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Enable string_view keys in operator[] by updating is_usable_as_basic_json_key_type (#3912) #4958

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Enable string_view keys in operator[] by updating is_usable_as_basic_json_key_type (#3912) #4958

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13 changes: 8 additions & 5 deletions include/nlohmann/detail/meta/type_traits.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -621,11 +621,14 @@ using is_usable_as_key_type = typename std::conditional <
template<typename BasicJsonType, typename KeyTypeCVRef, bool RequireTransparentComparator = true,
bool ExcludeObjectKeyType = RequireTransparentComparator, typename KeyType = uncvref_t<KeyTypeCVRef>>
using is_usable_as_basic_json_key_type = typename std::conditional <
is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
RequireTransparentComparator, ExcludeObjectKeyType>::value
&& !is_json_iterator_of<BasicJsonType, KeyType>::value,
std::true_type,
(is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
RequireTransparentComparator, ExcludeObjectKeyType>::value
&& !is_json_iterator_of<BasicJsonType, KeyType>::value)
#ifdef JSON_HAS_CPP_17
|| std::is_convertible<KeyType, std::string_view>::value
#endif
, std::true_type,
std::false_type >::type;

template<typename ObjectType, typename KeyType>
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13 changes: 8 additions & 5 deletions single_include/nlohmann/json.hpp
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Expand Up @@ -4164,11 +4164,14 @@ using is_usable_as_key_type = typename std::conditional <
template<typename BasicJsonType, typename KeyTypeCVRef, bool RequireTransparentComparator = true,
bool ExcludeObjectKeyType = RequireTransparentComparator, typename KeyType = uncvref_t<KeyTypeCVRef>>
using is_usable_as_basic_json_key_type = typename std::conditional <
is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
RequireTransparentComparator, ExcludeObjectKeyType>::value
&& !is_json_iterator_of<BasicJsonType, KeyType>::value,
std::true_type,
(is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
RequireTransparentComparator, ExcludeObjectKeyType>::value
&& !is_json_iterator_of<BasicJsonType, KeyType>::value)
#ifdef JSON_HAS_CPP_17
|| std::is_convertible<KeyType, std::string_view>::value
#endif
, std::true_type,
std::false_type >::type;

template<typename ObjectType, typename KeyType>
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52 changes: 52 additions & 0 deletions tests/src/unit-element_access2.cpp
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Expand Up @@ -1790,3 +1790,55 @@ TEST_CASE_TEMPLATE("element access 2 (additional value() tests)", Json, nlohmann
#endif
}
}

#ifdef JSON_HAS_CPP_17
TEST_CASE("operator[] with user-defined std::string_view-convertible types")
{
using json = nlohmann::json;

class TestClass
{
std::string key_data_ = "foo";

public:
operator std::string_view() const
{
return key_data_;
}
};

struct TestStruct
{
operator std::string_view() const
{
return "bar";
}
};

json j = {{"foo", "from_class"}, {"bar", "from_struct"}};
TestClass foo_obj;
TestStruct bar_obj;

SECTION("read access")
{
CHECK(j[foo_obj] == "from_class");
CHECK(j[TestClass{}] == "from_class");
CHECK(j[bar_obj] == "from_struct");
CHECK(j[TestStruct{}] == "from_struct");
}

SECTION("write access")
{
j[TestClass{}] = "updated_class";
j[TestStruct{}] = "updated_struct";
CHECK(j["foo"] == "updated_class");
CHECK(j["bar"] == "updated_struct");

SECTION("direct std::string_view access")
{
CHECK(j[std::string_view{"foo"}] == "updated_class");
CHECK(j[std::string_view{"bar"}] == "updated_struct");
}
}
}
#endif