Vix.cpp

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JSON Parsers Guide ​

Beginner-friendly guide to the JSON body parser middleware.

This middleware:

  • Validates Content-Type
  • Parses request body into nlohmann::json
  • Stores parsed JSON into request state
  • Returns structured errors automatically

What the middleware does ​

When installed:

  1. Checks body size (optional)
  2. Validates Content-Type: application/json
  3. Parses JSON
  4. Stores result in:
cpp
    req.state<middleware::parsers::JsonBody>()

Minimal Example ​

Save as: json_app_simple.cpp

cpp
#include <vix.hpp>
#include <vix/middleware/app/presets.hpp>

using namespace vix;

int main()
{
  App app;

  app.use("/json", middleware::app::json_dev(
      256,   // max_bytes
      true,  // allow_empty
      true   // require_content_type
  ));

  app.post("/json", [](Request& req, Response& res)
  {
    auto& jb = req.state<middleware::parsers::JsonBody>();

    res.json({
      "ok", true,
      "received", jb.value
    });
  });

  app.run(8080);
}

Run:

bash
vix run json_app_simple.cpp

Test with curl ​

Valid JSON ​

bash
curl -i -X POST http://localhost:8080/json   -H "Content-Type: application/json"   --data '{"x":1}'

Result: 200 OK

Invalid JSON ​

bash
curl -i -X POST http://localhost:8080/json   -H "Content-Type: application/json"   --data '{"x":}'

Result: 400 invalid_json

Wrong Content-Type ​

bash
curl -i -X POST http://localhost:8080/json   -H "Content-Type: text/plain"   --data '{"x":1}'

Result: 415 unsupported_media_type

Strict Mode Example ​

Disallow empty body:

cpp
app.use("/json", middleware::app::json_dev(
    256,
    false,  // allow_empty = false
    true
));

Now:

Empty body -> 400 empty_body

Error Codes ​

400 empty_body 400 invalid_json 413 payload_too_large 415 unsupported_media_type

Production Tips ​

✔ Always keep require_content_type = true ✔ Set max_bytes to prevent abuse ✔ Combine with body_limit middleware globally ✔ Combine with CSRF for browser APIs

Complete Copy-Paste Example ​

cpp
#include <vix.hpp>
#include <vix/middleware/app/presets.hpp>

using namespace vix;

int main()
{
  App app;

  app.use("/api", middleware::app::json_dev(
      512,
      false,
      true
  ));

  app.post("/api/data", [](Request& req, Response& res)
  {
    auto& jb = req.state<middleware::parsers::JsonBody>();

    if (!jb.value.contains("name"))
    {
      res.status(400).json({
        "ok", false,
        "error", "name is required"
      });
      return;
    }

    res.json({
      "ok", true,
      "name", jb.value["name"]
    });
  });

  app.run(8080);
}

Test:

bash
curl -i -X POST http://localhost:8080/api/data   -H "Content-Type: application/json"   --data '{"name":"Gaspard"}'

You now understand how JSON parsing works internally in Vix.cpp.