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c++inheritanceconstructordynamic-castobject-lifetime

Why can't I dynamic_cast "sideways" during multiple inheritence?

The following code throws std::bad_cast

struct Foo {
    void foo () {}
};

struct Bar {
    Bar () {
        dynamic_cast <Foo &> (*this) .foo ();
    }
    virtual ~ Bar () {}
};

struct Baz : public Foo, public Bar {
};

int main ()
{
    Baz b;
}

I remember once reading how dynamic_cast has implementation performance trade-offs because "it traverses the full inheritence lattice" in order to evaluate correctly. What the compiler needs to do here is first cast up and then down again.

Is it possible to make the above work or do I need to add virtual Foo* Bar::as_foo()=0; ?

Solution

  • There are no virtual functions in Foo, so dynamic_cast is perfectly obliged to fail. There needs to be a virtual function. It's also a bad idea to do so during construction, as you're going to run into construction order issues.