Chapter 12: JSON Parsing and Generation

An HTTP body is raw bytes, JsonCpp performs parsing, and JsonHelper provides a small set of convenience wrappers.

1. HTTP and JSON Boundaries

request.bodyView() returns a view of the body; it does not prove that the bytes are valid JSON. Validation covers Content-Type, body limits, parsing success, object shape, and field types.

2. Explicit parsing in a server handler

Json::CharReaderBuilder builder;
Json::Value input;
std::string errors;

// bodyView() is non-owning; parse it while the current request is alive.
const std::string_view body = request.bodyView();
std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
if(!reader->parse(body.data(), body.data() + body.size(), &input, &errors))
    return client.sendText("invalid json", "400 Bad Request") ? -1 : -2;

if(!input.isMember("name") || !input["name"].isString())
    return client.sendText("name is required", "422 Unprocessable Entity") ? -1 : -2;

3. JsonHelper contracts

API	Behavior	Failure or boundary
`toString(Value)`	Compact JSON serialization	A JSON string value includes JSON quotes
`toJsonArray(text)`	Parses into `Json::Value`	Prints an error and returns an empty value on failure; server code normally parses explicitly
`createJson(...)`	Creates a flat object from key/value pairs	Arguments are passed as key/value pairs; booleans remain JSON booleans
`createArray(...)`	Creates an array	Use Json::Value directly for complex nesting
`jsonAdd(a,b)`	Merges objects or appends arrays	Returns an empty string on parse failure or mixed container types
`getValue()`	Extracts a field or array element	`-1` failure, `0` scalar, `1` object/array

4. Field types and validation

const std::string text = stt::data::JsonHelper::createJson(
    "ok", true,
    "count", 3,
    "message", "ready");

Json::Value value = stt::data::JsonHelper::toJsonArray(text);
if(value.isObject() && value["ok"].isBool())
    std::cout << value["ok"].asBool() << '
';

Manual JSON concatenation is prone to escaping, type, and structural errors. Escaping, Unicode, numbers, and booleans are easy to corrupt. Body-size limits, object structure, and field types form the validation boundary before field access.

Complete JSON utility demo →