Do not use Qt's keyword macros as identifiers

The prefix tree used by unflatten() (#5836) had a member named slots, which
Qt defines as an empty macro, so including a Qt header before this library
failed to compile. Rename it to table and add a test that includes the
library with Qt's keyword macros (slots, signals, emit, foreach, forever)
defined.

Fixes #5865

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

docs/mkdocs/docs/community/quality_assurance.md

@@ -135,6 +135,9 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
   headers are included.
 - [x] On Windows, the library is compiled with `<Windows.h>` being included to detect and avoid common bugs (see
   [`unit-windows_h.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-windows_h.cpp)).
+- [x] The library is compiled with Qt's keyword macros (`slots`, `signals`, `emit`, `foreach`, `forever`) being defined
+  to make sure it does not use them as identifiers (see
+  [`unit-qt_keywords.cpp`](https://github.com/nlohmann/json/blob/develop/tests/src/unit-qt_keywords.cpp)).
 - [x] The library is compiled with exceptions disabled to support alternative means of error handling.
 
 ## Stable public API

include/nlohmann/detail/json_pointer.hpp

@@ -525,7 +525,7 @@ class json_pointer
             {}
         };
 
-        std::vector<slot> slots;
+        std::vector<slot> table;
         std::size_t used = 0;
 
         static std::size_t hash_of(std::size_t parent, const string_t& token) noexcept
@@ -540,19 +540,19 @@ class json_pointer
 
         void grow()
         {
-            std::vector<slot> old(slots.size() * 2);
-            old.swap(slots);
-            const std::size_t mask = slots.size() - 1;
+            std::vector<slot> old(table.size() * 2);
+            old.swap(table);
+            const std::size_t mask = table.size() - 1;
             for (const auto& s : old)
             {
                 if (s.token != nullptr)
                 {
                     std::size_t i = s.hash & mask;
-                    while (slots[i].token != nullptr)
+                    while (table[i].token != nullptr)
                     {
                         i = (i + 1) & mask;
                     }
-                    slots[i] = s;
+                    table[i] = s;
                 }
             }
         }
@@ -565,7 +565,7 @@ class json_pointer
 
         // start with the empty prefix only
         prefix_tree()
-            : slots(16)
+            : table(16)
             , parent(1, 0)
             , is_array(1, false)
         {}
@@ -575,20 +575,20 @@ class json_pointer
         std::size_t add_child(std::size_t id, const string_t& reference_token)
         {
             const std::size_t hash = hash_of(id, reference_token);
-            const std::size_t mask = slots.size() - 1;
+            const std::size_t mask = table.size() - 1;
             std::size_t i = hash & mask;
-            while (slots[i].token != nullptr)
+            while (table[i].token != nullptr)
             {
-                if (slots[i].hash == hash && slots[i].parent == id && *slots[i].token == reference_token)
+                if (table[i].hash == hash && table[i].parent == id && *table[i].token == reference_token)
                 {
-                    return slots[i].id;
+                    return table[i].id;
                 }
                 i = (i + 1) & mask;
             }
 
             // new prefix
             const std::size_t child = parent.size();
-            slots[i] = slot(hash, id, &reference_token, child);
+            table[i] = slot(hash, id, &reference_token, child);
             if (reference_token == "0")
             {
                 is_array[id] = true;
@@ -597,7 +597,7 @@ class json_pointer
             is_array.push_back(false);
 
             // keep the load factor below 1/2
-            if (++used * 2 > slots.size())
+            if (++used * 2 > table.size())
             {
                 grow();
             }

single_include/nlohmann/json.hpp

@@ -21497,7 +21497,7 @@ class json_pointer
             {}
         };
 
-        std::vector<slot> slots;
+        std::vector<slot> table;
         std::size_t used = 0;
 
         static std::size_t hash_of(std::size_t parent, const string_t& token) noexcept
@@ -21512,19 +21512,19 @@ class json_pointer
 
         void grow()
         {
-            std::vector<slot> old(slots.size() * 2);
-            old.swap(slots);
-            const std::size_t mask = slots.size() - 1;
+            std::vector<slot> old(table.size() * 2);
+            old.swap(table);
+            const std::size_t mask = table.size() - 1;
             for (const auto& s : old)
             {
                 if (s.token != nullptr)
                 {
                     std::size_t i = s.hash & mask;
-                    while (slots[i].token != nullptr)
+                    while (table[i].token != nullptr)
                     {
                         i = (i + 1) & mask;
                     }
-                    slots[i] = s;
+                    table[i] = s;
                 }
             }
         }
@@ -21537,7 +21537,7 @@ class json_pointer
 
         // start with the empty prefix only
         prefix_tree()
-            : slots(16)
+            : table(16)
             , parent(1, 0)
             , is_array(1, false)
         {}
@@ -21547,20 +21547,20 @@ class json_pointer
         std::size_t add_child(std::size_t id, const string_t& reference_token)
         {
             const std::size_t hash = hash_of(id, reference_token);
-            const std::size_t mask = slots.size() - 1;
+            const std::size_t mask = table.size() - 1;
             std::size_t i = hash & mask;
-            while (slots[i].token != nullptr)
+            while (table[i].token != nullptr)
             {
-                if (slots[i].hash == hash && slots[i].parent == id && *slots[i].token == reference_token)
+                if (table[i].hash == hash && table[i].parent == id && *table[i].token == reference_token)
                 {
-                    return slots[i].id;
+                    return table[i].id;
                 }
                 i = (i + 1) & mask;
             }
 
             // new prefix
             const std::size_t child = parent.size();
-            slots[i] = slot(hash, id, &reference_token, child);
+            table[i] = slot(hash, id, &reference_token, child);
             if (reference_token == "0")
             {
                 is_array[id] = true;
@@ -21569,7 +21569,7 @@ class json_pointer
             is_array.push_back(false);
 
             // keep the load factor below 1/2
-            if (++used * 2 > slots.size())
+            if (++used * 2 > table.size())
             {
                 grow();
             }

tests/src/unit-qt_keywords.cpp

@@ -0,0 +1,62 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+// Qt defines its keywords as macros (see qtmetamacros.h and qforeach.h) unless
+// QT_NO_KEYWORDS is defined, and it is common to include Qt headers before this
+// library; it must therefore not use these words as identifiers (see #5865)
+#define slots
+#define signals public
+#define emit
+#define Q_FOREACH(variable, container) for (variable : container)
+#define foreach Q_FOREACH
+#define forever for (;;)
+
+#include <nlohmann/json.hpp>
+using nlohmann::json;
+
+TEST_CASE("Qt keyword macros")
+{
+    SECTION("parse and dump")
+    {
+        const json j = json::parse(R"({"a": [1, 2.5, "x", null, true]})");
+        CHECK(j.dump() == R"({"a":[1,2.5,"x",null,true]})");
+    }
+
+    SECTION("flatten and unflatten")
+    {
+        const json j = {{"a", {{"b", {1, 2}}, {"c", "d"}}}, {"e", nullptr}};
+        CHECK(j.flatten().unflatten() == j);
+    }
+
+    SECTION("JSON pointer")
+    {
+        json j = {{"a", {{"b", {1, 2}}}}};
+        CHECK(j.at(json::json_pointer("/a/b/1")) == 2);
+        j[json::json_pointer("/a/c")] = 3;
+        CHECK(j.contains(json::json_pointer("/a/c")));
+    }
+
+    SECTION("binary formats")
+    {
+        const json j = {{"a", {1, 2, 3}}, {"b", "c"}};
+        CHECK(json::from_cbor(json::to_cbor(j)) == j);
+        CHECK(json::from_msgpack(json::to_msgpack(j)) == j);
+        CHECK(json::from_ubjson(json::to_ubjson(j)) == j);
+        CHECK(json::from_bjdata(json::to_bjdata(j)) == j);
+        CHECK(json::from_bson(json::to_bson(j)) == j);
+    }
+
+    SECTION("patch and diff")
+    {
+        const json source = {{"a", 1}, {"b", {1, 2}}};
+        const json target = {{"a", 2}, {"c", "d"}};
+        CHECK(source.patch(json::diff(source, target)) == target);
+    }
+}