Add more code examples to QUrl documentation

Code examples make it much easier to learn how an API behaves.
One area that the patch tries to address is the distinction
between a relative URL and a relative path.

Change-Id: Ife52172816b89afb6cd810b07d3573480e2cd747
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qurl.cpp

@@ -90,11 +90,6 @@
     fromPercentEncoding() and toPercentEncoding() which deal with
     percent encoding and decoding of QString objects.
 
-    Calling isRelative() will tell whether or not the URL is
-    relative. A relative URL can be resolved by passing it as argument
-    to resolved(), which returns an absolute URL. isParentOf() is used
-    for determining whether one URL is a parent of another.
-
     fromLocalFile() constructs a QUrl by parsing a local
     file path. toLocalFile() converts a URL to a local file path.
 
@@ -116,6 +111,37 @@
     from freedesktop.org, provided that the locale encodes file names using
     UTF-8 (required by IDN).
 
+    \section2 Relative URLs vs Relative Paths
+
+    Calling isRelative() will return whether or not the URL is relative.
+    A relative URL has no \l {scheme}. For example:
+
+    \code
+    qDebug() << QUrl("main.qml").isRelative();          // true: no scheme
+    qDebug() << QUrl("qml/main.qml").isRelative();      // true: no scheme
+    qDebug() << QUrl("file:main.qml").isRelative();     // false: has "file" scheme
+    qDebug() << QUrl("file:qml/main.qml").isRelative(); // false: has "file" scheme
+    \endcode
+
+    Notice that a URL can be absolute while containing a relative path, and
+    vice versa:
+
+    \code
+    // Absolute URL, relative path
+    QUrl url("file:file.txt");
+    qDebug() << url.isRelative();                 // false: has "file" scheme
+    qDebug() << QDir::isAbsolutePath(url.path()); // false: relative path
+
+    // Relative URL, absolute path
+    url = QUrl("/home/user/file.txt");
+    qDebug() << url.isRelative();                 // true: has no scheme
+    qDebug() << QDir::isAbsolutePath(url.path()); // true: absolute path
+    \endcode
+
+    A relative URL can be resolved by passing it as an argument to resolved(),
+    which returns an absolute URL. isParentOf() is used for determining whether
+    one URL is a parent of another.
+
     \section2 Error checking
 
     QUrl is capable of detecting many errors in URLs while parsing it or when
@@ -2539,6 +2565,12 @@ void QUrl::setPath(const QString &path, ParsingMode mode)
 /*!
     Returns the path of the URL.
 
+    \code
+    qDebug() << QUrl("file:file.txt").path();                   // "file.txt"
+    qDebug() << QUrl("/home/user/file.txt").path();             // "/home/user/file.txt"
+    qDebug() << QUrl("http://www.example.com/test/123").path(); // "/test/123"
+    \endcode
+
     The \a options argument controls how to format the path component. All
     values produce an unambiguous result. With QUrl::FullyDecoded, all
     percent-encoded sequences are decoded; otherwise, the returned value may
@@ -2549,6 +2581,31 @@ void QUrl::setPath(const QString &path, ParsingMode mode)
     sequences are present. It is recommended to use that value when the result
     will be used in a non-URL context, such as sending to an FTP server.
 
+    An example of data loss is when you have non-Unicode percent-encoded sequences
+    and use FullyDecoded (the default):
+
+    \code
+    qDebug() << QUrl("/foo%FFbar").path();
+    \endcode
+
+    In this example, there will be some level of data loss because the \c %FF cannot
+    be converted.
+
+    Data loss can also occur when the path contains sub-delimiters (such as \c +):
+
+    \code
+    qDebug() << QUrl("/foo+bar%2B").path(); // "/foo+bar+"
+    \endcode
+
+    Other decoding examples:
+
+    \code
+    const QUrl url("/tmp/Mambo %235%3F.mp3");
+    qDebug() << url.path(QUrl::FullyDecoded);  // "/tmp/Mambo #5?.mp3"
+    qDebug() << url.path(QUrl::PrettyDecoded); // "/tmp/Mambo #5?.mp3"
+    qDebug() << url.path(QUrl::FullyEncoded);  // "/tmp/Mambo%20%235%3F.mp3"
+    \endcode
+
     \sa setPath()
 */
 QString QUrl::path(ComponentFormattingOptions options) const
@@ -3257,6 +3314,8 @@ QUrl QUrl::resolved(const QUrl &relative) const
     equivalent to calling scheme().isEmpty().
 
     Relative references are defined in RFC 3986 section 4.2.
+
+    \sa {Relative URLs vs Relative Paths}
 */
 bool QUrl::isRelative() const
 {
@@ -3796,6 +3855,41 @@ bool QUrl::isDetached() const
 
     An empty \a localFile leads to an empty URL (since Qt 5.4).
 
+    \code
+    qDebug() << QUrl::fromLocalFile("file.txt");            // QUrl("file:file.txt")
+    qDebug() << QUrl::fromLocalFile("/home/user/file.txt"); // QUrl("file:///home/user/file.txt")
+    qDebug() << QUrl::fromLocalFile("file:file.txt");       // doesn't make sense; expects path, not url with scheme
+    \endcode
+
+    In the first line in snippet above, a file URL is constructed from a
+    local, relative path. A file URL with a relative path only makes sense
+    if there is a base URL to resolve it against. For example:
+
+    \code
+    QUrl url = QUrl::fromLocalFile("file.txt");
+    QUrl baseUrl = QUrl("file:/home/user/");
+    // wrong: prints QUrl("file:file.txt"), as url already has a scheme
+    qDebug() << baseUrl.resolved(url);
+    \endcode
+
+    To resolve such a URL, it's necessary to remove the scheme beforehand:
+
+    \code
+    // correct: prints QUrl("file:///home/user/file.txt")
+    url.setScheme(QString());
+    qDebug() << baseUrl.resolved(url);
+    \endcode
+
+    For this reason, it is better to use a relative URL (that is, no scheme)
+    for relative file paths:
+
+    \code
+    QUrl url = QUrl("file.txt");
+    QUrl baseUrl = QUrl("file:/home/user/");
+    // prints QUrl("file:///home/user/file.txt")
+    qDebug() << baseUrl.resolved(url);
+    \endcode
+
     \sa toLocalFile(), isLocalFile(), QDir::toNativeSeparators()
 */
 QUrl QUrl::fromLocalFile(const QString &localFile)
@@ -3840,6 +3934,12 @@ QUrl QUrl::fromLocalFile(const QString &localFile)
     returned value in the form found on SMB networks (for example,
     "//servername/path/to/file.txt").
 
+    \code
+    qDebug() << QUrl("file:file.txt").toLocalFile();            // "file:file.txt"
+    qDebug() << QUrl("file:/home/user/file.txt").toLocalFile(); // "file:///home/user/file.txt"
+    qDebug() << QUrl("file.txt").toLocalFile();                 // ""; wasn't a local file as it had no scheme
+    \endcode
+
     Note: if the path component of this URL contains a non-UTF-8 binary
     sequence (such as %80), the behaviour of this function is undefined.