refactor(cli): improve option handling with defaults and protected inheritance

Update OptionUtils to include name placeholders for default values, switch cli
classes to protected inheritance, and use at() for safer option retrieval.

src/client/echo_client_cli.cpp

@@ -1,4 +1,5 @@
 #include "echo_client_cli.hpp"
+#include <boost/program_options/value_semantic.hpp>
 
 EchoClientCli::EchoClientCli()
     : CliBase("Echo Client Options")
@@ -12,24 +13,22 @@ void EchoClientCli::setupOptions()
 {
     // clang-format off
     desc_.add_options()
-        ("help,h", "Show help")
-        ("host,H",    withDefault(host_),    "Server host (IP or DNS name)")
-        ("port,p",    withDefault(port_),    "Server port")
-        ("message,m", withDefault(message_), "Message to send");
+        ("help,h",    "Show help")
+        ("host,H",    withDefault(host_, "<HOST>"),   "Server host (IP or DNS)")
+        ("port,p",    withDefault(port_, "<PORT>"),   "Server port")
+        ("message,m", withDefault(message_, "<TEXT>"), "Message to send");
     // clang-format on
 }
 
 std::string EchoClientCli::getHost() const
 {
-    return vm_["host"].as<std::string>();
+    return vm_.at("host").as<std::string>();
 }
-
 int EchoClientCli::getPort() const
 {
-    return vm_["port"].as<int>();
+    return vm_.at("port").as<int>();
 }
-
 std::string EchoClientCli::getMessage() const
 {
-    return vm_["message"].as<std::string>();
+    return vm_.at("message").as<std::string>();
 }

src/client/echo_client_cli.hpp

@@ -4,7 +4,7 @@
 #include "option_utils.hpp"
 #include <string>
 
-class EchoClientCli : public CliBase, public OptionUtils
+class EchoClientCli : public CliBase, protected OptionUtils
 {
   public:
     EchoClientCli();
@@ -17,6 +17,7 @@ class EchoClientCli : public CliBase, public OptionUtils
     void setupOptions() override;
 
   private:
+    // defaults (used only to show defaults in help via withDefault)
     std::string host_;
     int port_;
     std::string message_;

src/common/option_utils.hpp

@@ -4,11 +4,17 @@
 
 class OptionUtils
 {
+  public:
+    template <typename T>
+    using OptionValue = boost::program_options::typed_value<T> *;
+
+    template <typename T>
+    static OptionValue<T>
+    withDefault(const T &value, const char *name = nullptr);
+
   protected:
     OptionUtils() = default;
     ~OptionUtils() = default;
-
-    template <typename T> static auto withDefault(T value);
 };
 
 #include "option_utils.tpp"

src/common/option_utils.tpp

@@ -1,8 +1,13 @@
 #pragma once
-
 #include "option_utils.hpp"
 
-template <typename T> auto OptionUtils::withDefault(T value)
+template <typename T>
+OptionUtils::OptionValue<T>
+OptionUtils::withDefault(const T &value, const char *name)
 {
-    return boost::program_options::value<T>()->default_value(value);
+    auto v = boost::program_options::value<T>()->default_value(value);
+    if (name) {
+        v->value_name(name); // Optional
+    }
+    return v;
 }

src/common/server.hpp

@@ -1,9 +0,0 @@
-#pragma once
-
-class Server
-{
-  public:
-    virtual ~Server() = default;
-    virtual void start() = 0;
-    virtual void stop() = 0;
-};

src/server/echo_server_cli.cpp

@@ -11,18 +11,17 @@ void EchoServerCli::setupOptions()
 {
     // clang-format off
     desc_.add_options()
-        ("help,h", "Show help")
-        ("port,p",    withDefault(port_),    "Port to listen on")
-        ("address,a", withDefault(address_), "Bind address (e.g. 0.0.0.0 or 127.0.0.1)");
+        ("help,h",    "Show help")
+        ("port,p",    withDefault(port_, "<PORT>"),       "Port to listen on")
+        ("address,a", withDefault(address_, "<ADDRESS>"), "Bind address (e.g. 0.0.0.0)");
     // clang-format on
 }
 
 int EchoServerCli::getPort() const
 {
-    return vm_["port"].as<int>();
+    return vm_.at("port").as<int>();
 }
-
 std::string EchoServerCli::getAddress() const
 {
-    return vm_["address"].as<std::string>();
+    return vm_.at("address").as<std::string>();
 }

src/server/echo_server_cli.hpp

@@ -4,7 +4,7 @@
 #include "option_utils.hpp"
 #include <string>
 
-class EchoServerCli : public CliBase, public OptionUtils
+class EchoServerCli : public CliBase, protected OptionUtils
 {
   public:
     EchoServerCli();