Make json_pointer usable as map key (again) (#3685)

* Make json_pointer usable as map key

* Add unit tests

include/nlohmann/detail/json_pointer.hpp

@@ -847,7 +847,7 @@ class json_pointer
     }
 
   public:
-#ifdef JSON_HAS_CPP_20
+#if JSON_HAS_THREE_WAY_COMPARISON
     /// @brief compares two JSON pointers for equality
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_eq/
     template<typename RefStringTypeRhs>
@@ -862,6 +862,13 @@ class json_pointer
     {
         return *this == json_pointer(rhs);
     }
+
+    /// @brief 3-way compares two JSON pointers
+    template<typename RefStringTypeRhs>
+    std::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept // *NOPAD*
+    {
+        return  reference_tokens <=> rhs.reference_tokens; // *NOPAD*
+    }
 #else
     /// @brief compares two JSON pointers for equality
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_eq/
@@ -904,6 +911,12 @@ class json_pointer
     // NOLINTNEXTLINE(readability-redundant-declaration)
     friend bool operator!=(const StringType& lhs,
                            const json_pointer<RefStringTypeRhs>& rhs);
+
+    /// @brief compares two JSON pointer for less-than
+    template<typename RefStringTypeLhs, typename RefStringTypeRhs>
+    // NOLINTNEXTLINE(readability-redundant-declaration)
+    friend bool operator<(const json_pointer<RefStringTypeLhs>& lhs,
+                          const json_pointer<RefStringTypeRhs>& rhs) noexcept;
 #endif
 
   private:
@@ -911,7 +924,7 @@ class json_pointer
     std::vector<string_t> reference_tokens;
 };
 
-#ifndef JSON_HAS_CPP_20
+#if !JSON_HAS_THREE_WAY_COMPARISON
 // functions cannot be defined inside class due to ODR violations
 template<typename RefStringTypeLhs, typename RefStringTypeRhs>
 inline bool operator==(const json_pointer<RefStringTypeLhs>& lhs,
@@ -958,6 +971,13 @@ inline bool operator!=(const StringType& lhs,
 {
     return !(lhs == rhs);
 }
+
+template<typename RefStringTypeLhs, typename RefStringTypeRhs>
+inline bool operator<(const json_pointer<RefStringTypeLhs>& lhs,
+                      const json_pointer<RefStringTypeRhs>& rhs) noexcept
+{
+    return lhs.reference_tokens < rhs.reference_tokens;
+}
 #endif
 
 NLOHMANN_JSON_NAMESPACE_END

single_include/nlohmann/json.hpp

@@ -14507,7 +14507,7 @@ class json_pointer
     }
 
   public:
-#ifdef JSON_HAS_CPP_20
+#if JSON_HAS_THREE_WAY_COMPARISON
     /// @brief compares two JSON pointers for equality
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_eq/
     template<typename RefStringTypeRhs>
@@ -14522,6 +14522,13 @@ class json_pointer
     {
         return *this == json_pointer(rhs);
     }
+
+    /// @brief 3-way compares two JSON pointers
+    template<typename RefStringTypeRhs>
+    std::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept // *NOPAD*
+    {
+        return  reference_tokens <=> rhs.reference_tokens; // *NOPAD*
+    }
 #else
     /// @brief compares two JSON pointers for equality
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_eq/
@@ -14564,6 +14571,12 @@ class json_pointer
     // NOLINTNEXTLINE(readability-redundant-declaration)
     friend bool operator!=(const StringType& lhs,
                            const json_pointer<RefStringTypeRhs>& rhs);
+
+    /// @brief compares two JSON pointer for less-than
+    template<typename RefStringTypeLhs, typename RefStringTypeRhs>
+    // NOLINTNEXTLINE(readability-redundant-declaration)
+    friend bool operator<(const json_pointer<RefStringTypeLhs>& lhs,
+                          const json_pointer<RefStringTypeRhs>& rhs) noexcept;
 #endif
 
   private:
@@ -14571,7 +14584,7 @@ class json_pointer
     std::vector<string_t> reference_tokens;
 };
 
-#ifndef JSON_HAS_CPP_20
+#if !JSON_HAS_THREE_WAY_COMPARISON
 // functions cannot be defined inside class due to ODR violations
 template<typename RefStringTypeLhs, typename RefStringTypeRhs>
 inline bool operator==(const json_pointer<RefStringTypeLhs>& lhs,
@@ -14618,6 +14631,13 @@ inline bool operator!=(const StringType& lhs,
 {
     return !(lhs == rhs);
 }
+
+template<typename RefStringTypeLhs, typename RefStringTypeRhs>
+inline bool operator<(const json_pointer<RefStringTypeLhs>& lhs,
+                      const json_pointer<RefStringTypeRhs>& rhs) noexcept
+{
+    return lhs.reference_tokens < rhs.reference_tokens;
+}
 #endif
 
 NLOHMANN_JSON_NAMESPACE_END

tests/src/unit-json_pointer.cpp

@@ -15,6 +15,7 @@ using nlohmann::json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
+#include <map>
 #include <sstream>
 
 TEST_CASE("JSON pointers")
@@ -697,6 +698,32 @@ TEST_CASE("JSON pointers")
         }
     }
 
+    SECTION("less-than comparison")
+    {
+        auto ptr1 = json::json_pointer("/foo/a");
+        auto ptr2 = json::json_pointer("/foo/b");
+
+        CHECK(ptr1 < ptr2);
+        CHECK_FALSE(ptr2 < ptr1);
+
+        // build with C++20
+        // JSON_HAS_CPP_20
+#if JSON_HAS_THREE_WAY_COMPARISON
+        CHECK((ptr1 <=> ptr2) == std::strong_ordering::less); // *NOPAD*
+        CHECK(ptr2 > ptr1);
+#endif
+    }
+
+    SECTION("usable as map key")
+    {
+        auto ptr = json::json_pointer("/foo");
+        std::map<json::json_pointer, int> m;
+
+        m[ptr] = 42;
+
+        CHECK(m.find(ptr) != m.end());
+    }
+
     SECTION("backwards compatibility and mixing")
     {
         json j = R"(