There should be separation of roles to avoid creation of GOD classes with unclear API:

• decoder instance for exact input stream data. New data stream - new instance. No need to support multiple read calls/etc (because nobody would test these cases).
• decoder as format decoder. With some decoding settings/properties.
• decoder factory/registry.

@alalek, I think, removal close() from ReadData(), so that it's only called in destructor or in 'open()' does not relate to GOD concept, as you call it. close() will be called in any case.

The user might call this method more than once.

The GOD concept is about this.
Current code is not designed for that. Also there are no tests, so we can't check this proposal anyway (especially besides of JPEG/PNG codecs).

@ocpalo Please pay attention that Decoder classes are not designed to call methods multiple times and it;s not tested.

@asmorkalov I see. Yet this calls are still unnecessary. Consider readHeader method fails. result will be false and readHeader method will call close() method. Since codecs are also RAII classes, destructor of this codecs calls close() method too! If i am not missing any other point, this function calls is redundant.