Add qgsAsConst(...)

This is the equivalent of std::as_const (which requires c++17) or
qAsConst (which requires Qt 5.7), neither of which we have
as minimum dependancies.

By wrapping implicitly shared Qt containers in qgsAsConst we can
safely use c++ for ranged loops instead of Q_FOREACH. (Since
Q_FOREACH's future is shaky)

See https://www.kdab.com/goodbye-q_foreach/ for further details
on why for causes a detach for Qt containers and why Q_FOREACH
is being removed from Qt.

python/core/qgis.sip

@@ -161,6 +161,8 @@ double qgsRound( double number, double places );
  :rtype: float
 %End
 
+
+
 double qgsPermissiveToDouble( QString string, bool &ok );
 %Docstring
  Converts a string to a double in a permissive way, e.g., allowing for incorrect

src/core/qgis.h

@@ -250,6 +250,31 @@ inline double qgsRound( double number, double places )
   return static_cast<double>( static_cast<qlonglong>( number * scaleFactor + 0.5 ) ) / scaleFactor;
 }
 
+
+#ifndef SIP_RUN
+
+///@cond PRIVATE
+
+/**
+ * Adds const to non-const objects.
+ *
+ * To be used as a proxy for std::as_const until we target c++17 minimum.
+ *
+ * \since QGIS 3.0
+ * \note not available in Python bindings
+ */
+// TODO - remove when we target c++17 minimum and can use std::as_const
+template <typename T> struct QgsAddConst { typedef const T Type; };
+
+template <typename T>
+constexpr typename QgsAddConst<T>::Type &qgsAsConst( T &t ) noexcept { return t; }
+
+template <typename T>
+void qgsAsConst( const T && ) = delete;
+
+///@endcond
+#endif
+
 /** Converts a string to a double in a permissive way, e.g., allowing for incorrect
  * numbers of digits between thousand separators
  * \param string string to convert

tests/src/core/testqgis.cpp

@@ -41,6 +41,7 @@ class TestQgis : public QObject
     void signalBlocker();
     void qVariantCompare_data();
     void qVariantCompare();
+    void testQgsAsConst();
 
   private:
     QString mReport;
@@ -278,6 +279,32 @@ void TestQgis::qVariantCompare()
   QCOMPARE( qgsVariantGreaterThan( lhs, rhs ), greaterThan );
 }
 
+class ConstTester
+{
+  public:
+
+    void doSomething()
+    {
+      mVal = 1;
+    }
+
+    void doSomething() const
+    {
+      mVal = 2;
+    }
+
+    mutable int mVal = 0;
+};
+
+void TestQgis::testQgsAsConst()
+{
+  ConstTester ct;
+  ct.doSomething();
+  QCOMPARE( ct.mVal, 1 );
+  qgsAsConst( ct ).doSomething();
+  QCOMPARE( ct.mVal, 2 );
+}
+
 
 QGSTEST_MAIN( TestQgis )
 #include "testqgis.moc"