Merge pull request #6 from gjbex/development

Add example code for introspection

Author: gjbex

source-code/Pointers/README.md

@@ -16,3 +16,6 @@ Some illustrations of using pointers in C++.
   * `tree_uptr.cpp`: main file to test node implementation.
 * `DoubleLinkedList`: implementation of a double linked list
   that requires using raw pointers.
+* `runtime_vs_compile_time.cpp`: illustrates how to determine the
+  type of a derived class object at runtime when it is assigned to
+  a pointer of the parent class.

source-code/Pointers/runtime_vs_compile_time.cpp

@@ -0,0 +1,51 @@
+#include <iostream>
+#include <memory>
+#include <typeinfo>
+
+struct A {
+    virtual void f() const = 0;
+    void p() const { std::cout << "A::p()" << std::endl; }
+    ~A() = default;
+};
+
+struct B : public A {
+    void f() const override { std::cout << "B::f()" << std::endl; }
+    void p() const { std::cout << "B::p()" << std::endl; }
+};
+
+struct C : public A {
+    void f() const override { std::cout << "C::f()" << std::endl; }
+    void p() const { std::cout << "C::p()" << std::endl; }
+};
+
+int main() {
+    B b;
+    std::cout << "B: ";
+    b.f();
+    b.p();
+    C c;
+    std::cout << "C: ";
+    c.f();
+    c.p();
+    std::cout << "A = B: ";
+    std::unique_ptr<A> a = std::make_unique<B>();
+    a->f();
+    a->p();
+    if (dynamic_cast<B*>(a.get())) {
+        std::cout << "a is a B: ";
+        dynamic_cast<B*>(a.get())->p();
+    }
+    if (!dynamic_cast<C*>(a.get()))
+        std::cout << "a is not a C\n";
+    std::cout << "A = B: ";
+    a = std::make_unique<C>();
+    a->f();
+    a->p();
+    if (typeid(*a.get()) != typeid(B))
+        std::cout << "a is not a B\n";
+    if (typeid(*a.get()) == typeid(C)) {
+        std::cout << "a is a C: ";
+        static_cast<C*>(a.get())->p();
+    }
+    return 0;
+}