fix double includes and constness

include/shapes/Rectangle.hpp

@@ -1,12 +1,15 @@
+#ifndef RECTANGLE
+#define RECTANGLE
+
 #include "Shape.hpp"
 
-struct Rectangle : public Shape
-{
+struct Rectangle : public Shape {
   Rectangle(const double sideLen);
 
-  double area() override;
+  double area() const override;
 
 private:
   double sideLen{};
 };
 
+#endif

include/shapes/Shape.hpp

@@ -1,11 +1,14 @@
+#ifndef SHAPE
+#define SHAPE
+
 #include <cstdint>
 #include <iostream>
 
 struct Shape
 {
-  virtual double area() = 0;
+  virtual double area() const = 0;
 
-  double getArea();
+  double getArea() const;
 
 protected:
   Shape() = delete;
@@ -13,3 +16,5 @@ protected:
 
   size_t numSides{};
 };
+
+#endif

include/shapes/Triangle.hpp

@@ -1,11 +1,16 @@
+#ifndef TRIANGLE
+#define TRIANGLE
+
 #include "Shape.hpp"
 
 struct Triangle : public Shape
 {
   Triangle(const double sideLen);
 
-  double area() override;
+  double area() const override;
 
 private:
   double sideLen{};
 };
+
+#endif