different behavior of static_cast and dynamic_cast in a specific scenario

Question

I don't figure out the real difference between static_cast and dynamic_cast in below scenario:

 **///with static_cast///** class Foo{}; class Bar: public Foo { public: void func() { return; } }; int main(int argc, char** argv) { Foo* f = new Foo; Bar* b = static_cast<Bar*>(f); b->func(); return 0; }

Output:

Successfully Build and Compiled!

 **///with dynamic_cast///** class Foo{}; class Bar: public Foo { public: void func() { return; } }; int main(int argc, char** argv) { Foo* f = new Foo; Bar* b = dynamic_cast<Bar*>(f); b->func(); return 0; }

Output:

main.cpp: In function 'int main(int, char**)': main.cpp:26:34: error: cannot dynamic_cast 'f' (of type 'class Foo*') to type 'class Bar*' (source type is not polymorphic) Bar* b = dynamic_cast(f);

I'd be appreciated if someone could help me understand this!

While the first builds and runs, it will lead to undefined behavior, because f (and therefore b) isn't a pointer to a Bar object. — Some programmer dude
– Some programmer dude, Commented Feb 6, 2018 at 8:20
You can't cast a pointer to a base class object to a derived class pointer. dynamic_cast (in case of virtual members) will return nullptr. static_cast will compile but using such pointer will result in undefined behavior. — Daniel Langr
– Daniel Langr, Commented Feb 6, 2018 at 8:49

Olaf Dietsche · Accepted Answer · 2018-02-06 08:24:39Z

The hint is in the part

(source type is not polymorphic)

It means, for dynamic_cast to work, it needs a polymorphic base class, i.e. have a virtual method

class Foo { public: virtual ~Foo() {} };

Apart from that, it will not work, because f does not point to a Bar object. dynamic_cast will return a nullptr in this case, which you must check

Foo* f = new Foo; Bar* b = dynamic_cast<Bar*>(f); if (b != nullptr) b->func();

Just to expand on this a bit: The main reason the polymorphic base class is required is to perform this run time check. A lesser reason is to enable downcasting over a virtual inheritance relation. static_cast cannot handle this latter case because the offset of the base class subobject is not fixed among the derived class and all its further derived classes.

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