Compiler Warning (level 1) C4540
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Compiler Warning (level 1) C4540

Error Message

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
}
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