CODEWITHSUNDEEP
c++language-lawyerconstantsdynamic-cast
florestan
florestan

Reputation: 4655

What is the const-ness of dynamic cast of a pointer-type class member

The following simple programm fails to compile with MSVC but not with GCC and Clang? (i.e. MSVC triggers the static_assert).

#include <utility>

class Element {
    virtual ~Element() = default;
};


struct Proxy {

    Element* e{};
};

int main() {

    Proxy p;
    static_assert(std::is_same<decltype(dynamic_cast<const Element*>(p.e)), const Element*>::value, "not const???" );

}

I think the standard is quite clear:

The result of the expression dynamic_cast(v) is the result of converting the expression v to type T. T shall be a pointer or reference to a complete class type, or “pointer to cv void.” The dynamic_cast operator shall not cast away constness ([expr.const.cast]).

Is there some other part in the standard that supports MSVC here, or is it a compiler bug? Or even UB? But if so, why?

Live code here.

Upvotes: 3

Views: 137

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170055

It looks like a compiler bug. [expr.dynamic.cast]/3 implies the result should be a pointer to a const-qualified class type.

If the type of v is the same as T, or it is the same as T except that the class object type in T is more cv-qualified than the class object type in v, the result is v (converted if necessary).

I emphasized "converted if necessary" because it applies to the const qualification of the class type, I believe.

Upvotes: 1

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