Compiler warning (level 1 and level 4, off) C4437

dynamic_cast from virtual base 'base_class' to 'derived_class' could fail in some contexts

This warning is off by default. For more information, see Compiler Warnings That Are Off by Default.

A dynamic_cast operation is used when:

• The cast is from a base class pointer to a derived class pointer.
• The derived class virtually inherits the base class.
• The derived class doesn't have a vtordisp field for the virtual base.
• The cast is found in a constructor or destructor of the derived class, or a class that inherits from the derived class. Otherwise, compiler warning C4436 is emitted issued of C4435.

This warning indicates that the dynamic_cast might not perform correctly when applied to a partially constructed object. This situation occurs when the enclosing function is called from a constructor or destructor of a class that inherits from derived_class. You can ignore the error if derived_class is never further derived, or the enclosing function isn't called during object construction or destruction.

Example

The following sample generates C4437 and demonstrates the code generation issue that arises from the missing vtordisp field:

// C4437.cpp
// To see the warning and runtime assert, compile with: /W4
// To eliminate the warning and assert, compile with: /W4 /vd2
//       or compile with: /W4 /DFIX
#pragma warning(default : 4437)
#include <cassert>

struct A
{
public:
    virtual ~A() {}
};

#if defined(FIX)
#pragma vtordisp(push, 2)
#endif
struct B : virtual A
{
    B()
    {
        func();
    }

    void func()
    {
        A* a = static_cast<A*>(this);
        B* b = dynamic_cast<B*>(a);     // C4437
        assert(this == b);              // assert unless compiled with /vd2
    }
};
#if defined(FIX)
#pragma vtordisp(pop)
#endif

struct C : B
{
    int i;
};

int main()
{
    C c;
}

See also

dynamic_cast Operator
vtordisp
Compiler Warning (level 1) C4436