QBezier: replace out parameters by return-by-value in split()

At least QBezier itself is calling the old function with *this aliased
to one of the arguments. Consequently, the implementation looks rather
... shuffled, to avoid writing into members that it will read once
more later.

Fix by returning a std::pair<QBezier, QBezier> instead. This
simplifies the code that doesn't actually pass existing objects in,
and avoids aliasing problems cropping up under seemingly innocuous
reorderings of statements in the implementation going forward.

While I'm usually vehemently against use std::pair or std::tuple in
APIs, preferring simple structs with aptly-named members instead, this
is one case where the .first and .second actually fit, and pair allows
us to use std::tie, which was handy in qbezier.cpp.

This patch preserves the body of the function as much as possible. A
follow-up patch will clean it up.

Change-Id: I017dfee4a0e69a2e171ce21b89ba04654772c33d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/gui/painting/qbezier_p.h b/src/gui/painting/qbezier_p.h
index f88e3b35b3..02b29c5ba7 100644
--- a/src/gui/painting/qbezier_p.h
+++ b/src/gui/painting/qbezier_p.h
@@ -107,7 +107,7 @@ public:
     inline QLineF endTangent() const;
 
     inline void parameterSplitLeft(qreal t, QBezier *left);
-    inline void split(QBezier *firstHalf, QBezier *secondHalf) const;
+    inline std::pair<QBezier, QBezier> split() const;
 
     int shifted(QBezier *curveSegments, int maxSegmets,
                 qreal offset, float threshold) const;
@@ -223,10 +223,11 @@ inline QPointF QBezier::secondDerivedAt(qreal t) const
                        a * y1 + b * y2 + c * y3 + d * y4);
 }
 
-inline void QBezier::split(QBezier *firstHalf, QBezier *secondHalf) const
+std::pair<QBezier, QBezier> QBezier::split() const
 {
-    Q_ASSERT(firstHalf);
-    Q_ASSERT(secondHalf);
+    std::pair<QBezier, QBezier> r;
+    auto firstHalf = &r.first;
+    auto secondHalf = &r.second;
 
     qreal c = (x2 + x3)*.5;
     firstHalf->x2 = (x1 + x2)*.5;
@@ -245,6 +246,8 @@ inline void QBezier::split(QBezier *firstHalf, QBezier *secondHalf) const
     firstHalf->y3 = (firstHalf->y2 + c)*.5;
     secondHalf->y2 = (secondHalf->y3 + c)*.5;
     firstHalf->y4 = secondHalf->y1 = (firstHalf->y3 + secondHalf->y2)*.5;
+
+    return r;
 }
 
 inline void QBezier::parameterSplitLeft(qreal t, QBezier *left)