Make various gfx classes more amenable to use as compile-time constants.

ui/gfx/geometry/insets.h

Index: ui/gfx/geometry/insets.h
diff --git a/ui/gfx/geometry/insets.h b/ui/gfx/geometry/insets.h
index 2fc0d62412b5e962f549e889c327996f83446eac..0318aa490237bdbe7c1d68464d04d8385748955f 100644
--- a/ui/gfx/geometry/insets.h
+++ b/ui/gfx/geometry/insets.h
@@ -14,25 +14,29 @@ namespace gfx {
 
 class GFX_EXPORT Insets {
  public:
-  Insets();
-  explicit Insets(int all);
-  Insets(int vertical, int horizontal);
-  Insets(int top, int left, int bottom, int right);
-
-  ~Insets();
-
-  int top() const { return top_; }
-  int left() const { return left_; }
-  int bottom() const { return bottom_; }
-  int right() const { return right_; }
+  constexpr Insets() : top_(0), left_(0), bottom_(0), right_(0) {}
+  constexpr explicit Insets(int all)
+      : top_(all), left_(all), bottom_(all), right_(all) {}
+  constexpr Insets(int vertical, int horizontal)
+      : top_(vertical),
+        left_(horizontal),
+        bottom_(vertical),
+        right_(horizontal) {}
+  constexpr Insets(int top, int left, int bottom, int right)
+      : top_(top), left_(left), bottom_(bottom), right_(right) {}
+
+  constexpr int top() const { return top_; }
+  constexpr int left() const { return left_; }
+  constexpr int bottom() const { return bottom_; }
+  constexpr int right() const { return right_; }
 
   // Returns the total width taken up by the insets, which is the sum of the
   // left and right insets.
-  int width() const { return left_ + right_; }
+  constexpr int width() const { return left_ + right_; }
 
   // Returns the total height taken up by the insets, which is the sum of the
   // top and bottom insets.
-  int height() const { return top_ + bottom_; }
+  constexpr int height() const { return top_ + bottom_; }
 
   // Returns true if the insets are empty.
   bool IsEmpty() const { return width() == 0 && height() == 0; }
@@ -60,6 +64,13 @@ class GFX_EXPORT Insets {
     right_ += insets.right_;
   }
 
+  void operator-=(const Insets& insets) {

[danakj, 2016/06/10 00:36:47]

Can you add a unit test for each new operator?

[Peter Kasting, 2016/06/10 05:50:06]

Good idea. Done.

+    top_ -= insets.top_;
+    left_ -= insets.left_;
+    bottom_ -= insets.bottom_;
+    right_ -= insets.right_;
+  }
+
   Insets operator-() const {
     return Insets(-top_, -left_, -bottom_, -right_);
   }
@@ -90,6 +101,16 @@ class GFX_EXPORT Insets {
   int right_;
 };
 
+inline Insets operator+(Insets lhs, const Insets& rhs) {
+  lhs += rhs;

[danakj, 2016/06/10 00:36:47]

These methods could be constexpr (or at least not depend on mutating an Insets)
if you reverse the dependency.

ie. This method does:

return Insets(lhs.top + rhs.top, ...);

And operator+= can do:
*this = *this + rhs;

wdyt?

[Peter Kasting, 2016/06/10 05:50:06]

I don't have a strong opinion.  I took this format from
http://en.cppreference.com/w/cpp/language/operators (see the section on "Binary
arithmetic operators").  They mention something about optimizing chained
addition.  I don't know whether your proposal is equally optimal, or if we even
care.  Intuitively, it seems like if += is more common than +, the way this is
structured now might be better, but my instincts as to what the compiler can
optimize are very bad.

I guess what I'm saying is, glance at that stuff, figure out if you still think
your suggestion is better, and I'll go whichever route you prefer.

[danakj, 2016/06/14 19:04:12]

I see. Interesting. I guess if we don't need constexpr on these then this is
nice for chaining, which we probably don't do much. I don't have much of an
opinion I guess until we need these to be constexpr, so this is fine.

+  return lhs;
+}
+
+inline Insets operator-(Insets lhs, const Insets& rhs) {
+  lhs -= rhs;
+  return lhs;
+}
+
 }  // namespace gfx
 
 #endif  // UI_GFX_GEOMETRY_INSETS_H_