckError{err};
+    }
+  }
+}