Struct <-> JSON Mapping

Demonstrates bi-directional serialization and deserialization between C++ structs and JSON payloads with zero dynamic document allocations.

Source: examples/StructJson/StructJson.ino

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Overview

By adding the REST_JSON_MAP macro inside your struct definition, you can: 1. Send structs directly in request bodies: client.post("/todos").body(newTodo); 2. Populate structs directly from HTTP responses: client.get("/todos/1").getBody(&todo); 3. Convert outside HTTP requests: ESP32HTTPClient::toJson(obj) and ESP32HTTPClient::fromJson(json, &obj).

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Full Sketch

#include <Arduino.h>
#include <WiFi.h>
#include "ESP32HTTPClient.h"

const char* ssid     = "YOUR_SSID";
const char* password = "YOUR_PASSWORD";

ESP32HTTPClient client("https://jsonplaceholder.typicode.com");

// Declare mapped struct
struct Todo {
    int id = 0;
    int userId = 0;
    char title[64] = {0};
    bool completed = false;

    REST_JSON_MAP(
        REST_FIELD(id),
        REST_FIELD(userId),
        REST_FIELD(title),
        REST_FIELD(completed)
    )
};

void setup() {
    Serial.begin(115200);

    WiFi.begin(ssid, password);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nConnected to WiFi");
}

void loop() {
    Todo todo;

    // 1. Fetching remote JSON directly into a struct
    Serial.println("\n--- [1] Fetching Todo as Struct ---");
    client.get("/todos/1").getBody(&todo);

    if (client.isSuccess()) {
        Serial.printf("Struct Populated -> ID: %d, User: %d, Title: %s, Completed: %s\n",
                      todo.id, todo.userId, todo.title, todo.completed ? "true" : "false");
    }

    // 2. Sending a struct as JSON request body
    Serial.println("\n--- [2] Posting Todo from Struct ---");
    Todo newTodo;
    newTodo.userId = 1;
    newTodo.id = 101;
    strncpy(newTodo.title, "Build awesome IoT device", sizeof(newTodo.title));
    newTodo.completed = false;

    Todo responseTodo;
    client.post("/todos")
          .body(newTodo)
          .getBody(&responseTodo);

    if (client.isSuccess()) {
        Serial.printf("Struct Posted -> Created Resource ID: %d\n", responseTodo.id);
    }

    // 3. Standalone JSON conversion
    Serial.println("\n--- [3] Standalone Struct to JSON String ---");
    String jsonString = ESP32HTTPClient::toJson(newTodo);
    Serial.printf("Serialized JSON: %s\n", jsonString.c_str());

    delay(15000);
}

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Expected Serial Output

Connected to WiFi

--- [1] Fetching Todo as Struct ---
Struct Populated -> ID: 1, User: 1, Title: delectus aut autem, Completed: false

--- [2] Posting Todo from Struct ---
Struct Posted -> Created Resource ID: 101

--- [3] Standalone Struct to JSON String ---
Serialized JSON: {"id":101,"userId":1,"title":"Build awesome IoT device","completed":false}

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Key Takeaways

• Missing fields in JSON responses leave the struct member default values intact.
• JSON null values are automatically handled without crashes or corruption.
• Extra unknown fields in JSON are safely ignored.