[Interest] Difference between QUrl::fromEncoded and QUrl::fromPercentEncoding ?

Thiago Macieira thiago.macieira at intel.com
Wed Aug 14 01:01:09 CEST 2013

On quarta-feira, 14 de agosto de 2013 05:30:44, Richard Turner wrote:
> Just one more question: could you elaborate on the idea that calling the 
> QUrl constructor taking a QString is a big no-no in Qt 4 code, but not 
> in Qt 5?

The problem is that the QUrl constructor in Qt 4 takes a fully-decoded URL, 
which is a broken concept. Fully-decoded URLs don't exist because you *need* 
the percents in some cases to make distinctions.

For example, the two URIs are not equivalent (as you've found out):


URIs (and URLs, which are a subset of URIs) only truly exist in percent-
encoded forms so that the two above forms can be distinguished from one 
another. But the QUrl constructor in Qt 4 assumes that all URLs are in fully 
decoded form. Then when you do:

	QUrl url("http%3A%2F%2Fexample.com")
	qDebug() << url.toEncoded();

because QUrl::toEncoded is the form you'd pass to other applications, you'll 

Which is a third URL, different from the other two above!

PS: I've just found a bug with this URL in Qt 5.[01]:
	QUrl url("http%3A%2F%2Fexample.com");
	qDebug() << url.path();  		// prints "http:%2F%2Fexample.com"
	qDebug() << url.isValid();		// prints false
	qDebug() << url.toEncoded();	// prints ""

There's a special error condition in URLs that QUrl in Qt 5 recognises: path 
contains a colon before the first slash. However, your URL does *not* contain a 
colon before the first slash, it contains a %3A and that's not the same thing. 
This is fixed in Qt 5.2 and I've just submitted a new unit test to prove it:

Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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