Qt support for XML is very strong, up to and including support for XML schema validation.
Qt Support for JSON appears to be less extensive. Nothing I can find seems to confirm or deny support for json-schema or any other kind of Json schema validation in Qt.
Is there a sound way to validate json in Qt / C++?
Edit: to be clear, this question is centered on Json schema validation, not just confirming if an arbitrary document is valid Json.
Indeed, it seems there are no Qt support for JSON Schema validation, even in Qt 5.1. Since writing your own would be very time-consuming, I would suggest:
If you as developer would have provided the Schema, then do not validate by using a JSON Schema validation, but instead perform a hard-coded validation of your parsed JSON (i.e., manually check that the required fields are present, are of the correct type, and are within the specified bounds)
If handling external JSON Schema is necessary (i.e., the Schema is not known in advance, possibly user-defined), then do not use Qt but an independent C/C++ JSON validator, like WJElement (the one linked on the json-schema website)