Fix return value of get_ptr for unsigned integers (#4525)

* 🐛 fix return value of get_ptr for unsigned integers

* 📝 update documentation

docs/mkdocs/docs/api/basic_json/get_ptr.md

@@ -35,7 +35,35 @@ Constant.
 
 !!! danger "Undefined behavior"
 
-    Writing data to the pointee of the result yields an undefined state.
+    The pointer becomes invalid if the underlying JSON object changes.
+
+    Consider the following example code where the pointer `ptr` changes after the array is resized. As a result, reading or writing to `ptr` after the array change would be undefined behavior. The address of the first array element changes, because the underlying `std::vector` is resized after adding a fifth element.
+
+    ```cpp
+    #include <iostream>
+    #include <nlohmann/json.hpp>
+
+    using json = nlohmann::json;
+
+    int main()
+    {
+        json j = {1, 2, 3, 4};
+        auto* ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+
+        j.push_back(5);
+
+        ptr = j[0].get_ptr<std::int64_t*>();
+        std::cout << "value at " << ptr << " is " << *ptr << std::endl;
+    }
+    ```
+
+    Output:
+
+    ```
+    value at 0x6000012fc1c8 is 1
+    value at 0x6000029fc088 is 1
+    ```
 
 ## Examples
 
@@ -54,6 +82,10 @@ Constant.
     --8<-- "examples/get_ptr.output"
     ```
 
+## See also
+
+- [get_ref()](get_ref.md) get a reference value
+
 ## Version history
 
 - Added in version 1.0.0.

docs/mkdocs/docs/api/basic_json/get_ref.md

@@ -40,7 +40,7 @@ Constant.
 
 !!! danger "Undefined behavior"
 
-    Writing data to the referee of the result yields an undefined state.
+    The reference becomes invalid if the underlying JSON object changes.
 
 ## Examples
 
@@ -58,6 +58,10 @@ Constant.
     --8<-- "examples/get_ref.output"
     ```
 
+## See also
+
+- [get_ptr()](get_ptr.md) get a pointer value
+
 ## Version history
 
 - Added in version 1.1.0.

include/nlohmann/json.hpp

@@ -1463,13 +1463,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// get a pointer to the value (integer number)
     number_integer_t* get_impl_ptr(number_integer_t* /*unused*/) noexcept
     {
-        return is_number_integer() ? &m_data.m_value.number_integer : nullptr;
+        return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
     /// get a pointer to the value (integer number)
     constexpr const number_integer_t* get_impl_ptr(const number_integer_t* /*unused*/) const noexcept
     {
-        return is_number_integer() ? &m_data.m_value.number_integer : nullptr;
+        return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
     /// get a pointer to the value (unsigned number)

single_include/nlohmann/json.hpp

@@ -20962,13 +20962,13 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// get a pointer to the value (integer number)
     number_integer_t* get_impl_ptr(number_integer_t* /*unused*/) noexcept
     {
-        return is_number_integer() ? &m_data.m_value.number_integer : nullptr;
+        return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
     /// get a pointer to the value (integer number)
     constexpr const number_integer_t* get_impl_ptr(const number_integer_t* /*unused*/) const noexcept
     {
-        return is_number_integer() ? &m_data.m_value.number_integer : nullptr;
+        return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
     /// get a pointer to the value (unsigned number)

tests/src/unit-pointer_access.cpp

@@ -326,7 +326,7 @@ TEST_CASE("pointer access")
         CHECK(value.get_ptr<json::array_t*>() == nullptr);
         CHECK(value.get_ptr<json::string_t*>() == nullptr);
         CHECK(value.get_ptr<json::boolean_t*>() == nullptr);
-        CHECK(value.get_ptr<json::number_integer_t*>() != nullptr);
+        CHECK(value.get_ptr<json::number_integer_t*>() == nullptr);
         CHECK(value.get_ptr<json::number_unsigned_t*>() != nullptr);
         CHECK(value.get_ptr<json::number_float_t*>() == nullptr);
         CHECK(value.get_ptr<json::binary_t*>() == nullptr);
@@ -355,7 +355,7 @@ TEST_CASE("pointer access")
         CHECK(value.get_ptr<const json::array_t*>() == nullptr);
         CHECK(value.get_ptr<const json::string_t*>() == nullptr);
         CHECK(value.get_ptr<const json::boolean_t*>() == nullptr);
-        CHECK(value.get_ptr<const json::number_integer_t*>() != nullptr);
+        CHECK(value.get_ptr<const json::number_integer_t*>() == nullptr);
         CHECK(value.get_ptr<const json::number_unsigned_t*>() != nullptr);
         CHECK(value.get_ptr<const json::number_float_t*>() == nullptr);
         CHECK(value.get_ptr<const json::binary_t*>() == nullptr);

tests/src/unit-reference_access.cpp

@@ -215,8 +215,8 @@ TEST_CASE("reference access")
                              "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number", json::type_error&);
         CHECK_THROWS_WITH_AS(value.get_ref<json::boolean_t&>(),
                              "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number", json::type_error&);
-        //CHECK_THROWS_WITH_AS(value.get_ref<json::number_integer_t&>(),
-        //    "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number", json::type_error&);
+        CHECK_THROWS_WITH_AS(value.get_ref<json::number_integer_t&>(),
+                             "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number", json::type_error&);
         CHECK_NOTHROW(value.get_ref<json::number_unsigned_t&>());
         CHECK_THROWS_WITH_AS(value.get_ref<json::number_float_t&>(), "[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number", json::type_error&);
     }