Interacting with RESTful APIs Using C++: Automating GET Requests and Handling Responses

Making GET Requests and Handling Responses

Welcome to the second lesson in our journey of Interacting with APIs in C++ . In the previous lesson, we laid a strong foundation by understanding RESTful APIs and how HTTP requests facilitate interactions with them. We used curl to manually craft and send a GET request to an endpoint. Now, we are transitioning to automating this process using C++. This lesson introduces the httplib library, which allows us to send HTTP requests effortlessly in our programs, making it an invaluable tool in the realm of web development and API integration.

Setting Up the Environment

To make HTTP requests in C++, we use the httplib library, a user-friendly and powerful library for handling HTTP requests. It is a header-only library, meaning you only need to include the header file to start using it. Additionally, we will utilize the nlohmann::json library for parsing JSON data. Downloading and Installing Libraries: httplib : Being a header-only library, you can directly download the httplib.h file: Run wget https://raw.githubusercontent.com/yhirose/cpp-httplib/master/httplib.h -O httplib.h to download it directly into your project directory. nlohmann::json : Linux (using apt): Run sudo apt install nlohmann-json3-dev to install the development package. Windows/Mac: Download the json.hpp file from the GitHub repository and include it in your project directory. Alternatively, use a package manager like vcpkg : Run vcpkg install nlohmann-json . Once you have the libraries installed, include them in your C++ code: C++ #include "httplib.h" #include <nlohmann/json.hpp> #include <iostream> #include "httplib.h" #include <nlohmann/json.hpp> #include <iostream> With this setup, you'll be equipped to automate requests. Note that in the CodeSignal IDE environment, these libraries are already pre-configured, allowing you to focus solely on learning and practicing HTTP requests without worrying about the setup process.

Defining the Base URL

When interacting with an API, it's helpful to define a base URL for the service you're communicating with. This makes your code more modular and easy to maintain, allowing you to change the root of your API URL in one place without modifying each request: C++ // Base URL for the server const char* base_url = "localhost"; int port = 8000; // Base URL for the server const char* base_url = "localhost"; int port = 8000; By setting a base URL, we can easily access different endpoints within the API, making our code cleaner and more adaptable. APIs often run on specific ports to separate different services or applications on the same server. In local development, "localhost" typically refers to the local loopback address (127.0.0.1). However, the API server must be listening on the specified port for successful communication. While the default HTTP and HTTPS ports are 80 and 443 respectively, many development APIs use custom ports, such as 8000 in this case. It's crucial to ensure that the API server is running on the correct port before making requests, as using an incorrect port will lead to connection failures.

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Performing a Basic GET Request

Let's dive into the process of fetching data from an API using C++. Our goal is to retrieve a list of to-do items from the /todos endpoint using the GET method.

C++

httplib::Client client(base_url, port);
auto res = client.Get("/todos");

std::cout << "Raw Response:" << std::endl;
if (res) {
    std::cout << res->body << std::endl;
} else {
    std::cerr << "Failed to fetch data." << std::endl;
    return 1;
}

httplib::Client client(base_url, port);
auto res = client.Get("/todos");

std::cout << "Raw Response:" << std::endl;
if (res) {
    std::cout << res->body << std::endl;
} else {
    std::cerr << "Failed to fetch data." << std::endl;
    return 1;
}

First, we create an HTTP client object using httplib::Client, which takes two parameters:

base_url: A string (const char*) representing the host name or IP address of the server (in our case, "localhost")
port: An integer specifying the port number where the server is listening (in our case, 8000)

Next, we send a GET request to the "/todos" endpoint using the client. The response is stored in the res variable, which contains all information returned by the server.

We then print a header text to indicate we're about to show the raw response.

Finally, we check if the request was successful. If res exists (not null), it prints the response body (the actual data). If the request failed (res is null), it prints an error message and exits the program with code 1.

By using the client.Get() method, we send a GET request to the constructed full URL. The response from the server, stored in res, is then printed. You'll see the raw JSON response from the server:

JSON

[
  {
    "description": "Milk, eggs, bread, and coffee",
    "done": false,
    "id": 1,
    "title": "Buy groceries"
  },
  {
    "description": "Check in and catch up",
    "done": true,
    "id": 2,
    "title": "Call mom"
  },
  {
    "description": "Summarize Q4 performance metrics",
    "done": false,
    "id": 3,
    "title": "Finish project report"
  },
  {
    "description": "30 minutes of cardio",
    "done": true,
    "id": 4,
    "title": "Workout"
  }
]

[
  {
    "description": "Milk, eggs, bread, and coffee",
    "done": false,
    "id": 1,
    "title": "Buy groceries"
  },
  {
    "description": "Check in and catch up",
    "done": true,
    "id": 2,
    "title": "Call mom"
  },
  {
    "description": "Summarize Q4 performance metrics",
    "done": false,
    "id": 3,
    "title": "Finish project report"
  },
  {
    "description": "30 minutes of cardio",
    "done": true,
    "id": 4,
    "title": "Workout"
  }
]

Handling Successful Requests (Status Code: 200)

To determine the outcome of an HTTP request, we check the status code returned by the server. When the server responds with a status code of 200, it signifies a successful interaction, meaning the server has correctly processed the request and returned the expected data. In such cases, we can parse the body in the desired format, which is JSON in our case, using the nlohmann::json library. This allows us to access and display each to-do item.

C++

if (res->status == 200) {
    std::cout << "Todos retrieved successfully:" << std::endl;
    auto todos = nlohmann::json::parse(res->body);
    for (const auto& todo : todos) {
        std::cout << "Title: " << todo["title"] << ", Description: " << todo["description"] << ", Done: " << todo["done"] << std::endl;
    }
}

if (res->status == 200) {
    std::cout << "Todos retrieved successfully:" << std::endl;
    auto todos = nlohmann::json::parse(res->body);
    for (const auto& todo : todos) {
        std::cout << "Title: " << todo["title"] << ", Description: " << todo["description"] << ", Done: " << todo["done"] << std::endl;
    }
}

Here is an example of what you might see when the request is successful:

text

Todos retrieved successfully:
Title: Buy groceries, Description: Milk, eggs, bread, and coffee, Done: False
Title: Call mom, Description: Check in and catch up, Done: True
Title: Finish project report, Description: Summarize Q4 performance metrics, Done: False
Title: Workout, Description: 30 minutes of cardio, Done: True

Todos retrieved successfully:
Title: Buy groceries, Description: Milk, eggs, bread, and coffee, Done: False
Title: Call mom, Description: Check in and catch up, Done: True
Title: Finish project report, Description: Summarize Q4 performance metrics, Done: False
Title: Workout, Description: 30 minutes of cardio, Done: True

By leveraging the nlohmann::json library, we can efficiently extract and work with the data returned by the server, making our programs more intuitive and powerful.

Note: While a status code of 200 indicates success, it does not always guarantee that the response contains the expected data. Some APIs may return an empty array ([]) when no records are found or a subset of records due to pagination. In such cases, make sure to implement corresponding conditional checks.

Making GET Requests and Handling Responses

Setting Up the Environment

Defining the Base URL

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Performing a Basic GET Request

Handling Successful Requests (Status Code: 200)

Handling Bad Requests (Status Code: 400)

Handling Unauthorized Requests (Status Code: 401)

Handling Not Found Errors (Status Code: 404)

Handling Internal Server Errors (Status Code: 500)

Handling Unexpected Status Codes

Conclusion, Key Takeaways, and Next Steps