Merge 14e3318929 into 0457de21cf

include/nlohmann/detail/conversions/from_json.hpp

@@ -28,6 +28,7 @@
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/value_t.hpp>
+#include <nlohmann/optional.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -43,6 +44,34 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
     n = nullptr;
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType, typename T>
+void from_json(const BasicJsonType& j, std::optional<T>& opt)
+{
+    if (j.is_null())
+    {
+        opt = std::nullopt;
+    }
+    else
+    {
+        opt = j.template get<T>();
+    }
+}
+
+template<typename BasicJsonType, typename T>
+void from_json(const BasicJsonType& j, optional<T>& opt)
+{
+    if (j.is_null())
+    {
+        opt = std::nullopt;
+    }
+    else
+    {
+        opt = j.template get<T>();
+    }
+}
+#endif
+
 // overloads for basic_json template parameters
 template < typename BasicJsonType, typename ArithmeticType,
            enable_if_t < std::is_arithmetic<ArithmeticType>::value&&

include/nlohmann/detail/conversions/to_json.hpp

@@ -23,6 +23,7 @@
 #include <nlohmann/detail/meta/std_fs.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
+#include <nlohmann/optional.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -260,6 +261,29 @@ struct external_constructor<value_t::object>
 // to_json //
 /////////////
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType, typename T,
+         enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
+{
+    if (opt.has_value())
+    {
+        j = *opt;
+    }
+    else
+    {
+        j = nullptr;
+    }
+}
+
+template<typename BasicJsonType, typename T,
+         enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
+void to_json(BasicJsonType& j, const optional<T>& opt) noexcept
+{
+    to_json(j, opt.base());
+}
+#endif
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
 inline void to_json(BasicJsonType& j, T b) noexcept

include/nlohmann/optional.hpp

@@ -0,0 +1,224 @@
+#pragma once
+
+#include <nlohmann/detail/macro_scope.hpp>
+
+#ifdef JSON_HAS_CPP_17
+
+#include <optional>
+#include <utility>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+
+template <typename T>
+class optional
+{
+    // *INDENT-OFF*
+
+    using base_type = std::optional<T>;
+    using value_type = T;
+
+    template <typename U, typename = optional>
+    struct has_conversion_operator : std::false_type { };
+
+    template <typename U>
+    struct has_conversion_operator<U,
+        decltype(std::declval<U>().operator optional())> : std::true_type { };
+
+    template <typename... U>
+    using is_base_constructible_from = std::is_constructible<base_type, U...>;
+
+    template <typename U>
+    using is_convertible_to_base = std::is_convertible<U, base_type>;
+
+    template <typename U>
+    using enable_int_if = std::enable_if_t<U::value, int>;
+
+    template <typename U>
+    using use_conversion_operator =
+        enable_int_if<
+            has_conversion_operator<U>
+        >;
+
+    template <typename U>
+    using use_implicit_forwarding =
+        enable_int_if<
+            std::conjunction<
+                std::negation<has_conversion_operator<U>>,
+                is_base_constructible_from<U>,
+                is_convertible_to_base<U>
+            >
+        >;
+
+    template <typename U>
+    using use_explicit_forwarding =
+        enable_int_if<
+            std::conjunction<
+                std::negation<has_conversion_operator<U>>,
+                is_base_constructible_from<U>,
+                std::negation<is_convertible_to_base<U>>
+            >
+        >;
+
+    template <typename... U>
+    using can_construct_in_place_from =
+        enable_int_if<
+            is_base_constructible_from<std::in_place_t, U...>
+        >;
+
+    struct noexcept_fix_t {}; // trick for GCC8.1 (see default constructor)
+
+    base_type base_value;
+
+  public:
+
+    base_type& base() & noexcept
+    {
+        return base_value;
+    }
+
+    const base_type& base() const & noexcept
+    {
+        return base_value;
+    }
+
+    base_type&& base() && noexcept
+    {
+        return std::move(base_value);
+    }
+
+    constexpr optional() noexcept(noexcept(std::optional<noexcept_fix_t>()))
+        : base_value() // explicitly initialized to mitigate -Werror=effc++
+    {
+    }
+
+    constexpr optional(std::nullopt_t /* unused */) noexcept
+        : base_value(std::nullopt)
+    {
+    }
+
+    template <typename U, use_conversion_operator<U> = 0>
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value).operator optional())
+         )) :
+            base_value(std::forward<U>(value).operator optional())
+    {
+    }
+
+    template <typename U = T, use_implicit_forwarding<U> = 0>
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value))
+        )) :
+            base_value(std::forward<U>(value))
+    {
+    }
+
+    template <typename U, use_explicit_forwarding<U> = 0>
+    explicit
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value))
+        )) :
+            base_value(std::forward<U>(value))
+    {
+    }
+
+    template <typename U, typename... Args, can_construct_in_place_from<U, Args...> = 0>
+    explicit
+    constexpr optional(std::in_place_t /* unused */, U&& u, Args&&... args)
+        noexcept(noexcept(
+            base_type(std::in_place, std::forward<U>(u), std::forward<Args>(args)...)
+        )) :
+            base_value(std::in_place, std::forward<U>(u), std::forward<Args>(args)...)
+    {
+    }
+
+    template <typename U, typename... Args, can_construct_in_place_from<std::initializer_list<U>&, Args...> = 0>
+    explicit
+    constexpr optional(std::in_place_t /* unused */, std::initializer_list<U> u, Args&&... args)
+        noexcept(noexcept(
+            base_type(std::in_place, u, std::forward<Args>(args)...)
+        )) :
+            base_value(std::in_place, u, std::forward<Args>(args)...)
+    {
+    }
+
+    constexpr T& operator *() & noexcept { return *base_value; }
+    constexpr const T& operator *() const& noexcept { return *base_value; }
+
+    constexpr T&& operator *() && noexcept { return static_cast<T&&>(*base_value); }
+    constexpr const T&& operator *() const&& noexcept { return static_cast<const T&&>(*base_value); }
+
+    constexpr T* operator ->() noexcept { return base_value.operator ->(); }
+    constexpr const T* operator ->() const noexcept { return base_value.operator ->(); }
+
+    operator base_type& () & noexcept { return base_value; }
+    operator base_type&& () && noexcept { return std::move(base_value); }
+
+    // *INDENT-ON*
+};
+
+namespace detail::opt
+{
+
+template <typename T> const T& cmp_val(const T& v)
+{
+    return v;
+}
+template <typename T> const std::optional<T>& cmp_val(const optional<T>& v)
+{
+    return v.base();
+}
+
+}  // namespace detail::opt
+
+#define JSON_OPTIONAL_COMPARISON_EXPR(OP) \
+    detail::opt::cmp_val(lhs) OP detail::opt::cmp_val(rhs)
+
+#define JSON_OPTIONAL_COMPARISON(OP, LHS, RHS) \
+    template <typename A, typename B> \
+    auto operator OP (const LHS& lhs, const RHS& rhs) \
+    -> decltype(JSON_OPTIONAL_COMPARISON_EXPR(OP)) \
+    { \
+        return JSON_OPTIONAL_COMPARISON_EXPR(OP); \
+    }
+
+#ifdef JSON_HAS_CPP_20
+
+// *INDENT-OFF*
+
+JSON_OPTIONAL_COMPARISON( <=>, optional<A>, B)
+
+// *INDENT-ON*
+
+JSON_OPTIONAL_COMPARISON( ==, optional<A>, B)
+JSON_OPTIONAL_COMPARISON( ==, optional<A>, std::optional<B>)
+JSON_OPTIONAL_COMPARISON( ==, std::optional<A>, optional<B>)
+
+#else // JSON_HAS_CPP_20
+
+#define JSON_OPTIONAL_COMPARISON_OP(OP) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, std::optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, std::optional<A>, optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, B) \
+    JSON_OPTIONAL_COMPARISON(OP, A, optional<B>)
+
+JSON_OPTIONAL_COMPARISON_OP( == )
+JSON_OPTIONAL_COMPARISON_OP( != )
+JSON_OPTIONAL_COMPARISON_OP( < )
+JSON_OPTIONAL_COMPARISON_OP( <= )
+JSON_OPTIONAL_COMPARISON_OP( > )
+JSON_OPTIONAL_COMPARISON_OP( >= )
+
+#undef JSON_OPTIONAL_COMPARISON_OP
+
+#endif // JSON_HAS_CPP_20
+
+#undef JSON_OPTIONAL_COMPARISON
+#undef JSON_OPTIONAL_COMPARISON_EXPR
+
+NLOHMANN_JSON_NAMESPACE_END
+
+#endif // JSON_HAS_CPP_17

single_include/nlohmann/json.hpp

@@ -4655,6 +4655,233 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/value_t.hpp>
 
+// #include <nlohmann/optional.hpp>
+
+
+// #include <nlohmann/detail/macro_scope.hpp>
+
+
+#ifdef JSON_HAS_CPP_17
+
+#include <optional>
+#include <utility>
+
+NLOHMANN_JSON_NAMESPACE_BEGIN
+
+template <typename T>
+class optional
+{
+    // *INDENT-OFF*
+
+    using base_type = std::optional<T>;
+    using value_type = T;
+
+    template <typename U, typename = optional>
+    struct has_conversion_operator : std::false_type { };
+
+    template <typename U>
+    struct has_conversion_operator<U,
+        decltype(std::declval<U>().operator optional())> : std::true_type { };
+
+    template <typename... U>
+    using is_base_constructible_from = std::is_constructible<base_type, U...>;
+
+    template <typename U>
+    using is_convertible_to_base = std::is_convertible<U, base_type>;
+
+    template <typename U>
+    using enable_int_if = std::enable_if_t<U::value, int>;
+
+    template <typename U>
+    using use_conversion_operator =
+        enable_int_if<
+            has_conversion_operator<U>
+        >;
+
+    template <typename U>
+    using use_implicit_forwarding =
+        enable_int_if<
+            std::conjunction<
+                std::negation<has_conversion_operator<U>>,
+                is_base_constructible_from<U>,
+                is_convertible_to_base<U>
+            >
+        >;
+
+    template <typename U>
+    using use_explicit_forwarding =
+        enable_int_if<
+            std::conjunction<
+                std::negation<has_conversion_operator<U>>,
+                is_base_constructible_from<U>,
+                std::negation<is_convertible_to_base<U>>
+            >
+        >;
+
+    template <typename... U>
+    using can_construct_in_place_from =
+        enable_int_if<
+            is_base_constructible_from<std::in_place_t, U...>
+        >;
+
+    struct noexcept_fix_t {}; // trick for GCC8.1 (see default constructor)
+
+    base_type base_value;
+
+  public:
+
+    base_type& base() & noexcept
+    {
+        return base_value;
+    }
+
+    const base_type& base() const & noexcept
+    {
+        return base_value;
+    }
+
+    base_type&& base() && noexcept
+    {
+        return std::move(base_value);
+    }
+
+    constexpr optional() noexcept(noexcept(std::optional<noexcept_fix_t>()))
+        : base_value() // explicitly initialized to mitigate -Werror=effc++
+    {
+    }
+
+    constexpr optional(std::nullopt_t /* unused */) noexcept
+        : base_value(std::nullopt)
+    {
+    }
+
+    template <typename U, use_conversion_operator<U> = 0>
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value).operator optional())
+         )) :
+            base_value(std::forward<U>(value).operator optional())
+    {
+    }
+
+    template <typename U = T, use_implicit_forwarding<U> = 0>
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value))
+        )) :
+            base_value(std::forward<U>(value))
+    {
+    }
+
+    template <typename U, use_explicit_forwarding<U> = 0>
+    explicit
+    constexpr optional(U&& value)
+        noexcept(noexcept(
+            base_type(std::forward<U>(value))
+        )) :
+            base_value(std::forward<U>(value))
+    {
+    }
+
+    template <typename U, typename... Args, can_construct_in_place_from<U, Args...> = 0>
+    explicit
+    constexpr optional(std::in_place_t /* unused */, U&& u, Args&&... args)
+        noexcept(noexcept(
+            base_type(std::in_place, std::forward<U>(u), std::forward<Args>(args)...)
+        )) :
+            base_value(std::in_place, std::forward<U>(u), std::forward<Args>(args)...)
+    {
+    }
+
+    template <typename U, typename... Args, can_construct_in_place_from<std::initializer_list<U>&, Args...> = 0>
+    explicit
+    constexpr optional(std::in_place_t /* unused */, std::initializer_list<U> u, Args&&... args)
+        noexcept(noexcept(
+            base_type(std::in_place, u, std::forward<Args>(args)...)
+        )) :
+            base_value(std::in_place, u, std::forward<Args>(args)...)
+    {
+    }
+
+    constexpr T& operator *() & noexcept { return *base_value; }
+    constexpr const T& operator *() const& noexcept { return *base_value; }
+
+    constexpr T&& operator *() && noexcept { return static_cast<T&&>(*base_value); }
+    constexpr const T&& operator *() const&& noexcept { return static_cast<const T&&>(*base_value); }
+
+    constexpr T* operator ->() noexcept { return base_value.operator ->(); }
+    constexpr const T* operator ->() const noexcept { return base_value.operator ->(); }
+
+    operator base_type& () & noexcept { return base_value; }
+    operator base_type&& () && noexcept { return std::move(base_value); }
+
+    // *INDENT-ON*
+};
+
+namespace detail::opt
+{
+
+template <typename T> const T& cmp_val(const T& v)
+{
+    return v;
+}
+template <typename T> const std::optional<T>& cmp_val(const optional<T>& v)
+{
+    return v.base();
+}
+
+}  // namespace detail::opt
+
+#define JSON_OPTIONAL_COMPARISON_EXPR(OP) \
+    detail::opt::cmp_val(lhs) OP detail::opt::cmp_val(rhs)
+
+#define JSON_OPTIONAL_COMPARISON(OP, LHS, RHS) \
+    template <typename A, typename B> \
+    auto operator OP (const LHS& lhs, const RHS& rhs) \
+    -> decltype(JSON_OPTIONAL_COMPARISON_EXPR(OP)) \
+    { \
+        return JSON_OPTIONAL_COMPARISON_EXPR(OP); \
+    }
+
+#ifdef JSON_HAS_CPP_20
+
+// *INDENT-OFF*
+
+JSON_OPTIONAL_COMPARISON( <=>, optional<A>, B)
+
+// *INDENT-ON*
+
+JSON_OPTIONAL_COMPARISON( ==, optional<A>, B)
+JSON_OPTIONAL_COMPARISON( ==, optional<A>, std::optional<B>)
+JSON_OPTIONAL_COMPARISON( ==, std::optional<A>, optional<B>)
+
+#else // JSON_HAS_CPP_20
+
+#define JSON_OPTIONAL_COMPARISON_OP(OP) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, std::optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, std::optional<A>, optional<B>) \
+    JSON_OPTIONAL_COMPARISON(OP, optional<A>, B) \
+    JSON_OPTIONAL_COMPARISON(OP, A, optional<B>)
+
+JSON_OPTIONAL_COMPARISON_OP( == )
+JSON_OPTIONAL_COMPARISON_OP( != )
+JSON_OPTIONAL_COMPARISON_OP( < )
+JSON_OPTIONAL_COMPARISON_OP( <= )
+JSON_OPTIONAL_COMPARISON_OP( > )
+JSON_OPTIONAL_COMPARISON_OP( >= )
+
+#undef JSON_OPTIONAL_COMPARISON_OP
+
+#endif // JSON_HAS_CPP_20
+
+#undef JSON_OPTIONAL_COMPARISON
+#undef JSON_OPTIONAL_COMPARISON_EXPR
+
+NLOHMANN_JSON_NAMESPACE_END
+
+#endif // JSON_HAS_CPP_17
+
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -4670,6 +4897,34 @@ inline void from_json(const BasicJsonType& j, typename std::nullptr_t& n)
     n = nullptr;
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType, typename T>
+void from_json(const BasicJsonType& j, std::optional<T>& opt)
+{
+    if (j.is_null())
+    {
+        opt = std::nullopt;
+    }
+    else
+    {
+        opt = j.template get<T>();
+    }
+}
+
+template<typename BasicJsonType, typename T>
+void from_json(const BasicJsonType& j, optional<T>& opt)
+{
+    if (j.is_null())
+    {
+        opt = std::nullopt;
+    }
+    else
+    {
+        opt = j.template get<T>();
+    }
+}
+#endif
+
 // overloads for basic_json template parameters
 template < typename BasicJsonType, typename ArithmeticType,
            enable_if_t < std::is_arithmetic<ArithmeticType>::value&&
@@ -5400,6 +5655,8 @@ class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
 
 // #include <nlohmann/detail/value_t.hpp>
 
+// #include <nlohmann/optional.hpp>
+
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -5637,6 +5894,29 @@ struct external_constructor<value_t::object>
 // to_json //
 /////////////
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType, typename T,
+         enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
+void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
+{
+    if (opt.has_value())
+    {
+        j = *opt;
+    }
+    else
+    {
+        j = nullptr;
+    }
+}
+
+template<typename BasicJsonType, typename T,
+         enable_if_t<std::is_constructible<BasicJsonType, T>::value, int> = 0>
+void to_json(BasicJsonType& j, const optional<T>& opt) noexcept
+{
+    to_json(j, opt.base());
+}
+#endif
+
 template<typename BasicJsonType, typename T,
          enable_if_t<std::is_same<T, typename BasicJsonType::boolean_t>::value, int> = 0>
 inline void to_json(BasicJsonType& j, T b) noexcept

tests/src/unit-conversions.cpp

@@ -31,6 +31,15 @@ using nlohmann::json;
 // NLOHMANN_JSON_SERIALIZE_ENUM uses a static std::pair
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
 DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wunused-macros")
+
+// For testing copy-initialization of std::optional and nlohmann::optional
+// (clang doesn't need the suppressing)
+#define SUPPRESS_CONVERSION_WARNING \
+    DOCTEST_GCC_SUPPRESS_WARNING_WITH_PUSH("-Wconversion")
+
+#define RESTORE_CONVERSION_WARNING \
+    DOCTEST_GCC_SUPPRESS_WARNING_POP
 
 TEST_CASE("value conversion")
 {
@@ -1569,4 +1578,147 @@ TEST_CASE("JSON to enum mapping")
     }
 }
 
+#ifdef JSON_HAS_CPP_17
+TEST_CASE("std::optional")
+{
+    SECTION("null")
+    {
+        json j_null;
+        std::optional<std::string> const opt_null;
+
+        CHECK(json(opt_null) == j_null);
+        CHECK(j_null.get<std::optional<std::string>>() == std::nullopt);
+
+        using opt_int = std::optional<int>;
+
+        auto opt1 = []() -> opt_int { return json().get<opt_int>(); };
+        auto opt2 = []() -> opt_int { return opt_int(json()); }; // NOLINT(modernize-return-braced-init-list)
+
+        CHECK(opt1() == std::nullopt);
+        CHECK_THROWS_AS(opt2(), json::type_error&);
+
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        SUPPRESS_CONVERSION_WARNING
+        auto opt3 = []() -> opt_int { return json(); };
+        RESTORE_CONVERSION_WARNING
+        CHECK_THROWS_AS(opt3(), json::type_error&);
+#endif
+    }
+
+    SECTION("string")
+    {
+        json j_string = "string";
+        std::optional<std::string> opt_string = "string";
+
+        CHECK(json(opt_string) == j_string);
+        CHECK(std::optional<std::string>(j_string) == opt_string);
+    }
+
+    SECTION("bool")
+    {
+        json j_bool = true;
+        std::optional<bool> opt_bool = true;
+
+        CHECK(json(opt_bool) == j_bool);
+        CHECK(std::optional<bool>(j_bool) == opt_bool);
+    }
+
+    SECTION("number")
+    {
+        json j_number = 1;
+        std::optional<int> opt_int = 1;
+
+        CHECK(json(opt_int) == j_number);
+        CHECK(std::optional<int>(j_number) == opt_int);
+    }
+
+    SECTION("array")
+    {
+        json j_array = {1, 2, nullptr};
+        std::vector<std::optional<int>> opt_array = {{1, 2, std::nullopt}};
+
+        CHECK(json(opt_array) == j_array);
+        CHECK(j_array.get<std::vector<std::optional<int>>>() == opt_array);
+    }
+
+    SECTION("object")
+    {
+        json j_object = {{"one", 1}, {"two", 2}, {"zero", nullptr}};
+        std::map<std::string, std::optional<int>> opt_object {{"one", 1}, {"two", 2}, {"zero", std::nullopt}};
+
+        CHECK(json(opt_object) == j_object);
+        CHECK(j_object.get<std::map<std::string, std::optional<int>>>() == opt_object);
+    }
+}
+
+TEST_CASE("nlohmann::optional")
+{
+    SECTION("null")
+    {
+        json j_null;
+        nlohmann::optional<std::string> const opt_null;
+
+        CHECK(json(opt_null) == j_null);
+        CHECK(j_null.get<nlohmann::optional<std::string>>() == std::nullopt);
+
+        using opt_int = nlohmann::optional<int>;
+
+        auto opt1 = []() -> opt_int { return json().get<opt_int>(); };
+        auto opt2 = []() -> opt_int { return opt_int(json()); }; // NOLINT(modernize-return-braced-init-list)
+        SUPPRESS_CONVERSION_WARNING
+        auto opt3 = []() -> opt_int { return json(); };
+        RESTORE_CONVERSION_WARNING
+
+        CHECK(opt1() == std::nullopt);
+        CHECK(opt2() == std::nullopt);
+        CHECK(opt3() == std::nullopt);
+    }
+
+    SECTION("string")
+    {
+        json j_string = "string";
+        nlohmann::optional<std::string> opt_string = "string";
+
+        CHECK(json(opt_string) == j_string);
+        CHECK(nlohmann::optional<std::string>(j_string) == opt_string);
+    }
+
+    SECTION("bool")
+    {
+        json j_bool = true;
+        nlohmann::optional<bool> opt_bool = true;
+
+        CHECK(json(opt_bool) == j_bool);
+        CHECK(nlohmann::optional<bool>(j_bool) == opt_bool);
+    }
+
+    SECTION("number")
+    {
+        json j_number = 1;
+        nlohmann::optional<int> opt_int = 1;
+
+        CHECK(json(opt_int) == j_number);
+        CHECK(nlohmann::optional<int>(j_number) == opt_int);
+    }
+
+    SECTION("array")
+    {
+        json j_array = {1, 2, nullptr};
+        std::vector<nlohmann::optional<int>> opt_array = {{1, 2, std::nullopt}};
+
+        CHECK(json(opt_array) == j_array);
+        CHECK(j_array.get<std::vector<nlohmann::optional<int>>>() == opt_array);
+    }
+
+    SECTION("object")
+    {
+        json j_object = {{"one", 1}, {"two", 2}, {"zero", nullptr}};
+        std::map<std::string, nlohmann::optional<int>> opt_object{{"one", 1}, {"two", 2}, {"zero", std::nullopt}};
+
+        CHECK(json(opt_object) == j_object);
+        CHECK(j_object.get<std::map<std::string, nlohmann::optional<int>>>() == opt_object);
+    }
+}
+#endif
+
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-optional.cpp

@@ -0,0 +1,181 @@
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#ifdef JSON_HAS_CPP_17
+
+DOCTEST_GCC_SUPPRESS_WARNING_PUSH
+DOCTEST_GCC_SUPPRESS_WARNING("-Wuseless-cast")
+
+#include <memory>
+#include <vector>
+#include <utility>
+
+using std::nullopt;
+using std::in_place;
+using nlohmann::optional;
+
+using opt_int = optional<int>;
+using opt_vec = optional<std::vector<int>>;
+using opt_ptr = optional<std::unique_ptr<int>>;
+
+using std_opt_int = std::optional<int>;
+using std_opt_vec = std::optional<std::vector<int>>;
+using std_opt_ptr = std::optional<std::unique_ptr<int>>;
+
+// NOLINTBEGIN(bugprone-unchecked-optional-access,bugprone-use-after-move,hicpp-invalid-access-moved,clang-analyzer-cplusplus.Move)
+
+TEST_CASE("nlohmann::optional comparison")
+{
+    CHECK(opt_int() == nullopt);
+    CHECK(nullopt == opt_int());
+    CHECK(opt_int() == opt_int(nullopt));
+    CHECK(opt_int(0) == opt_int(0));
+    CHECK_FALSE(opt_int(0) == nullopt);
+    CHECK_FALSE(nullopt == opt_int(0));
+    CHECK_FALSE(opt_int(0) == opt_int(1));
+
+    CHECK(opt_int() == std_opt_int());
+    CHECK(opt_int(0) == std_opt_int(0));
+    CHECK_FALSE(opt_int(0) == std_opt_int(1));
+
+    CHECK(std_opt_int() == opt_int());
+    CHECK(std_opt_int(0) == opt_int(0));
+    CHECK_FALSE(std_opt_int(0) == opt_int(1));
+
+    CHECK(opt_int(0) != nullopt);
+    CHECK(nullopt != opt_int(0));
+    CHECK(opt_int(0) != opt_int(1));
+    CHECK_FALSE(opt_int() != nullopt);
+    CHECK_FALSE(nullopt != opt_int());
+    CHECK_FALSE(opt_int() != opt_int(nullopt));
+    CHECK_FALSE(opt_int(0) != opt_int(0));
+
+    CHECK_FALSE(opt_int() != std_opt_int());
+    CHECK_FALSE(opt_int(0) != std_opt_int(0));
+    CHECK(opt_int(0) != std_opt_int(1));
+
+    CHECK_FALSE(std_opt_int() != opt_int());
+    CHECK_FALSE(std_opt_int(0) != opt_int(0));
+    CHECK(std_opt_int(0) != opt_int(1));
+
+    CHECK(opt_int(0) > nullopt);
+    CHECK(opt_int(1) > opt_int(0));
+    CHECK_FALSE(nullopt > opt_int(0));
+    CHECK_FALSE(opt_int(0) > opt_int(1));
+
+    CHECK(opt_int(0) > std_opt_int());
+    CHECK(std_opt_int(0) > opt_int());
+
+    CHECK(opt_int(0) >= nullopt);
+    CHECK(opt_int(1) >= opt_int(0));
+    CHECK_FALSE(nullopt >= opt_int(0));
+    CHECK_FALSE(opt_int(0) >= opt_int(1));
+
+    CHECK(opt_int(0) >= std_opt_int());
+    CHECK(std_opt_int(0) >= opt_int());
+
+    CHECK(nullopt < opt_int(0));
+    CHECK(opt_int(0) < opt_int(1));
+    CHECK_FALSE(opt_int(0) < nullopt);
+    CHECK_FALSE(opt_int(1) < opt_int(0));
+
+    CHECK_FALSE(opt_int(0) < std_opt_int());
+    CHECK_FALSE(std_opt_int(0) < opt_int());
+
+    CHECK(nullopt <= opt_int(0));
+    CHECK(opt_int(0) <= opt_int(1));
+    CHECK_FALSE(opt_int(0) <= nullopt);
+    CHECK_FALSE(opt_int(1) <= opt_int(0));
+
+    CHECK_FALSE(opt_int(0) <= std_opt_int());
+    CHECK_FALSE(std_opt_int(0) <= opt_int());
+}
+
+TEST_CASE("nlohmann::optional constructors")
+{
+    struct S1
+    {
+        operator int() noexcept
+        {
+            return 0;
+        }
+    };
+    struct S2 : S1
+    {
+        operator opt_int() noexcept
+        {
+            return nullopt;
+        }
+    };
+
+    CHECK(opt_int(S1()) == opt_int(0));
+    CHECK(opt_int(S2()) == nullopt);
+
+    CHECK(opt_int(S1()) == std_opt_int(S1()));
+    CHECK(opt_int(S2()) != std_opt_int(S2()));
+
+    CHECK(opt_int(std_opt_int(0)) == opt_int(0));
+    CHECK(std_opt_int(opt_int(0)) == opt_int(0));
+
+    CHECK(opt_int(in_place) == std_opt_int(in_place));
+    CHECK(opt_vec(in_place) == std_opt_vec(in_place));
+    CHECK(opt_ptr(in_place) == std_opt_ptr(in_place));
+
+    CHECK(opt_vec(in_place, 5)->size() == 5);
+    CHECK(opt_vec(in_place, {1, 2, 3}) == std_opt_vec(in_place, {1, 2, 3}));
+    CHECK(**opt_ptr(in_place, new int{42}) == **std_opt_ptr(in_place, new int{42}));
+
+    std::vector<int> vec{1, 2, 3};
+    CHECK(*opt_vec(in_place, vec.begin(), vec.end()) == vec);
+
+    CHECK(opt_vec({1, 2, 3})->size() == 3);
+}
+
+TEST_CASE("nlohmann::optional copy")
+{
+    opt_int opt1 = 111;
+    std_opt_int opt2 = 222;
+
+    SECTION("1")
+    {
+        opt1 = std::as_const(opt2);
+        CHECK(*opt1 == 222);
+        CHECK(*opt_int(std::as_const(opt1).base()) == 222);
+    }
+
+    SECTION("2")
+    {
+        opt2 = std::as_const(opt1).base();
+        CHECK(*opt2 == 111);
+        CHECK(*opt_int(std::as_const(opt2)) == 111);
+    }
+}
+
+TEST_CASE("nlohmann::optional move")
+{
+    opt_ptr opt1(new int(111));
+    std_opt_ptr opt2(new int(222));
+
+    SECTION("1")
+    {
+        opt1 = std::move(opt2);
+        CHECK(*opt2 == nullptr);
+        CHECK(**opt1 == 222);
+        CHECK(**opt_ptr(std::move(opt1)) == 222);
+    }
+
+    SECTION("2")
+    {
+        opt2 = std::move(opt1);
+        CHECK(*opt1 == nullptr);
+        CHECK(**opt2 == 111);
+        CHECK(**opt_ptr(std::move(opt2)) == 111);
+    }
+}
+
+// NOLINTEND(bugprone-unchecked-optional-access,bugprone-use-after-move,hicpp-invalid-access-moved,clang-analyzer-cplusplus.Move)
+
+DOCTEST_GCC_SUPPRESS_WARNING_POP
+
+#endif  // JSON_HAS_CPP_17