Docs Update for C++ [3/4] (#4163)

# Description of Changes

In order to give reviewers a chance to finish in one session the docs
are getting split up for the C++ update. This pass contains the
following section update:
- Functions

# API and ABI breaking changes

N/A

# Expected complexity level and risk

1 - Just documentation additions

# Testing

- [x] Manually tested each block

Author: JasonAtClockwork

docs/docs/00200-core-concepts/00200-functions/00300-reducers/00300-reducers.md

@@ -108,6 +108,31 @@ use spacetimedb::Table;
 If you see errors like "no method named `try_insert` found", add this import.
 :::
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+Use the `SPACETIMEDB_REDUCER` macro on a function:
+
+```cpp
+#include <spacetimedb.h>
+using namespace SpacetimeDB;
+
+SPACETIMEDB_REDUCER(create_user, ReducerContext ctx, std::string name, std::string email) {
+    // Validate input
+    if (name.empty()) {
+        return Err("Name cannot be empty");
+    }
+    
+    // Modify tables
+    User user{0, name, email};  // 0 for id - auto-increment will assign
+    ctx.db[user].insert(user);
+    
+    return Ok();
+}
+```
+
+Reducers must take `ReducerContext ctx` as their first parameter. Additional parameters can be any registered types. Reducers return `ReducerResult` (which is `Outcome<void>`): use `Ok()` on success or `Err(message)` on error for convenience.
+
 </TabItem>
 </Tabs>
 
@@ -161,6 +186,17 @@ ctx.db.user().insert(User {
 });
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+ctx.db[user].insert(User{
+    0,  // auto-increment will assign
+    "Alice",
+    "[email protected]"
+});
+```
+
 </TabItem>
 </Tabs>
 
@@ -204,6 +240,17 @@ if let Some(user) = ctx.db.user().id().find(123) {
 let by_email = ctx.db.user().email().find("[email protected]");
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+if (auto user = ctx.db[user_id].find(123)) {
+    LOG_INFO("Found: " + user->name);
+}
+
+auto by_email = ctx.db[user_email].find("[email protected]");
+```
+
 </TabItem>
 </Tabs>
 
@@ -239,6 +286,15 @@ for user in ctx.db.user().name().filter("Alice") {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+for (const auto& user : ctx.db[user_name].filter("Alice")) {
+    LOG_INFO("User " + std::to_string(user.id) + ": " + user.email);
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -279,6 +335,16 @@ if let Some(mut user) = ctx.db.user().id().find(123) {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+if (auto user = ctx.db[user_id].find(123)) {
+    user->name = "Bob";
+    ctx.db[user_id].update(*user);
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -322,6 +388,22 @@ let deleted = ctx.db.user().name().delete("Alice");
 log::info!("Deleted {} row(s)", deleted);
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+// Delete by primary key
+ctx.db[user_id].delete_by_key(123);
+
+// Delete all matching an indexed column
+uint32_t deleted = 0;
+for (const auto& user : ctx.db[user_name].filter("Alice")) {
+    ctx.db[user_id].delete_by_key(user.id);
+    deleted++;
+}
+LOG_INFO("Deleted " + std::to_string(deleted) + " row(s)");
+```
+
 </TabItem>
 </Tabs>
 
@@ -357,6 +439,15 @@ for user in ctx.db.user().iter() {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+for (const auto& user : ctx.db[user]) {
+    LOG_INFO(std::to_string(user.id) + ": " + user.name);
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -388,6 +479,14 @@ let total = ctx.db.user().count();
 log::info!("Total users: {}", total);
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+auto total = ctx.db[user].count();
+LOG_INFO("Total users: " + std::to_string(total));
+```
+
 </TabItem>
 </Tabs>
 
@@ -553,6 +652,48 @@ fn queue_fetch(ctx: &ReducerContext, url: String) {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+#define SPACETIMEDB_UNSTABLE_FEATURES
+#include <spacetimedb.h>
+using namespace SpacetimeDB;
+
+// Define a table to store scheduled tasks
+struct FetchSchedule {
+    uint64_t scheduled_id;
+    ScheduleAt scheduled_at;
+    std::string url;
+};
+SPACETIMEDB_STRUCT(FetchSchedule, scheduled_id, scheduled_at, url);
+SPACETIMEDB_TABLE(FetchSchedule, fetch_schedule, Private);
+FIELD_PrimaryKeyAutoInc(fetch_schedule, scheduled_id);
+
+// Register the table for scheduling (column 1 = scheduled_at field, 0-based index)
+SPACETIMEDB_SCHEDULE(fetch_schedule, 1, fetch_external_data);
+
+// The procedure to be scheduled - called automatically when the time arrives
+SPACETIMEDB_PROCEDURE(uint32_t, fetch_external_data, ProcedureContext ctx, FetchSchedule schedule) {
+    LOG_INFO("Fetching data from: " + schedule.url);
+    // Process response...
+    return 0;  // Success
+}
+
+// From a reducer, schedule the procedure by inserting into the schedule table
+SPACETIMEDB_REDUCER(queue_fetch, ReducerContext ctx, std::string url) {
+    auto scheduled_at = ScheduleAt(TimeDuration::from_seconds(0));  // Run immediately
+    FetchSchedule fetch_task{
+        0,                // scheduled_id - auto-increment will assign
+        scheduled_at,     // When to execute
+        url
+    };
+    ctx.db[fetch_schedule].insert(fetch_task);
+    LOG_INFO("Fetch scheduled for URL: " + url);
+    return Ok();
+}
+```
+
 </TabItem>
 </Tabs>
 

docs/docs/00200-core-concepts/00200-functions/00300-reducers/00400-reducer-context.md

@@ -81,6 +81,27 @@ fn create_user(ctx: &ReducerContext, name: String) {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+#include <spacetimedb.h>
+using namespace SpacetimeDB;
+
+struct User {
+    uint64_t id;
+    std::string name;
+};
+SPACETIMEDB_STRUCT(User, id, name);
+SPACETIMEDB_TABLE(User, user, Public);
+FIELD_PrimaryKeyAutoInc(user, id);
+
+SPACETIMEDB_REDUCER(create_user, ReducerContext ctx, std::string name) {
+    ctx.db[user].insert(User{0, name});
+    return Ok();
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -184,6 +205,35 @@ fn update_score(ctx: &ReducerContext, new_score: u32) {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+#include <spacetimedb.h>
+using namespace SpacetimeDB;
+
+struct Player {
+    Identity identity;
+    std::string name;
+    uint32_t score;
+};
+SPACETIMEDB_STRUCT(Player, identity, name, score);
+SPACETIMEDB_TABLE(Player, player, Public);
+FIELD_PrimaryKey(player, identity);
+
+SPACETIMEDB_REDUCER(update_score, ReducerContext ctx, uint32_t new_score) {
+    // Get the caller's identity
+    auto caller = ctx.sender;
+    
+    // Find and update their player record
+    if (auto player = ctx.db[player_identity].find(caller)) {
+        player->score = new_score;
+        ctx.db[player_identity].update(*player);
+    }
+    return Ok();
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -298,6 +348,36 @@ fn send_reminder(ctx: &ReducerContext, task: ScheduledTask) {
 }
 ```
 
+</TabItem>
+<TabItem value="cpp" label="C++">
+
+```cpp
+#include <spacetimedb.h>
+using namespace SpacetimeDB;
+
+struct ScheduledTask {
+    uint64_t task_id;
+    ScheduleAt scheduled_at;
+    std::string message;
+};
+SPACETIMEDB_STRUCT(ScheduledTask, task_id, scheduled_at, message);
+SPACETIMEDB_TABLE(ScheduledTask, scheduled_task, Private);
+FIELD_PrimaryKeyAutoInc(scheduled_task, task_id);
+
+// Register the table for scheduling (column 1 = scheduled_at field, 0-based index)
+SPACETIMEDB_SCHEDULE(scheduled_task, 1, send_reminder);
+
+SPACETIMEDB_REDUCER(send_reminder, ReducerContext ctx, ScheduledTask task) {
+    // Only allow the scheduler (module identity) to call this
+    if (ctx.sender != ctx.identity()) {
+        return Err("This reducer can only be called by the scheduler");
+    }
+    
+    LOG_INFO("Reminder: " + task.message);
+    return Ok();
+}
+```
+
 </TabItem>
 </Tabs>
 
@@ -346,4 +426,23 @@ TypeScript uses `Math.random()` for random number generation, which is automatic
 - `random<T>() -> T` - Generate a single random value
 - `sender_auth() -> &AuthCtx` - Get authorization context for the caller (includes JWT claims and internal call detection)
 </TabItem>
+<TabItem value="cpp" label="C++">
+
+| Property        | Type                           | Description                                     |
+| --------------- | ------------------------------ | ----------------------------------------------- |
+| `db[table]`     | `Table<T>`                     | Access to a specific table's operations         |
+| `sender`        | `Identity`                     | Identity of the caller                          |
+| `timestamp`     | `Timestamp`                    | Time when the reducer was invoked               |
+| `connection_id` | `std::optional<ConnectionId>`  | Connection ID of the caller, if available       |
+
+**Methods:**
+
+- `identity() -> Identity` - Get the module's identity
+- `rng() -> StdbRng&` - Get the random number generator (deterministic and reproducible)
+- `sender_auth() -> const AuthCtx&` - Get authorization context for the caller (includes JWT claims and internal call detection)
+
+:::note
+C++ uses the `std::optional` type for the `connection_id` to represent values that may not be present. The `rng()` method returns a deterministic random number generator that is seeded consistently across all nodes.
+:::
+</TabItem>
 </Tabs>