README.md

# Software Engineering Project - Wizard
Welcome to **Wizard**, a C++ implementation of the classic card game. Compete with your friends in exciting multiplayer 
gameplay!

<div style="display: flex; align-items: center;">
  <img src="./assets/wizard_logo.png" alt="Wizard Logo" style="height: 150px; margin-right: 20px;">
  <img src="./assets/cards.png" alt="Cards" style="height: 75px;">
</div>



## 🚀 Features
- 🌐 **Multiplayer Gameplay:** Play with 3 to 6 players.
- 🔄 **Synchronized Game State:** Real-time updates for all players.
- 🖼 **Elegant User Interface:** A visually appealing UI powered by wxWidgets.

---

You can read the game's rules [here](https://www.amigo.games/content/ap/rule/19420--031-2019-Wizard_Manual_002_LAYOUT[1].pdf). The implementation features a client/server architecture for multiplayer 
scenarios. It uses [wxWidgets](https://www.wxwidgets.org/) for the GUI, [sockpp](https://github.com/fpagliughi/sockpp) for the network interface, [rapidjson](https://rapidjson.org/md_doc_tutorial.html) for object 
serialization, and [googletest](https://github.com/google/googletest) for the unit tests. 

---

## 1 Overview

The game and source files are available on GitLab on the main branch. The game was developed based on the provided LAMA 
example project game. This project consists of a **server** and a **client**, each with their own main.cpp file. Each 
player can run their own client and connect to the same server in the same local network.

---

## 2 Compile Instructions

This project only works on UNIX systems (Linux / macOS). We thus only explain the how to compile the game on these 
systems. The following description was tested on Ubuntu 20.04 and on macOS Sequoia.

### 2.1 Prepare OS Environment

If your OS does not yet have git installed, install git by running `sudo apt-get install git` on Ubuntu or by 
running `xcode-select --install` on macOS (this installs git as well). The Wizard repository can then be cloned by 
running `git clone https://gitlab.ethz.ch/beckermar/wizard.git` (clone with HTTPS) or by running
`git clone git@gitlab.ethz.ch:beckermar/wizard.git` (clone with SSH) inside the directory that should contain the game.
Cloning the game as a first step also makes provided scripts for preparing the OS environment and for compiling the code
available.

#### 2.1.1 Ubuntu 20.04

The necessary packages and software can either be installed manually or by running the provided script. 

To use the provided script, run `bash scripts/prepare_ubuntu.sh` inside the **wizard** directory.

To manually prepare Ubuntu, execute the following commands:
1. `sudo apt-get update` (update package list)
2. `sudo apt-get install build-essential` (install software to build from source)
3. `sudo apt-get install libwxgtk3.0-gtk3-dev` (install wxwidgets, use libwxgtk3.2-dev on Ubuntu 24.04)
4. `sudo apt-get install cmake` (install cmake)

#### 2.1.2 macOS Sequoia

The necessary packages and software can either be installed manually or by running the provided script.

To use the provided script, run `zsh scripts/prepare_macos.sh` inside the **wizard** directory.

To manually prepare macOS, execute the following commands: 
1. `/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"` (install homebrew)
2. `brew install cmake` (install cmake)
3. `brew install wxwidgets` (install wxwidgets)

### 2.2 Compile Code

Compiling the code creates executables for the client (Wizard-client) and for the server (Wizard-server).

#### 2.2.1 Ubuntu 20.04

Compiling the code can either be done manually or by running the provided script.

To use the provided script, run `bash scripts/compile_ubuntu.sh` inside the **wizard** directory.

To manually compile the code on Ubuntu, perform the following steps:
1. Move into the **wizard** directory if not yet done: `cd /your/path/to/wizard/`
2. Move into the **sockpp** directory: `cd sockpp/`
3. Create the following new directory: `mkdir cmake-build-debug`
4. Move into the **cmake-build-debug** directory: `cd cmake-build-debug`
5. Run cmake: `cmake ..`
6. Run make: `make`
7. Move back into the **wizard** directory: `cd ../..`
8. Create the following new directory: `mkdir cmake-build-debug`
9. Move into the **cmake-build-debug** directory: `cd cmake-build-debug`
10. Run cmake: `cmake ..`
11. Run make: `make`

#### 2.2.1 macOS Sequoia

Compiling the code can either be done manually or by running the provided script.

To use the provided script, run `zsh scripts/compile_macos.sh` inside the **wizard** directory.

To manually compile the code on Ubuntu, perform the following steps:
1. Move into the **wizard** directory if not yet done: `cd /your/path/to/wizard/`
2. Move into the **sockpp** directory: `cd sockpp/`
3. Create the following new directory: `mkdir cmake-build-debug`
4. Move into the **cmake-build-debug** directory: `cd cmake-build-debug`
5. Run cmake: `cmake ..`
6. Run make: `make`
7. Move back into the **wizard** directory: `cd ../..`
8. Create the following new directory: `mkdir cmake-build-debug`
9. Move into the **cmake-build-debug** directory: `cd cmake-build-debug`
10. Run cmake: `cmake ..`
11. Run make: `make`

---

## 3 Run the Game

After compiling the code, navigate into the **cmake-build-debug** directory (if not yet there). To start a server, run
`./Wizard-server`. In new consoles, you can now start as many clients as you wish by running `./Wizard-client`.

---

## 4 Play the Game

Enjoy the game!








### 4.1 Connection Panel 
The client renders the GUI that is presented to the player, whereas the server is a console application without a user interface. Every action a player performs in the client application (for example playing a card) is sent as a formatted message to the server application, which processes the request.   
- If the **player's move was valid**, the server will update the game state (e.g. move a card from the player's hand to the discard pile) and broadcast this new game state to all players of the game. Whenever the client application receives a game state update, it will re-render the GUI accordingly and allow new interactions.   
- If the **move was invalid**, the game state will not be updated and only the requesting player will get a response containing the error message. 

### Network Interface
Everything that is passed between client and server are objects of type `client_request` and `server_response`. Since the underlying network protocol works with TCP, these `client_request` and `server_response` objects are transformed into a **[JSON](https://wiki.selfhtml.org/wiki/JSON) string**, which can then be sent over the network. The receiving end reads the JSON string and constructs an object of type `client_request` resp. `server_response` that reflects the exact parameters that are specified in the JSON string. This process is known as **serialization** (object to string) and **deserialization** (string to object). If you want to read more about serialization, [read me on Wikipedia](https://en.wikipedia.org/wiki/Serialization).

![client-server-diagram](./docs/img/client-server-diagram.png?raw=true)

#### Serialization & Deserialization of messages
Both, the `client_request` and `server_response` base classes, implement the abstract class `serializable` with its `write_into_json(...)` function. It allows to serialize the object instance into a JSON string. Additionally, they have a static function `from_json(...)`, which allows creating an object instance from a JSON string.

```cpp
// All request types of your imlementation
// IMPORTANT: Add your own types here (and remove unused ones)
enum RequestType {
    join_game,
    start_game,
    estimate_tricks,
    decide_trump_color,
    leave_game,
    play_card,
};

class client_request : public serializable {
protected:
    RequestType _type;   // stores the type of request, such that the receiving end knows how to deserialize it
    std::string _req_id; // unique id of this request
    std::string _player_id; // id of the player sending the request
    std::string _game_id;   // id of the game this request is for

    ...
private:
    // for deserializing RequestType (contains mappings from string to RequestType)
    // IMPORTANT: Add mapping for your own RequestTypes to this unordered_map
    static const std::unordered_map<std::string, RequestType> _string_to_request_type;
    
    // for serializing RequestType (contains mappings from RequestType to string)
    // IMPORTANT: Add mapping for your own RequestTypes to this unordered_map
    static const std::unordered_map<RequestType, std::string> _request_type_to_string;

public:
    // DESERIALIZATION: Attempts to create the specific client_request from the provided json.
    static client_request* from_json(const rapidjson::Value& json);

    // SERIALIZATION: Serializes the client_request into a json object that can be sent over the network
    virtual void write_into_json(rapidjson::Value& json, rapidjson::Document::AllocatorType& allocator) const override;
};
```

##### Serialization

When you implement your own specializations of `client_request` (and `server_response`, if necessary) you will have to implement the `write_into_json(...)` functions yourself. Your subclass always has to call the `write_into_json(...)` function of its base-class, such that the parameters of the base-class are written into the JSON document: 

Here is the **base-class** implementation:
```cpp
// Implementation in the base-class client_request
void client_request::write_into_json(rapidjson::Value &json,
                                     rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> &allocator) const {
    // Look up string value of this client_request's RequestType and store it in the json document
    rapidjson::Value type_val(_request_type_to_string.at(this->_type).c_str(), allocator);
    json.AddMember("type", type_val, allocator);

    // Save player_id in the JSON document
    rapidjson::Value player_id_val(_player_id.c_str(), allocator);
    json.AddMember("player_id", player_id_val, allocator);

    // Save game_id in the JSON document
    rapidjson::Value game_id_val(_game_id.c_str(), allocator);
    json.AddMember("game_id", game_id_val, allocator);
    ...
}
```
And here is the **subclass** implementation (for the `play_card_request` class), where an additional field `_card_id` is serialized.
```cpp
// Implementation in the subclass play_card_request 
void play_card_request::write_into_json(rapidjson::Value &json,
                                        rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> &allocator) const {
    // IMPORTANT: call base-class, such that the parameters of the base-class are written into the 'json' variable
    client_request::write_into_json(json, allocator);

    // Add parameters to the JSON that are unique to the play_card_request
    rapidjson::Value card_id_val(_card_id.c_str(), allocator);
    json.AddMember("card_id", card_id_val,allocator);
}
```

##### Deserialization

The deserialization of `client_request` JSONs always goes through the `from_json(...)` function of the `client_request` class. In this function, the "type" field, stored in the JSON, is inspected to determine, which subclass should be called to perform the deserialization: 

```cpp
if (json.HasMember("type") && json["type"].IsString()) {
        // Get the RequestType stored as a string in the JSON
        const std::string type = json["type"].GetString();
        // Lookup the actual RequestType per string from a pre-defined unordered_map
        const RequestType request_type = client_request::_string_to_request_type.at(type);

        // Call the correct from_json() specialization
        if (request_type == RequestType::play_card) {
            return play_card_request::from_json(json);
        }
        else if (request_type == RequestType::draw_card) {
            return draw_card_request::from_json(json);
        }
        else if (...) {
            ...
        } else {
            throw WizardException("Encountered unknown ClientRequest type " + type);
        }
    }
    throw WizardException("Could not determine type of ClientRequest. JSON was:\n" + json_utils::to_string(&json));
```

Therefore, when you implement your own `client_request` subclasses, remember to add a new element into the `RequestType` enum to define your new request type. You will also have to add an entry for this new RequestType in the two unordered_maps `_string_to_request_type`, resp. `_request_type_to_string` in the `client_request` base-class. Once this is done, you can add a check for your new `RequestType` element in the `from_json(...)` function of the `client_request` base-class and call the specialized `from_json(...)` function of your subclass from there. 

Also, don't forget to set the correct `RequestType` in the public constructor of your new `client_request` subclass, here examplified at the `play_card_request` class:

```cpp
// Public constructor
play_card_request::play_card_request(std::string game_id, std::string player_id, std::string card_id)
        : client_request(client_request::create_base_class_properties(  // call base-class constructor
                                                    RequestType::play_card, // IMPORTANT: set the RequestType of your subclass
                                                    uuid_generator::generate_uuid_v4(), 
                                                    player_id, 
                                                    game_id) ),
        _card_id(card_id)   // set subclass specific parameters
{ }
```

The deserialization in your subclass will look something like this:

```cpp
// private constructor for deserialization
play_card_request::play_card_request(client_request::base_class_properties props, std::string card_id) :
        client_request(props),  // call base-class constructor
        _card_id(card_id)   // set subclass specific parameters
{ }

// Deserialization
play_card_request* play_card_request::from_json(const rapidjson::Value& json) {
    // extract base-class properties from the json
    base_class_properties props = client_request::extract_base_class_properties(json);

    // get subclass specific properties
    if (json.HasMember("card_id")) {
        // invoke deserialization constructor
        return new play_card_request(props, json["card_id"].GetString());
    } else {
        throw WizardException("Could not find 'card_id' in play_card_request");
    }
}
```

There are plenty of examples of subclasses in the network/requests folder, where you can see how the serialization/deserialization scheme works.

#### 4.2.2 Sending messages
#### Client -> Server:
All you have to do is use the static class `ClientNetworkManager` on the client side and then invoke its `sendRequest(const client_request& request)` function with the `client_request` that you want to send. The server's response will arrive as an object of type `request_response` and the `ClientNetworkManager` will invoke the `Process()` function of that `request_response` object automatically.

#### Server -> Client:
All messages arriving at the server are being deserialized and then passed on to the `handle_request(client_request* req)` function of the `request_handler` singleton class. This function returns a pointer to an object of type `request_response` (a subclass of `server_response`), which is then automatically sent back to the requesting client. In your game implementation you should extend the `handle_request(client_request* req)` function of the `request_handler`, such that it can handle the `client_request` that you add to your game and return an object of type `request_response` with all parameters you want to send. 

If the `client_request` causes an update of the game_state you should also update all other players of that game about the game_state change. This happens in the `game_instance` class, here examplified at the case where a `start_game_request` calls the `start_game(...)` function on the respective `game_instance` on the server side:

```cpp
bool game_instance::start_game(player* player, std::string &err) {
    modification_lock.lock();   // make sure only one request can modify the game_state at a time

    // Try to start the game
    if (_game_state->start_game(err)) { 
        // create a full_state_response (subclass of server_response) with the full game_state inside
        full_state_response state_update_msg = full_state_response(this->get_id(), *_game_state);
        // BROADCAST new game_state to all other players
        server_network_manager::broadcast_message(state_update_msg, _game_state->get_players(), player);

        modification_lock.unlock(); // allow other threads to modify the game_state
        return true;
    }
    modification_lock.unlock();
    return false;
}
```

#### 4.2.3 Debugging Messages

By default, the server (specifically, the server_network_manager) will print every valid message that it receives to the console. In order for this to work in your project as well, you have to make sure that your CMake file contains a line, where the preprocessor variable PRINT_NETWORK_MESSAGES is defined for your server executable. 

```
target_compile_definitions(Wizard-server PRIVATE PRINT_NETWORK_MESSAGES=1)
```

If a wrongly formatted message arrives at the server, it will print an error message with the received message string to the console, no matter if PRINT_NETWORK_MESSAGES is defined or not. 

If you want to manually print one of your serialized messages (or any other serialized object for that matter), you can use the helper function `json_utils::to_string(const rapidjson::Value* json)` as follows. 

```cpp
#include "src/common/serialization/json_utils.h"
#include "rapidjson/include/rapidjson/document.h"

...
// Create a request to serialize
join_game_request* req = new join_game_request(player->get_id(), player->get_player_name());

// serialize the request object
rapidjson::Document* req_json = req->to_json();

// print serialization to the console.
std::cout << json_utils::to_string(req_json) << std::endl;
```


### 4.3 Game State

The `game_state` class stores all parameters that are required to represent the game on the client (resp. server) side. In order to synchronize this `game_state` among all players, the `game_state` can also be **serialized** and **deserialized**. If a `client_request` was successfully executed on the server, then the `request_response` that is sent back to the client contains a serialized version of the updated `game_state`. All other players receive the updated `game_state` at the same time through a `full_state_response`.

To serialize the `game_state`, the same `write_into_json(...)` function is used as for the `client_request`. 

```cpp
class game_state : public unique_serializable {
private:
    // Properties
    std::vector<player*> _players;
    draw_pile* _draw_pile;
    discard_pile* _discard_pile;
    serializable_value<bool>* _is_started;
    serializable_value<bool>* _is_finished;
    serializable_value<int>* _round_number;
    serializable_value<int>* _current_player_idx;
    serializable_value<int>* _starting_player_idx;

    // deserialization constructor
    game_state(
            std::string id,
            draw_pile* draw_pile,
            discard_pile* discard_pile,
            std::vector<player*>& players,
            serializable_value<bool>* is_started,
            serializable_value<bool>* is_finished,
            serializable_value<int>* current_player_idx,
            serializable_value<int>* round_number,
            serializable_value<int>* starting_player_idx);

public:
    game_state();

    ...
    // SERIALIZATION
    virtual void write_into_json(rapidjson::Value& json, rapidjson::Document::AllocatorType& allocator) const override;
    // DESERIALIZATION
    static game_state* from_json(const rapidjson::Value& json);
};
```

The `game_state` inherits from `unique_serializable`, which essentially requires the `write_into_json()` function and adds a unique `id` to the object, such that it can be uniquely identified. Similarly, each parameter nested inside the `game_state` (e.g. players, draw_pile, etc.) also inherit from `unique_serializable` and therefore have their own `id` and serialization, resp. deserialization functions.

On the client side, the new `game_state` is then passed to the `updateGameState(game_state*)` function of the `GameController` class, which performs a redraw of the GUI.

Since you will have to add your own properties to the `game_state` class (and probably create other classes that inherit from `unique_serializable` to add to your game_state), we want to shortly elaborate how the serialization and deserialization works for subclasses of `unique_serializable`. It's very similar to the `client_request` class discussed earlier. Here is how the `write_into_json(...)` function is implemented in the `game_state` class of Wizard. **Don't be shocked by the lengthy code. It's only a lot of repetition for each class property** :

```cpp
void game_state::write_into_json(rapidjson::Value &json,
                                 rapidjson::MemoryPoolAllocator<rapidjson::CrtAllocator> &allocator) const {
    // call base-class to write id and object name into the json
    unique_serializable::write_into_json(json, allocator);

    // write all properties of this game_state instance into the JSON
    rapidjson::Value is_finished_val(rapidjson::kObjectType);   // create an empty rapidjson::Value that can hold an ObjectType
    _is_finished->write_into_json(is_finished_val, allocator);  // write class property '_is_finished' into the created rapidjson::Value
    json.AddMember("is_finished", is_finished_val, allocator);  // add the rapidjson::Value 'is_finished_val' to the game_state json

    rapidjson::Value is_started_val(rapidjson::kObjectType);
    _is_started->write_into_json(is_started_val, allocator);
    json.AddMember("is_started", is_started_val, allocator);

    rapidjson::Value current_player_idx_val(rapidjson::kObjectType);
    _current_player_idx->write_into_json(current_player_idx_val, allocator);
    json.AddMember("current_player_idx", current_player_idx_val, allocator);

    rapidjson::Value starting_player_idx_val(rapidjson::kObjectType);
    _starting_player_idx->write_into_json(starting_player_idx_val, allocator);
    json.AddMember("starting_player_idx", starting_player_idx_val, allocator);

    rapidjson::Value round_number_val(rapidjson::kObjectType);
    _round_number->write_into_json(round_number_val, allocator);
    json.AddMember("round_number", round_number_val, allocator);

    rapidjson::Value draw_pile_val(rapidjson::kObjectType);
    _draw_pile->write_into_json(draw_pile_val, allocator);
    json.AddMember("draw_pile", draw_pile_val, allocator);

    rapidjson::Value discard_pile_val(rapidjson::kObjectType);
    _discard_pile->write_into_json(discard_pile_val, allocator);
    json.AddMember("discard_pile", discard_pile_val, allocator);

    // Special helper function to serialize vector of pointers
    // The pointers inside the vector have to inherit from serializable (or unique_serializable)
    json.AddMember("players", vector_utils::serialize_vector(_players, allocator), allocator);
}
```

For **deserialization**, the `from_json(...)` function is used, which is implemented as follows:

```cpp
// DESERIALIZATION CONSTRUCTOR receives pointers for all its properties and stores them
game_state::game_state(std::string id, draw_pile *draw_pile, discard_pile *discard_pile,
                       std::vector<player *> &players, serializable_value<bool> *is_started,
                       serializable_value<bool> *is_finished, serializable_value<int> *current_player_idx,
                       serializable_value<int>* round_number, serializable_value<int> *starting_player_idx)
        : unique_serializable(id),  // initialize the unique_serializable base-class
          _draw_pile(draw_pile),
          _discard_pile(discard_pile),
          _players(players),
          _is_started(is_started),
          _is_finished(is_finished),
          _current_player_idx(current_player_idx),
          _round_number(round_number),
          _starting_player_idx(starting_player_idx)
{ }

// DESERIALIZATION 
// returns a pointer to the new game_state
game_state* game_state::from_json(const rapidjson::Value &json) {
    // Make sure the json contains all required information
    if (json.HasMember("is_finished")
        && json.HasMember("is_started")
        && json.HasMember("current_player_idx")
        && json.HasMember("round_number")
        && json.HasMember("starting_player_idx")
        && json.HasMember("players")
        && json.HasMember("draw_pile")
        && json.HasMember("discard_pile"))
    {
        // deserialize all players
        std::vector<player*> deserialized_players;
        for (auto &serialized_player : json["players"].GetArray()) {
            deserialized_players.push_back(player::from_json(serialized_player.GetObject()));
        }
        // Invoke deserialization constructor
        return new game_state(unique_serializable::extract_id(json),   // extract base_params from JSON
                              draw_pile::from_json(json["draw_pile"].GetObject()),  // deserialize the draw_pile
                              discard_pile::from_json(json["discard_pile"].GetObject()),
                              deserialized_players,
                              serializable_value<bool>::from_json(json["is_started"].GetObject()),
                              serializable_value<bool>::from_json(json["is_finished"].GetObject()),
                              serializable_value<int>::from_json(json["current_player_idx"].GetObject()),
                              serializable_value<int>::from_json(json["round_number"].GetObject()),
                              serializable_value<int>::from_json(json["starting_player_idx"].GetObject()));
    } else {
        throw WizardException("Failed to deserialize game_state. Required entries were missing.");
    }
}
```

A similar scheme is applied in all other objects that inherit from `unique_serializable`. Namely, these are:
- `player`
- `hand`
- `card`
- `draw_pile`
- `discard_pile`
- `serializable_value`


### 4.4 GUI with wxWidgets

The GUI of the example project was built using the cross-platform GUI library [wxWidgets](https://www.wxwidgets.org/). In order to build a project using wxWidget elements, you will first need to install wxWidgets on your system (see Section 1.1 above). 

#### 4.4.1 Structure & Important Classes

Here is a list of the most important elements that you will need to create your GUI. This is just meant as an overview, you will need to look up their correct usage in wxWidget's [documentation](https://docs.wxwidgets.org/3.0/index.html).

* __Application core__
    * __`wxIMPLEMENT_APP()`__: In order to properly interact with the operating system's GUI, wxWidgets takes over the control flow of your application. wxWidgets therefore has its own `main()` function, that you can reference with the macro `wxIMPLEMENT_APP(wxApp*)`.
    * __`wxApp`__: The core class of your application must inherit from the `wxApp` class. wxWidgets will call the `OnInit()` function when starting the application. You can find the example project's implementation in `src/client/app/Wizard`.
* __Windows__
    * __`wxFrame`__: Each window of your application must inherit from the `wxFrame` class. The example project has one window which you can find here: `src/client/windows/GameWindow`
* __GUI elements__
    * __`wxPanel`__: Panels serve as containers for elements within a window. All panels must instantiate or inherit from the `wxPanel` class. A panel can contain one or more subpanels.
    * __`wxBoxSizer`__: Box sizers allow you to layout your panels within a window, either horizontally or vertically. By nesting box sizers, you can create complex layouts. Have a look at `src/client/panels/ConnectionPanel` for an example.
    * __`wxStaticText`__: This class displays text in your GUI.
    * __`wxButton`__: This class creates a clickable button in your GUI.
* __Pop-ups__
    * __`wxMessageBox()`__: You can use this function to display a small pop-up window with text in front of the your current main window. This is useful to display error or status messages.


#### 4.4.2 Events

Like in most GUI environments, objects in wxWidgets trigger __events__ when they are interacted with by the user. For instance, a button will trigger a `wxEVT_BUTTON` event when clicked. Similarly, a panel will trigger a `wxEVT_LEFT_UP` event when clicked. There are many other events that can be triggered - for example when a keyboard key is pressed, when a window is resized, or when the cursor moves over an element.

In order to make the GUI interactive, we must specify the effect of an event. The easiest way is to __bind__ an event to a lambda function. A lambda function is an unnamed function that can be used as an r-value. In C++, lambda functions have the following syntax:

```
[ external_variables... ]( function_parameters ... ) {
    function_body...
}

```

Here is an example which binds a lambda function to a button click event:

```
wxButton* myButton = new wxButton(parentPanel, wxID_ANY, "Click me!");
int myVariable = 42;
myButton->Bind(wxEVT_BUTTON, [myVariable](wxCommandEvent& event) {
    doSomething(myVariable, event);
});
```

In C++, we need to specify which variables from outside the lambda function's scope should be accessible within it. In the example above, `myVariable` is declared outside of the lambda function but is used by the `doSomething()` function call within the lambda function. We must therefore list `myVariable` within the square brackets at the beginning of the lambda function definition, in order to make it accessible from within the lambda function's scope.