compiler-warning-level-1-c4540.md

description	title	ms.date	f1_keywords	helpviewer_keywords	ms.assetid
Learn more about: Compiler Warning (level 1) C4540	Compiler Warning (level 1) C4540	11/04/2016	C4540	C4540	8085e748-5f4d-43c2-b06d-eaf794edbf72

Compiler Warning (level 1) C4540

dynamic_cast used to convert to inaccessible or ambiguous base; run-time test will fail ('type1' to 'type2')

You used dynamic_cast to convert from one type to another. The compiler determined that the cast would always fail (return NULL) because a base class is inaccessible (private, for instance) or ambiguous (appears more than once in the class hierarchy, for instance).

The following shows an example of this warning. Class B is derived from class A. The program uses dynamic_cast to cast from class B (the derived class) to class A, which will always fail because class B is private and thus inaccessible. Changing the access of A to public will resolve the warning.

// C4540.cpp
// compile with: /W1

struct A {
   virtual void g() {}
};

struct B : private A {
   virtual void g() {}
};

int main() {
   B b;
   A * ap = dynamic_cast<A*>(&b);   // C4540
}