inheritance - Does this C++ program invoke undefined behavior? -

i reading static_cast operator.

consider following example:

#include <iostream> class b { }; class d : public b  {     public:         void fun()         {             std::cout<<"fun() called\n";         } }; void f(b* pb,d* pd) {     d* pd2=static_cast<d*>(pb);     b* pb2=static_cast<b*>(pd);     pd2->fun(); } int main() {     b b;     d d;     f(&b,&d); } 

it says that:

in example follows, line d* pd2 = static_cast(pb); not safe because d can have fields , methods not in b. however, line b* pb2 = static_cast(pd); safe conversion because d contains of b.

in contrast dynamic_cast, no run-time check made on static_cast conversion of pb. object pointed pb may not object of type d, in case use of *pd2 disastrous. instance, calling function member of d class, not b class, result in access violation.

i tried on gcc 4.8.1 & msvs 2010 & output fun() called. program invoke undefined behavior? can program crash @ runtime? c++ standard says this? please correct me if understood incorrectly.

yes, of course is.

you invoking member function of d on object that's b, forcing conversion b* d*.

it "appears work" because function involved not attempt access actual data and, therefore, there no memory access computer notice , complain @ runtime.

---

in fact, don't need bother talking whether function call has undefined behaviour; cast bad enough:

[c++14: 5.4.9/11]: a prvalue of type “pointer cv1 b,” b class type, can converted prvalue of type “pointer cv2 d,” d class derived (clause 10) b, if valid standard conversion “pointer d” “pointer b” exists (4.10), cv2 same cv-qualification as, or greater cv-qualification than, cv1, , b neither virtual base class of d nor base class of virtual base class of d. null pointer value (4.10) converted null pointer value of destination type. if prvalue of type “pointer cv1 b” points b subobject of object of type d, resulting pointer points enclosing object of type d. otherwise, behavior undefined.