Add a proxy class to return from operator[]

src/env_base.cpp

@@ -16,9 +16,26 @@
  */
 
 #include "env_base.hpp"
+#include "string_view_cat.hpp"
 
 namespace duck {
 bool EnvBase::empty() const noexcept {
 	return 0 == this->size();
 }
+
+std::string_view EnvBase::operator[] (std::string_view name) const noexcept {
+	return this->get(name, "");
+}
+
+detail::EnvVarProxy<std::string_view> EnvBase::operator[] (std::string_view name) noexcept {
+	return {name, this};
+}
+
+detail::EnvVarProxy<std::string> EnvBase::operator[] (std::string&& name) noexcept {
+	return {std::move(name), this};
+}
+
+detail::EnvVarProxy<std::string> EnvBase::operator[] (const std::string& name) noexcept {
+	return {to_string(name), this};
+}
 } //namespace duck

src/env_base.hpp

@@ -22,6 +22,23 @@
 #include <utility>
 
 namespace duck {
+	class EnvBase;
+
+	namespace detail {
+		template <typename S>
+		class EnvVarProxy {
+		public:
+			template <typename S1>
+			EnvVarProxy (S1&& name, EnvBase* env) noexcept;
+			EnvVarProxy& operator= (std::string_view new_val);
+			operator std::string_view() const noexcept;
+
+		private:
+			S m_name;
+			EnvBase* m_env;
+		};
+	} //namespace detail
+
 	class EnvBase {
 	public:
 		typedef std::string_view mapped_type;
@@ -33,7 +50,11 @@ namespace duck {
 		EnvBase() = default;
 		virtual ~EnvBase() noexcept = default;
 
-		virtual std::string_view operator[] (std::string_view name) const noexcept = 0;
+		std::string_view operator[] (std::string_view name) const noexcept;
+		detail::EnvVarProxy<std::string_view> operator[] (std::string_view name) noexcept;
+		detail::EnvVarProxy<std::string> operator[] (std::string&& name) noexcept;
+		detail::EnvVarProxy<std::string> operator[] (const std::string& name) noexcept;
+
 		virtual std::optional<std::string_view> get (std::string_view name) const noexcept = 0;
 		virtual std::string_view get (std::string_view name, std::string_view def) const noexcept = 0;
 		virtual void set (std::string_view name, std::string_view value) = 0;
@@ -43,4 +64,26 @@ namespace duck {
 		virtual bool empty() const noexcept;
 		virtual void clear() noexcept = 0;
 	};
+
+	namespace detail {
+		template <typename S>
+		template <typename S1>
+		inline EnvVarProxy<S>::EnvVarProxy (S1&& name, EnvBase* env) noexcept :
+			m_name(std::forward<S1>(name)),
+			m_env(env)
+		{
+		}
+
+		template <typename S>
+		inline auto EnvVarProxy<S>::operator= (std::string_view new_val) -> EnvVarProxy& {
+			m_env->set(m_name, new_val);
+			return *this;
+		}
+
+		template <typename S>
+		inline EnvVarProxy<S>::operator std::string_view() const noexcept {
+			return (*m_env)[m_name];
+		}
+	} //namespace detail
+
 } //namespace duck

src/env_real.cpp

@@ -165,10 +165,6 @@ EnvReal::EnvReal() :
 {
 }
 
-std::string_view EnvReal::operator[] (std::string_view name) const noexcept {
-	return this->get(name, "");
-}
-
 std::optional<std::string_view> EnvReal::get (std::string_view name) const noexcept {
 	const char* const ret = this->raw_fetch_env(name);
 	if (ret)

src/env_real.hpp

@@ -26,7 +26,6 @@ namespace duck {
 		EnvReal();
 		virtual ~EnvReal() noexcept = default;
 
-		virtual std::string_view operator[] (std::string_view name) const noexcept override;
 		virtual std::optional<std::string_view> get (std::string_view name) const noexcept override;
 		virtual std::string_view get (std::string_view name, std::string_view def) const noexcept override;
 		virtual void set (std::string_view name, std::string_view value) override;