Get the length of a Serialized JSON #2213
Description
I often find situation where I'd like to know the length of the serialised output; and then usually do something like this:
struct NullWriter {
size_t write(uint8_t c) { return 1; };
size_t write(const uint8_t *buffer, size_t length) { return length; };
} _nullwriter;
size_t len = serializeJson(jsonDoc, _nullwriter);
May be nice to have serializeJson(jsonDoc, NULL); be the equivalent for this.
As a background - here is a typical case were the custom writer really helps; but you need to have the length ahead o f time:
PubSubClient _client; // MQTT client
.... subscribe MQTT, etc...
size_t len = serializeJson(jsonDoc, _nullwriter);
if (_client.beginPublish("/foo/bar", len, false)) {
struct PubSubWriter {
PubSubClient * _psc;
size_t write(uint8_t c) { return _psc->write(c); };
size_t write(const uint8_t *buffer, size_t length) { return _psc->write(buffer,length); };
} _pswriter = { ._psc = &_client };
size_t actual = serializeJson(jsonDoc, _pswriter);
_client.endPublish();
if (actual != len)
Log.printf("Only wrote %d bytes of a %d report to mqtt", actual, len);
} else {
Log.printf("Could not post %d bytes to topic %s to mqtt", len, topic);
};