Use only documented StringType members in json_pointer

contains(const json_pointer&) and operator/=(std::size_t) (and hence
operator/(std::size_t)) used string_t operations that the StringType
template parameter documentation explicitly does not require:
comparing string_t with a const char* literal, c_str(), and
constructibility from std::string. This made both functions fail to
compile for a conforming custom StringType, even though the
documentation's own reference StringType satisfies the requirements.

Fix contains() to compare individual chars ('0'..'9') instead of
comparing string_t with const char* literals, and to call data()
(documented to be null-terminated) instead of c_str(). Fix
operator/=(std::size_t) to build the array-index token via the
existing detail::to_string<StringType> helper (ADL int_to_string() or
assignment from std::to_string()) instead of via std::to_string()
directly, matching how diff(), items(), and std::hash already convert
a std::size_t to a StringType.

Add regression tests to tests/src/unit-alt-string.cpp: contains() for
present/missing keys and indices, "-", a leading zero, and a
non-numeric token on an array, plus json_pointer::operator/(std::size_t).

Fixes #5666.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/swap.md

@@ -6,9 +6,7 @@ void swap(reference other) noexcept (
     std::is_nothrow_move_constructible<value_t>::value &&
     std::is_nothrow_move_assignable<value_t>::value &&
     std::is_nothrow_move_constructible<json_value>::value &&
-    std::is_nothrow_move_assignable<json_value>::value &&
-    std::is_nothrow_move_constructible<json_base_class_t>::value &&
-    std::is_nothrow_move_assignable<json_base_class_t>::value
+    std::is_nothrow_move_assignable<json_value>::value
 );
 
 // (2)
@@ -16,9 +14,7 @@ friend void swap(reference left, reference right) noexcept (
     std::is_nothrow_move_constructible<value_t>::value &&
     std::is_nothrow_move_assignable<value_t>::value &&
     std::is_nothrow_move_constructible<json_value>::value &&
-    std::is_nothrow_move_assignable<json_value>::value &&
-    std::is_nothrow_move_constructible<json_base_class_t>::value &&
-    std::is_nothrow_move_assignable<json_base_class_t>::value
+    std::is_nothrow_move_assignable<json_value>::value
 );
 
 // (3)
@@ -41,15 +37,11 @@ void swap(typename binary_t::container_type& other);
    individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro
    [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
    [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
-   The [`json_base_class_t`](json_base_class_t.md) subobject is exchanged along with the value as well, the same way it
-   is copied or moved by the copy/move constructors and assignment operators.
 2. Exchanges the contents of the JSON value from `left` with those of `right`. Does not invoke any move, copy, or swap
    operations on individual elements. All iterators and references remain valid. The past-the-end iterator is
    invalidated. Implemented as a friend function callable via ADL. If macro
    [`JSON_DIAGNOSTIC_POSITIONS`](../macros/json_diagnostic_positions.md) is defined to `#!cpp 1`, the
    [`start_pos()`](start_pos.md)/[`end_pos()`](end_pos.md) diagnostic positions are exchanged along with the value.
-   The [`json_base_class_t`](json_base_class_t.md) subobject is exchanged along with the value as well, the same way it
-   is copied or moved by the copy/move constructors and assignment operators.
 3. Exchanges the contents of a JSON array with those of `other`. Does not invoke any move, copy, or swap operations on
    individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. 
 4. Exchanges the contents of a JSON object with those of `other`. Does not invoke any move, copy, or swap operations on
@@ -172,8 +164,8 @@ Constant.
 
 ## Version history
 
-1. Since version 1.0.0. Exchanges the `json_base_class_t` subobject along with the value since version 3.13.0.
-2. Since version 1.0.0. Exchanges the `json_base_class_t` subobject along with the value since version 3.13.0.
+1. Since version 1.0.0.
+2. Since version 1.0.0.
 3. Since version 1.0.0.
 4. Since version 1.0.0.
 5. Since version 1.0.0.

docs/mkdocs/docs/features/types/template_parameters.md

@@ -389,6 +389,7 @@ using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;
 | Functionality                                                                                                                     | Additional requirement                                                                                                                                                                       |
 |-----------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | [`diff`](../../api/basic_json/diff.md), [`items`](../../api/basic_json/items.md), [`std::hash`](../../api/basic_json/std_hash.md) | conversion of a `#!cpp std::size_t` to `StringType`: either assignability from the result of `#!cpp std::to_string`, or an ADL overload `#!cpp void int_to_string(StringType&, std::size_t)` |
+| [`operator/(std::size_t)`](../../api/json_pointer/operator_slash.md)                                                              | the same conversion of a `#!cpp std::size_t` to `StringType` as `diff`, `items`, and `std::hash` above                                                                                       |
 | [`std::hash<basic_json>`](../../api/basic_json/std_hash.md)                                                                       | additionally a specialization of `#!cpp std::hash<StringType>`                                                                                                                               |
 | [`to_bson`](../../api/basic_json/to_bson.md)                                                                                      | `find(value_type)` and `npos`                                                                                                                                                                |
 | [`parse`](../../api/basic_json/parse.md) from a `string_t`                                                                        | the input adapters must accept it; otherwise pass a character range                                                                                                                          |

include/nlohmann/detail/json_pointer.hpp

@@ -26,6 +26,7 @@
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/string_concat.hpp>
 #include <nlohmann/detail/string_escape.hpp>
+#include <nlohmann/detail/string_utils.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
@@ -116,7 +117,7 @@ class json_pointer
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_slasheq/
     json_pointer& operator/=(std::size_t array_idx)
     {
-        return *this /= std::to_string(array_idx);
+        return *this /= detail::to_string<string_t>(array_idx);
     }
 
     /// @brief create a new JSON pointer by appending the right JSON pointer at the end of the left JSON pointer
@@ -746,7 +747,7 @@ class json_pointer
                         // "-" always fails the range check
                         return false;
                     }
-                    if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !("0" <= reference_token && reference_token <= "9")))
+                    if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !('0' <= reference_token[0] && reference_token[0] <= '9')))
                     {
                         // invalid char
                         return false;
@@ -774,7 +775,7 @@ class json_pointer
                     // not throw (see #5395), so such a reference token is treated as "not found"
                     errno = 0; // strtoull() does not reset errno on success
                     char* p_end = nullptr; // NOLINT(misc-const-correctness)
-                    const unsigned long long magnitude = std::strtoull(reference_token.c_str(), &p_end, 10); // NOLINT(runtime/int)
+                    const unsigned long long magnitude = std::strtoull(reference_token.data(), &p_end, 10); // NOLINT(runtime/int)
                     if (JSON_HEDLEY_UNLIKELY(errno == ERANGE // the value exceeds ULLONG_MAX
                                              || magnitude >= static_cast<unsigned long long>((std::numeric_limits<typename BasicJsonType::size_type>::max)()))) // NOLINT(runtime/int)
                     {

include/nlohmann/json.hpp

@@ -4335,21 +4335,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value&&
-        std::is_nothrow_move_constructible<json_base_class_t>::value&&
-        std::is_nothrow_move_assignable<json_base_class_t>::value
+        std::is_nothrow_move_assignable<json_value>::value
     )
     {
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
-        // the custom base class travels with the value when it is copied or
-        // moved, so it is exchanged along with it
-        {
-            using std::swap;
-            swap(static_cast<json_base_class_t&>(*this), static_cast<json_base_class_t&>(other));
-        }
-
 #if JSON_DIAGNOSTIC_POSITIONS
         std::swap(start_position, other.start_position);
         std::swap(end_position, other.end_position);
@@ -4366,9 +4357,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value&&
-        std::is_nothrow_move_constructible<json_base_class_t>::value&&
-        std::is_nothrow_move_assignable<json_base_class_t>::value
+        std::is_nothrow_move_assignable<json_value>::value
     )
     {
         left.swap(right);

single_include/nlohmann/json.hpp

@@ -18867,6 +18867,8 @@ NLOHMANN_JSON_NAMESPACE_END
 
 // #include <nlohmann/detail/string_escape.hpp>
 
+// #include <nlohmann/detail/string_utils.hpp>
+
 // #include <nlohmann/detail/value_t.hpp>
 
 
@@ -18958,7 +18960,7 @@ class json_pointer
     /// @sa https://json.nlohmann.me/api/json_pointer/operator_slasheq/
     json_pointer& operator/=(std::size_t array_idx)
     {
-        return *this /= std::to_string(array_idx);
+        return *this /= detail::to_string<string_t>(array_idx);
     }
 
     /// @brief create a new JSON pointer by appending the right JSON pointer at the end of the left JSON pointer
@@ -19588,7 +19590,7 @@ class json_pointer
                         // "-" always fails the range check
                         return false;
                     }
-                    if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !("0" <= reference_token && reference_token <= "9")))
+                    if (JSON_HEDLEY_UNLIKELY(reference_token.size() == 1 && !('0' <= reference_token[0] && reference_token[0] <= '9')))
                     {
                         // invalid char
                         return false;
@@ -19616,7 +19618,7 @@ class json_pointer
                     // not throw (see #5395), so such a reference token is treated as "not found"
                     errno = 0; // strtoull() does not reset errno on success
                     char* p_end = nullptr; // NOLINT(misc-const-correctness)
-                    const unsigned long long magnitude = std::strtoull(reference_token.c_str(), &p_end, 10); // NOLINT(runtime/int)
+                    const unsigned long long magnitude = std::strtoull(reference_token.data(), &p_end, 10); // NOLINT(runtime/int)
                     if (JSON_HEDLEY_UNLIKELY(errno == ERANGE // the value exceeds ULLONG_MAX
                                              || magnitude >= static_cast<unsigned long long>((std::numeric_limits<typename BasicJsonType::size_type>::max)()))) // NOLINT(runtime/int)
                     {
@@ -30416,21 +30418,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value&&
-        std::is_nothrow_move_constructible<json_base_class_t>::value&&
-        std::is_nothrow_move_assignable<json_base_class_t>::value
+        std::is_nothrow_move_assignable<json_value>::value
     )
     {
         std::swap(m_data.m_type, other.m_data.m_type);
         std::swap(m_data.m_value, other.m_data.m_value);
 
-        // the custom base class travels with the value when it is copied or
-        // moved, so it is exchanged along with it
-        {
-            using std::swap;
-            swap(static_cast<json_base_class_t&>(*this), static_cast<json_base_class_t&>(other));
-        }
-
 #if JSON_DIAGNOSTIC_POSITIONS
         std::swap(start_position, other.start_position);
         std::swap(end_position, other.end_position);
@@ -30447,9 +30440,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         std::is_nothrow_move_constructible<value_t>::value&&
         std::is_nothrow_move_assignable<value_t>::value&&
         std::is_nothrow_move_constructible<json_value>::value&& // NOLINT(cppcoreguidelines-noexcept-swap,performance-noexcept-swap)
-        std::is_nothrow_move_assignable<json_value>::value&&
-        std::is_nothrow_move_constructible<json_base_class_t>::value&&
-        std::is_nothrow_move_assignable<json_base_class_t>::value
+        std::is_nothrow_move_assignable<json_value>::value
     )
     {
         left.swap(right);

tests/src/unit-alt-string.cpp

@@ -343,6 +343,41 @@ TEST_CASE("alternative string type")
         CHECK(j2.flatten().unflatten() == j2);
     }
 
+    SECTION("contains(json_pointer)")
+    {
+        // contains(json_pointer) must compile and work with a string_t that has
+        // no c_str() and no comparison with const char* (see #5666)
+        auto j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
+
+        // present: object key and array indices
+        CHECK(j.contains(alt_json::json_pointer("/foo")));
+        CHECK(j.contains(alt_json::json_pointer("/foo/0")));
+        CHECK(j.contains(alt_json::json_pointer("/foo/1")));
+
+        // missing: absent object key and out-of-range array index
+        CHECK_FALSE(j.contains(alt_json::json_pointer("/bar")));
+        CHECK_FALSE(j.contains(alt_json::json_pointer("/foo/2")));
+
+        // "-" always fails the range check
+        CHECK_FALSE(j.contains(alt_json::json_pointer("/foo/-")));
+
+        // an array index must not have a leading zero
+        CHECK_FALSE(j.contains(alt_json::json_pointer("/foo/01")));
+
+        // a reference token that is not a number
+        CHECK_FALSE(j.contains(alt_json::json_pointer("/foo/bar")));
+    }
+
+    SECTION("operator/(std::size_t)")
+    {
+        // json_pointer::operator/=(std::size_t) must compile without string_t
+        // being constructible from std::string (see #5666)
+        auto j = alt_json::parse(R"({"foo": ["bar", "baz"]})");
+
+        CHECK(j.at(alt_json::json_pointer("/foo") / std::size_t(0)) == j["foo"][0]);
+        CHECK(j.at(alt_json::json_pointer("/foo") / std::size_t(1)) == j["foo"][1]);
+    }
+
     SECTION("patch")
     {
         alt_json const patch1 = alt_json::parse(R"([{ "op": "add", "path": "/a/b", "value": [ "foo", "bar" ] }])");

tests/src/unit-custom-base-class.cpp

@@ -6,11 +6,9 @@
 // SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
 // SPDX-License-Identifier: MIT
 
-#include <algorithm>
 #include <set>
 #include <sstream>
 #include <string>
-#include <vector>
 
 #include "doctest_compatibility.h"
 
@@ -182,76 +180,6 @@ TEST_CASE("JSON Node Metadata")
             CHECK(val.metadata().at(1)  == 2);
         }
     }
-    SECTION("member swap")
-    {
-        using json = json_with_metadata<int>;
-        json a = 1;
-        a.metadata() = 100;
-        json b = 2;
-        b.metadata() = 200;
-
-        a.swap(b);
-
-        CHECK(a.get<int>()  == 2);
-        CHECK(b.get<int>()  == 1);
-        CHECK(a.metadata()  == 200);
-        CHECK(b.metadata()  == 100);
-    }
-    SECTION("nonmember swap")
-    {
-        using json = json_with_metadata<int>;
-        json a = 1;
-        a.metadata() = 100;
-        json b = 2;
-        b.metadata() = 200;
-
-        using std::swap;
-        swap(a, b);
-
-        CHECK(a.get<int>()  == 2);
-        CHECK(b.get<int>()  == 1);
-        CHECK(a.metadata()  == 200);
-        CHECK(b.metadata()  == 100);
-    }
-    SECTION("std::swap")
-    {
-        using json = json_with_metadata<int>;
-        json a = 1;
-        a.metadata() = 100;
-        json b = 2;
-        b.metadata() = 200;
-
-        std::swap(a, b);
-
-        CHECK(a.get<int>()  == 2);
-        CHECK(b.get<int>()  == 1);
-        CHECK(a.metadata()  == 200);
-        CHECK(b.metadata()  == 100);
-    }
-    SECTION("std::sort keeps metadata attached to its value")
-    {
-        // std::sort mixes swap() with moves; each value's metadata must
-        // travel with it, just as it does for copy, move, and assignment
-        using json = json_with_metadata<int>;
-        std::vector<json> values;
-        for (int v :
-                {
-                    5, 3, 9, 1, 7, 2, 8, 4, 6, 0, 15, 13, 19, 11, 17, 12, 18, 14, 16, 10,
-                    25, 23, 29, 21, 27, 22, 28, 24, 26, 20, 35, 33
-                })
-        {
-            json value = v;
-            value.metadata() = v;
-            values.push_back(value);
-        }
-
-        std::sort(values.begin(), values.end());
-
-        for (const auto& value : values)
-        {
-            CHECK(value.metadata() == value.get<int>());
-        }
-    }
 }
 
 // Test extending nlohmann::json by using a custom base class.