Welcome to this lesson on sending data with POST requests in C++. As you continue your exploration of interacting with RESTful APIs, you'll learn how to send data to a server using the POST method. POST requests are essential for creating new resources or submitting data, such as when filling out a web form or adding a new entry to a database. Unlike GET requests, which you have already encountered, POST requests do not solely retrieve information; they actually transmit data to an API.

Understanding these differences is crucial as you expand your skill set in HTTP methods. Let’s dive deeper into utilizing POST requests to comprehend how they stand apart from GET requests.

Before diving into POST requests, let's briefly compare them to GET requests:

• GET Requests:

  • Purpose: Retrieve data from a server.
  • Data Location: Data is sent in the URL as path or query parameters.
  • Success Status: Expect a 200 status code for successful data retrieval.
  • Security and Data Visibility: Data is appended to the URL, making it visible in browser history, server logs, and network tools. This makes GET unsuitable for sensitive data, such as passwords or personal information.

• POST Requests:

  • Purpose: Send data to a server to create or update a resource, such as submitting a form, uploading a file, or adding a new item to a database.
  • Data Location: Data is sent in the request body.
  • Success Status: Expect a 201 status code for successful resource creation.
  • Security and Data Visibility: Data is sent in the body, making it less exposed compared to GET requests, and more suitable for sensitive information.

These differences clarify when to use each method. POST requests, in particular, require careful handling of the request body.

For POST requests, the request body is crucial as it holds the data you want to send to the server. This data is usually structured in formats like JSON, XML, or form data, with JSON being a common choice due to its readability and compatibility.

Here's an example of a JSON request body using nlohmann/json:

This represents a new todo item, including a title, completion status, and description. Note that we are not sending an id, as this is typically managed by the server upon resource creation.

In this lesson, we will utilize cpp-httplib library, with a specific focus on how it facilitates POST requests. Ensure you have the base URL for your API endpoint, which in our scenario is http://localhost:8000. That said, here’s a quick reminder of what you need to get started:

This sets up the foundation for us to explore POST requests further.

A POST request consists of several essential components:

1. Endpoint URL: The specific URL where the request will be sent, typically indicating the resource or action being targeted.

2. Request Body: The data being sent to the server, often structured in formats like JSON, XML, or form data.

3. Headers: Additional information about the request. Common headers include Content-Type (specifies the format of the request body, like application/json or application/xml for XML-formatted data).

Let's walk through an example of how to craft a POST request to add a new todo item to our API using cpp-httplib:

First, you'll need to prepare the data you wish to send. Here, we'll be adding a new todo item with a specific title, a completion status, and a description. The data is structured as a JSON object in C++:

Using cpp-httplib, you can send a POST request like so:

Here, the endpoint URL is formed as "/todos", and newTodo.dump() converts the JSON object to a string, sending it as JSON data in the request body. Additionally, specifying application/json as the Content-Type ensures that the server correctly interprets the request. Some APIs reject requests that omit this header or use an incorrect content type.

Interpreting the response from a POST request is an integral part of the process. After sending the request, the server provides a response indicating whether the operation was successful. A 201 status code signifies successful resource creation.

We use the response handling capabilities of cpp-httplib like this:

The code checks if the server response has a 201 status code, confirming successful creation, allowing you to print and use the details of the new todo, typically included in the server's response. Otherwise, it handles potential errors by outputting relevant error information.

For example, if the operation is successful, the output will be:

A POST request may fail if required fields are missing. For example, omitting a "title" might lead to an error response:

Running this code would produce:

In this lesson, you have learned how to send data to an API using POST requests in C++. We explored how POST differs from GET in terms of creating new resources. Using libraries like cpp-httplib, you are equipped to craft and send POST requests, handle responses, and manage errors to ensure robust API interactions.

As you proceed to the practice exercises, you will have the opportunity to apply everything you have learned here. Practice creating POST requests to reinforce your understanding and discover first-hand the nuances of sending data to an API. Keep up the excellent work, as each lesson brings you closer to mastering API interactions with C++!