dboyliao · March 16, 2022 09:33 · Mar 16, 2022 · Mar 16, 2022 · Mar 16, 2022 · Mar 16, 2022
diff --git a/README.md b/README.md
@@ -4,4 +4,8 @@
 $ mkdir -p build
 $ cmake -B build -S .
 $ cmake --build build
-```
+```
+
+# References
+
+- https://stackoverflow.com/questions/149336/practical-uses-for-the-curiously-recurring-template-pattern
diff --git a/private.cpp b/private.cpp
@@ -3,6 +3,6 @@
 
 #include "public.hpp"
 
-float CartesianPoint::dist_impl() const { return std::hypot(v0(), v1()); }
+float CartesianPoint::dist() const { return std::hypot(v0(), v1()); }
 
-float PolarPoint::dist_impl() const { return std::abs(v0()); }
+float PolarPoint::dist() const { return std::abs(v0()); }
diff --git a/public.hpp b/public.hpp
@@ -12,12 +12,13 @@ class PointBase {
   float const &v1() const { return m_v1; }
   float &v1() { return m_v1; }
   float dist() const {
+    static_assert(&Derived::dist != &PointBase::dist,
+                  "derived class must overwrite dist");
     auto &obj = derived();
-    return obj.dist_impl();
+    return obj.dist();
   }
 
  private:
-  virtual float dist_impl() const = 0;
   Derived const &derived() const { return *static_cast<Derived const *>(this); }
   float m_v0, m_v1;
 
@@ -37,7 +38,7 @@ class CartesianPoint : public PointBase<CartesianPoint> {
   using PointBase<CartesianPoint>::PointBase;
 
  private:
-  float dist_impl() const;
+  float dist() const;
   friend class PointBase<CartesianPoint>;
 };
 
@@ -46,7 +47,7 @@ class PolarPoint : public PointBase<PolarPoint> {
   using PointBase<PolarPoint>::PointBase;
 
  private:
-  float dist_impl() const;
+  float dist() const;
   friend class PointBase<PolarPoint>;
 };
 
@@ -55,7 +56,6 @@ class BadPoint : public PointBase<BadPoint> {
   using PointBase<BadPoint>::PointBase;
 
  private:
-  float dist_impl() const { return -1.0; }
   friend class PointBase<BadPoint>;
 };
 
@@ -64,6 +64,5 @@ class BadBadPoint : public PointBase<BadPoint> {
   using PointBase<BadPoint>::PointBase;
 
  private:
-  float dist_impl() const { return -1.0; }
   friend class PointBase<BadPoint>;
 };
diff --git a/.gitignore b/.gitignore
@@ -1,3 +1,4 @@
 build
 lib
 bin
+.vscode
diff --git a/crtp.cpp b/crtp.cpp
@@ -1,5 +1,3 @@
-
-
 #include <cstdint>
 #include <utility>
 

diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -3,9 +3,9 @@ project(crtp_example LANGUAGES CXX)
 set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_VERBOSE_MAKEFILE ON)
 
-add_library(public_lib SHARED src/private.cpp)
+add_library(public_lib SHARED private.cpp)
 target_include_directories(public_lib
-    PUBLIC include
+    PUBLIC ${CMAKE_SOURCE_DIR}
 )
 set_target_properties(public_lib
     PROPERTIES

diff --git a/README.md b/README.md
@@ -0,0 +1,7 @@
+# Build
+
+```bash
+$ mkdir -p build
+$ cmake -B build -S .
+$ cmake --build build
+```
diff --git a/src/private.cpp → private.cpp b/src/private.cpp → private.cpp
diff --git a/include/public.hpp → public.hpp b/include/public.hpp → public.hpp
diff --git a/.gitignore b/.gitignore
@@ -0,0 +1,3 @@
+build
+lib
+bin
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -0,0 +1,20 @@
+cmake_minimum_required(VERSION 3.8)
+project(crtp_example LANGUAGES CXX)
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_VERBOSE_MAKEFILE ON)
+
+add_library(public_lib SHARED src/private.cpp)
+target_include_directories(public_lib
+    PUBLIC include
+)
+set_target_properties(public_lib
+    PROPERTIES
+    LIBRARY_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR}/lib
+)
+
+add_executable(${PROJECT_NAME} ${CMAKE_SOURCE_DIR}/crtp.cpp)
+target_link_libraries(${PROJECT_NAME} public_lib)
+set_target_properties(${PROJECT_NAME}
+    PROPERTIES
+    RUNTIME_OUTPUT_DIRECTORY ${CMAKE_SOURCE_DIR}/bin
+)
diff --git a/crtp.cpp b/crtp.cpp
@@ -1,70 +1,9 @@
-#include <algorithm>
-#include <cmath>
-#include <cstdint>
-#include <iostream>
-#include <utility>
-
-template <class Derived>
-class PointBase {
- public:
-  constexpr static const double PI = 3.14159265358979323846;
-  PointBase(float v0, float v1) : m_v0(v0), m_v1(v1) {
-    std::cout << "PointBase: " << this << std::endl;
-  }
-  float const &v0() const { return m_v0; }
-  float &v0() { return m_v0; }
-  float const &v1() const { return m_v1; }
-  float &v1() { return m_v1; }
-  float dist() const {
-    // Prevent the derived class from working if it doesn't define dist(),
-    // just like what a pure virtual function does.
-    static_assert(&PointBase<Derived>::dist != &Derived::dist,
-                  "derived class must define dist()");
-    return static_cast<const Derived *>(this)->dist();
-  }
-
- private:
-  Derived const &derived() const { return *static_cast<Derived const *>(this); }
-  float m_v0, m_v1;
-
- private:
-  // safe guard for invalid inheritance
-  PointBase() {}
-  friend Derived;
-}; /* end class PointBase */
 
-template <typename Derived>
-float use_dist(const PointBase<Derived> &pt) {
-  return pt.dist();
-}
 
-class CartesianPoint : public PointBase<CartesianPoint> {
- public:
-  using PointBase<CartesianPoint>::PointBase;
-  float dist() const { return std::hypot(v0(), v1()); }
-}; /* end class CartesianPoint */
-
-class PolarPoint : public PointBase<PolarPoint> {
- public:
-  using PointBase<PolarPoint>::PointBase;
-  float dist() const { return std::abs(v0()); }
-}; /* end class PolarPoint */
-
-class BadPoint : public PointBase<BadPoint> {
- public:
-  using PointBase<BadPoint>::PointBase;
-  // Intentionally omit dist().
-  /*float dist() const
-  {
-      return 0.0;
-  }*/
-}; /* end class BadPoint */
+#include <cstdint>
+#include <utility>
 
-// intentionally iheritant from wrong base class
-class BadBadPoint : public PointBase<BadPoint> {
- public:
-  using PointBase<BadPoint>::PointBase;
-};
+#include "public.hpp"
 
 int main(int, char **) {
   std::cout << "sizeof(CartesianPoint) = " << sizeof(CartesianPoint)

diff --git a/include/public.hpp b/include/public.hpp
@@ -0,0 +1,69 @@
+#include <iostream>
+
+template <class Derived>
+class PointBase {
+ public:
+  constexpr static const double PI = 3.14159265358979323846;
+  PointBase(float v0, float v1) : m_v0(v0), m_v1(v1) {
+    std::cout << "PointBase: " << this << std::endl;
+  }
+  float const &v0() const { return m_v0; }
+  float &v0() { return m_v0; }
+  float const &v1() const { return m_v1; }
+  float &v1() { return m_v1; }
+  float dist() const {
+    auto &obj = derived();
+    return obj.dist_impl();
+  }
+
+ private:
+  virtual float dist_impl() const = 0;
+  Derived const &derived() const { return *static_cast<Derived const *>(this); }
+  float m_v0, m_v1;
+
+ private:
+  // safe guard for invalid inheritance
+  PointBase() {}
+  friend Derived;
+}; /* end class PointBase */
+
+template <typename Derived>
+float use_dist(const PointBase<Derived> &pt) {
+  return pt.dist();
+}
+
+class CartesianPoint : public PointBase<CartesianPoint> {
+ public:
+  using PointBase<CartesianPoint>::PointBase;
+
+ private:
+  float dist_impl() const;
+  friend class PointBase<CartesianPoint>;
+};
+
+class PolarPoint : public PointBase<PolarPoint> {
+ public:
+  using PointBase<PolarPoint>::PointBase;
+
+ private:
+  float dist_impl() const;
+  friend class PointBase<PolarPoint>;
+};
+
+class BadPoint : public PointBase<BadPoint> {
+ public:
+  using PointBase<BadPoint>::PointBase;
+
+ private:
+  float dist_impl() const { return -1.0; }
+  friend class PointBase<BadPoint>;
+};
+
+class BadBadPoint : public PointBase<BadPoint> {
+ public:
+  using PointBase<BadPoint>::PointBase;
+
+ private:
+  float dist_impl() const { return -1.0; }
+  friend class PointBase<BadPoint>;
+};
diff --git a/src/private.cpp b/src/private.cpp
@@ -0,0 +1,8 @@
+#include <algorithm>
+#include <cmath>
+
+#include "public.hpp"
+
+float CartesianPoint::dist_impl() const { return std::hypot(v0(), v1()); }
+
+float PolarPoint::dist_impl() const { return std::abs(v0()); }
diff --git a/crtp.cpp b/crtp.cpp
@@ -0,0 +1,90 @@
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+#include <iostream>
+#include <utility>
+
+template <class Derived>
+class PointBase {
+ public:
+  constexpr static const double PI = 3.14159265358979323846;
+  PointBase(float v0, float v1) : m_v0(v0), m_v1(v1) {
+    std::cout << "PointBase: " << this << std::endl;
+  }
+  float const &v0() const { return m_v0; }
+  float &v0() { return m_v0; }
+  float const &v1() const { return m_v1; }
+  float &v1() { return m_v1; }
+  float dist() const {
+    // Prevent the derived class from working if it doesn't define dist(),
+    // just like what a pure virtual function does.
+    static_assert(&PointBase<Derived>::dist != &Derived::dist,
+                  "derived class must define dist()");
+    return static_cast<const Derived *>(this)->dist();
+  }
+
+ private:
+  Derived const &derived() const { return *static_cast<Derived const *>(this); }
+  float m_v0, m_v1;
+
+ private:
+  // safe guard for invalid inheritance
+  PointBase() {}
+  friend Derived;
+}; /* end class PointBase */
+
+template <typename Derived>
+float use_dist(const PointBase<Derived> &pt) {
+  return pt.dist();
+}
+
+class CartesianPoint : public PointBase<CartesianPoint> {
+ public:
+  using PointBase<CartesianPoint>::PointBase;
+  float dist() const { return std::hypot(v0(), v1()); }
+}; /* end class CartesianPoint */
+
+class PolarPoint : public PointBase<PolarPoint> {
+ public:
+  using PointBase<PolarPoint>::PointBase;
+  float dist() const { return std::abs(v0()); }
+}; /* end class PolarPoint */
+
+class BadPoint : public PointBase<BadPoint> {
+ public:
+  using PointBase<BadPoint>::PointBase;
+  // Intentionally omit dist().
+  /*float dist() const
+  {
+      return 0.0;
+  }*/
+}; /* end class BadPoint */
+
+// intentionally iheritant from wrong base class
+class BadBadPoint : public PointBase<BadPoint> {
+ public:
+  using PointBase<BadPoint>::PointBase;
+};
+
+int main(int, char **) {
+  std::cout << "sizeof(CartesianPoint) = " << sizeof(CartesianPoint)
+            << std::endl;
+  std::cout << "sizeof(PolarPoint) = " << sizeof(PolarPoint) << std::endl;
+
+  CartesianPoint cpt(1, 1);
+  PolarPoint ppt(1, PolarPoint::PI / 4);
+
+  std::cout << "CartesianPoint(1,1)::dist() = " << use_dist(cpt) << std::endl;
+  std::cout << "PolarPoint(1, pi/4)::dist() = " << use_dist(ppt) << std::endl;
+
+  // This won't compile
+  // error: static_assert failed "derived class must define dist()"
+  // BadPoint bp;
+  // use_dist(bp);
+
+  // This won't compile
+  // since the default constructor is not accessible to BadBadPoint (due to
+  // incorrect usage of CRTP)
+  // BadBadPoint bbp;
+  return 0;
+}
-Original file line number
+Diff line change
@@ -1,3 +1,4 @@
     build
     lib
     bin
+    .vscode
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