Demonstrate static_cast and purpose of it?

Question

I am trying to understand static_cast in regards to upcasting(child to parent). It is not making sense to me. I have to cast a child to a parent and demonstrate it. Upon looking at some code online and referencing books this is what I have.

Mustang *myMustang = new Mustang; Car *myCar = new Car; myMustang = static_cast<Mustang*>(myCar);

But frankly, it does not show anything. I have no verification that it even casted. I tried to add a public function to the Car class and have it called from the child, but... it is obviously inherited.

This also implies I currently do not see a purpose in this type of upcasting.

My question is, how do I verify this even casted and what is the purpose of this typing of casting?

update: The answers were a bit hard to follow due to the fact I do not have experience with this type of casting and virtual functions are a vague memory. My friend was able to help me. Below is the code in case anyone else has the same issue.

class Car { public: virtual void Greeting() { cout << "I am a car." << endl; }; }; class Focus : public Car{ public: void FocusGreeting() { cout << "Hello, I am a Ford Focus." << endl; } }; class Mustang : public Car { public: virtual void Greeting() override { cout << "I am a Ford Mustang." << endl; } }; // in main Mustang* myMustang = new Mustang; Car *myCar = new Car; myCar->Greeting(); cout << "Now "; myCar = static_cast<Car*>(myMustang); myCar->Greeting();

unless Mustang is a base class of Car, this is undefined behaviour — M.M
– M.M, Commented Oct 2, 2016 at 6:25
Also try using virtual functions that can be overriden by derivate types. — Paul Stelian
– Paul Stelian, Commented Oct 2, 2016 at 6:31
I am not too familiar with virtual functions and this type of casting. Thus it was hard to follow the examples. My friend ended up helping me. — asmcriminal
– asmcriminal, Commented Oct 6, 2016 at 8:00

Richard Hodges · Accepted Answer · 2016-10-02 06:49:02Z

An example of use in the CRTP pattern:

#include <type_traits> // // the general concept of being able to accelerate template<class T> struct acceleratable { auto accelerate() { static_assert(std::is_base_of<acceleratable<T>, T>::value, ""); // turn this in to a T, since we know that *this really is a T return static_cast<T*>(this)->do_accelerate(); } }; // // something that implementes the concept of being able to accelerate struct car : acceleratable<car> { private: friend acceleratable<car>; void do_accelerate() { // accelerate implementation here } }; // // free function which accelerates anything that's acceleratable template<class Thing> auto accelerate(Thing& t) { t.accelerate(); } int main() { car c; accelerate(c); }

skypjack · Accepted Answer · 2016-10-02 07:51:57Z

Another nontrivial example of use is the type-erasure:

class S { using FN = void(*)(void*); template<typename T> static void invoke(void *ptr) { static_cast<T*>(ptr)->foo(); } public: template<typename T> static S create(T *t) { S s; s.ptr = t; s.f = &invoke<T>; return s; } void run() { f(ptr); } private: void *ptr; FN f; }; struct A { void foo() {} }; struct B { void foo() {} }; int main() { A a; B b; S s1 = S::create(&a); S s2 = S::create(&b); s1.run(); s2.run(); }

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