add: [imp] result<void>

include/libimp/error.h

@@ -16,6 +16,7 @@
 LIBIMP_NAMESPACE_BEG_
 
 using error_code_t = std::uint64_t;
+constexpr error_code_t error_number_limit = error_code_t(-1);
 
 /**
  * \brief Serves as the base class for specific error category types.

include/libimp/result.h

@@ -42,7 +42,7 @@ struct generic_traits {
 
   /// \brief Custom initialization.
   constexpr static void init_code(storage_t &code) noexcept {
-    code = {0, -1/*make a default error code*/};
+    code = {0, error_number_limit/*make a default error code*/};
   }
   constexpr static void init_code(storage_t &code, T value, error_code const &ec) noexcept {
     code = {value, ec};
@@ -66,12 +66,39 @@ struct generic_traits {
   }
 };
 
+template <typename ___>
+struct default_traits<void, ___> {
+  /// \typedef Use the `error_code` as the storage type.
+  using storage_t = error_code;
+
+  /// \brief Custom initialization.
+  constexpr static void init_code(storage_t &code) noexcept {
+    code = error_number_limit/*make a default error code*/;
+  }
+  constexpr static void init_code(storage_t &code, error_code const &ec) noexcept {
+    code = ec;
+  }
+
+  /// \brief Custom type data acquisition.
+  constexpr static bool get_ok(storage_t const &code) noexcept {
+    return !code;
+  }
+  constexpr static error_code get_error(storage_t const &code) noexcept {
+    return code;
+  }
+
+  /// \brief Custom formatted output.
+  static std::string format(result<void> const &r) noexcept {
+    return fmt("error = ", r.error());
+  }
+};
+
 template <typename T>
 struct default_traits<T, std::enable_if_t<std::is_integral<T>::value>> : generic_traits<T> {
   /// \brief Custom initialization.
   constexpr static void init_code(storage_t &code, T value, bool ok) noexcept {
-    code = {value, static_cast<error_code_t>(ok ?  0 : 
-                    ((value == default_value()) ? -1 : value))};
+    code = {value, static_cast<error_code_t>(ok ? 0 : 
+                    ((value == default_value()) ? error_number_limit : value))};
   }
   using generic_traits<T>::init_code;
 
@@ -146,6 +173,33 @@ public:
   friend bool operator!=(result const &lhs, result const &rhs) noexcept { return !(lhs == rhs); }
 };
 
+template <typename TypeTraits>
+class result<void, TypeTraits> {
+public:
+  using type_traits_t = TypeTraits;
+  using storage_t     = typename type_traits_t::storage_t;
+
+private:
+  storage_t code_; ///< internal data
+
+public:
+  template <typename... A, 
+            typename = is_not_match<result, A...>,
+            typename = decltype(type_traits_t::init_code(std::declval<storage_t &>()
+                                                       , std::declval<A>()...))>
+  result(A &&... args) noexcept {
+    type_traits_t::init_code(code_, std::forward<A>(args)...);
+  }
+
+  bool       ok   () const noexcept { return type_traits_t::get_ok   (code_); }
+  error_code error() const noexcept { return type_traits_t::get_error(code_); }
+
+  explicit operator bool() const noexcept { return ok   (); }
+
+  friend bool operator==(result const &lhs, result const &rhs) noexcept { return lhs.code_ == rhs.code_; }
+  friend bool operator!=(result const &lhs, result const &rhs) noexcept { return !(lhs == rhs); }
+};
+
 /// \brief Custom defined fmt_to method for imp::fmt
 namespace detail {
 
@@ -154,5 +208,10 @@ bool tag_invoke(decltype(::LIBIMP::fmt_to), fmt_context &ctx, result<T, D> r) {
   return fmt_to(ctx, (r ? "succ" : "fail"), ", value = ", result<T, D>::type_traits_t::format(r));
 }
 
+template <typename D>
+bool tag_invoke(decltype(::LIBIMP::fmt_to), fmt_context &ctx, result<void, D> r) {
+  return fmt_to(ctx, (r ? "succ" : "fail"), ", ", result<void, D>::type_traits_t::format(r));
+}
+
 } // namespace detail
 LIBIMP_NAMESPACE_END_

src/libimp/error.cpp

@@ -17,7 +17,7 @@ public:
     return "generic";
   }
   std::string message(error_code_t const &r) const {
-    if (r == error_code_t(-1)) {
+    if (r == error_number_limit) {
       return "0, \"failure\"";
     }
     if (r == error_code_t(0)) {

test/imp/test_imp_result.cpp

@@ -48,6 +48,16 @@ TEST(result, ok) {
   EXPECT_TRUE(ret.ok());
   EXPECT_EQ(ret.value(), 4321);
 
+  imp::result<void> r1;
+  EXPECT_FALSE(r1);
+  r1 = 0;
+  EXPECT_TRUE(r1);
+  r1 = {};
+  EXPECT_FALSE(r1);
+  r1 = 9999;
+  EXPECT_FALSE(r1);
+  EXPECT_EQ(r1.error(), 9999);
+
   imp::result<int *> r2 {nullptr, 4321};
   EXPECT_NE(r2, nullptr); // imp::result<int *>{nullptr}
   EXPECT_EQ(*r2, nullptr);
@@ -102,4 +112,11 @@ TEST(result, fmt) {
     imp::result<std::int64_t> r1 {-123};
     EXPECT_EQ(imp::fmt(r1), imp::fmt("succ, value = ", -123));
   }
+  {
+    imp::result<void> r1;
+    EXPECT_EQ(imp::fmt(r1), "fail, error = [generic: 0, \"failure\"]");
+    r1 = 0;
+    EXPECT_TRUE(r1);
+    EXPECT_EQ(imp::fmt(r1), "succ, error = [generic: 0, \"success\"]");
+  }
 }