Add wxVector::shrink_to_fit() for C++11 compatibility

Also use this for wxArray::Shrink() implementation as it's more
efficient than the old swap-based implementation which requires an extra
memory allocation instead of really shrinking the existing one.

include/wx/vector.h

@@ -375,6 +375,12 @@ public:
         return m_capacity;
     }
 
+    void shrink_to_fit()
+    {
+        m_values = Ops::Realloc(m_values, m_size, m_size);
+        m_capacity = m_size;
+    }
+
     bool empty() const
     {
         return size() == 0;

interface/wx/vector.h

@@ -262,6 +262,18 @@ public:
     void resize(size_type n, const value_type& v);
     //@}
 
+    /**
+        Free unused memory allocated by the vector.
+
+        Reduces the memory used by the vector to the bare minimum required to
+        hold its current number of elements, possibly 0.
+
+        After calling this method, capacity() returns the same as size().
+
+        @since 3.1.1
+     */
+    void shrink_to_fit();
+
     /**
         Returns the size of the vector.
     */

tests/vectors/vectors.cpp

@@ -361,3 +361,27 @@ TEST_CASE("wxVector::reverse_iterator", "[vector][reverse_iterator]")
     CHECK( ri - rb == 2 );
     CHECK( re - ri == 8 );
 }
+
+TEST_CASE("wxVector::capacity", "[vector][capacity][shrink_to_fit]")
+{
+    wxVector<int> v;
+    CHECK( v.capacity() == 0 );
+
+    v.push_back(0);
+    // When using the standard library vector, we don't know what growth
+    // strategy it uses, so we can't rely on this check passing, but with our
+    // own one we can, allowing us to check that shrink_to_fit() really shrinks
+    // the capacity below.
+#if !wxUSE_STD_CONTAINERS
+    CHECK( v.capacity() > 1 );
+#endif
+
+    v.shrink_to_fit();
+    CHECK( v.capacity() == 1 );
+
+    v.erase(v.begin());
+    CHECK( v.capacity() == 1 );
+
+    v.shrink_to_fit();
+    CHECK( v.capacity() == 0 );
+}