c++ typecast

README.adoc

@@ -14644,6 +14644,13 @@ https://stackoverflow.com/questions/81656/where-do-i-find-the-current-c-or-c-sta
 
 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf
 
+[[cpp-type-casting]]
+==== C++ type casting
+
+link:userland/cpp/static_dynamic_reinterpret_cast.cpp[]
+
+https://stackoverflow.com/questions/332030/when-should-static-cast-dynamic-cast-const-cast-and-reinterpret-cast-be-used/60414256#60414256
+
 === POSIX
 
 Programs under link:userland/posix/[] are examples of POSIX C programming.

userland/cpp/static_dynamic_reinterpret_cast.cpp

@@ -0,0 +1,75 @@
+// https://cirosantilli.com/linux-kernel-module-cheat#cpp-type-casting
+
+#include <iostream>
+
+struct B1 {
+    B1(int int_in_b1) : int_in_b1(int_in_b1) {}
+    virtual ~B1() {}
+    void f0() {}
+    virtual int f1() { return 1; }
+    int int_in_b1;
+};
+
+struct B2 {
+    B2(int int_in_b2) : int_in_b2(int_in_b2) {}
+    virtual ~B2() {}
+    virtual int f2() { return 2; }
+    int int_in_b2;
+};
+
+struct D : public B1, public B2 {
+    D(int int_in_b1, int int_in_b2, int int_in_d)
+        : B1(int_in_b1), B2(int_in_b2), int_in_d(int_in_d) {}
+    void d() {}
+    int f2() { return 3; }
+    int int_in_d;
+};
+
+int main() {
+    B2 *b2s[2];
+    B2 b2{11};
+    D *dp;
+    D d{1, 2, 3};
+
+    // The memory layout must support the virtual method call use case.
+    b2s[0] = &b2;
+    // An upcast is an implicit static_cast<>().
+    b2s[1] = &d;
+    std::cout << "&d           " << &d           << std::endl;
+    std::cout << "b2s[0]       " << b2s[0]       << std::endl;
+    std::cout << "b2s[1]       " << b2s[1]       << std::endl;
+    std::cout << "b2s[0]->f2() " << b2s[0]->f2() << std::endl;
+    std::cout << "b2s[1]->f2() " << b2s[1]->f2() << std::endl;
+
+    // Now for some downcasts.
+
+    // Cannot be done implicitly
+    // error: invalid conversion from ‘B2*’ to ‘D*’ [-fpermissive]
+    // dp = (b2s[0]);
+
+    // Undefined behaviour to an unrelated memory address because this is a B2, not D.
+    dp = static_cast<D*>(b2s[0]);
+    std::cout << "static_cast<D*>(b2s[0])            " << dp           << std::endl;
+    std::cout << "static_cast<D*>(b2s[0])->int_in_d  " << dp->int_in_d << std::endl;
+
+    // OK
+    dp = static_cast<D*>(b2s[1]);
+    std::cout << "static_cast<D*>(b2s[1])            " << dp           << std::endl;
+    std::cout << "static_cast<D*>(b2s[1])->int_in_d  " << dp->int_in_d << std::endl;
+
+    // Segfault because dp is nullptr.
+    dp = dynamic_cast<D*>(b2s[0]);
+    std::cout << "dynamic_cast<D*>(b2s[0])           " << dp           << std::endl;
+    //std::cout << "dynamic_cast<D*>(b2s[0])->int_in_d " << dp->int_in_d << std::endl;
+
+    // OK
+    dp = dynamic_cast<D*>(b2s[1]);
+    std::cout << "dynamic_cast<D*>(b2s[1])           " << dp           << std::endl;
+    std::cout << "dynamic_cast<D*>(b2s[1])->int_in_d " << dp->int_in_d << std::endl;
+
+    // Undefined behaviour to an unrelated memory address because this
+    // did not calculate the offset to get from B2* to D*.
+    dp = reinterpret_cast<D*>(b2s[1]);
+    std::cout << "reinterpret_cast<D*>(b2s[1])           " << dp           << std::endl;
+    std::cout << "reinterpret_cast<D*>(b2s[1])->int_in_d " << dp->int_in_d << std::endl;
+}