Add QHttpHeaders::valueAt() function

We need a way for users to consume the complete contents of
QHttpHeaders.

For now, the only way is

    for (const auto &name : h.names())
        use(h.value/combinedValue(name));

which is quadratic.

Adding the usual iterators and operator[] would require us to
expose the underlying value_type, which we're not ready to do, yet.

So add valueAt() and (in a follow-up) nameAt() functions to enable
efficient indexed iteration without the need to expose a value_type.

Resulted from API-review

Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 4bb12dab6a13c6deee713a77efa9ce996adc97a9)
Change-Id: I2ee509d89e6f9080c17a4cef947f80f312d75180

src/network/access/qhttpheaders.cpp

@@ -987,6 +987,18 @@ QList<QByteArray> QHttpHeaders::values(WellKnownHeader name) const
     return values(headerNames[qToUnderlying(name)]);
 }
 
+/*!
+    Returns the header value at index \a i. The index \a i must be valid
+    (see \l size()).
+
+    \sa size(), value(), values(), combinedValue()
+*/
+QByteArrayView QHttpHeaders::valueAt(qsizetype i) const noexcept
+{
+    d->verify(i);
+    return d->headers.at(i).value;
+}
+
 /*!
     Returns the values of header \a name in a comma-combined string.
     Returns a \c null QByteArray if the header with \a name doesn't

src/network/access/qhttpheaders.h

@@ -235,6 +235,8 @@ public:
     Q_NETWORK_EXPORT QList<QByteArray> values(QAnyStringView name) const;
     Q_NETWORK_EXPORT QList<QByteArray> values(WellKnownHeader name) const;
 
+    Q_NETWORK_EXPORT QByteArrayView valueAt(qsizetype i) const noexcept;
+
     Q_NETWORK_EXPORT QByteArray combinedValue(QAnyStringView name) const;
     Q_NETWORK_EXPORT QByteArray combinedValue(WellKnownHeader name) const;
 

tests/auto/network/access/qhttpheaders/tst_qhttpheaders.cpp

@@ -186,6 +186,15 @@ void tst_QHttpHeaders::accessors()
     QCOMPARE(h1.size(), 5);
     QCOMPARE(h1.names().size(), 2);
 
+    // valueAt()
+    h1.clear();
+    h1.append(n1, v1);
+    h1.append(n2, v2);
+    h1.append(n3, v3);
+    QCOMPARE(h1.valueAt(0), v1);
+    QCOMPARE(h1.valueAt(1), v2);
+    QCOMPARE(h1.valueAt(2), v3);
+
     // removeAll()
     h1.clear();
     QVERIFY(h1.append(n1, v1));