Constructor is always used instead of explicit conversion operator

Question

I have the following class:

template <typename T1>
class Foo {
public:
    Foo(T1* obj) : obj(obj) {}

    template <typename T2>
    Foo(const Foo<T2>& other) : obj(other.obj) {}

    template <typename T2>
    explicit operator Foo<T2>() {
        return Foo<T2>(static_cast<T2*>(obj));
    }
    T1* obj;
};

The intention of the second constructor is that an implicit conversion from Foo<X> to Foo<Y> is allowed iff an implicit conversion from X* to Y* is allowed.

The conversion operator is there to allow an explicit conversion from Foo<X> to Foo<Y> using an explicit conversion from X* to Y*.

But I noticed that the conversion operator never gets used. The compiler always uses the second constructor even when I do an explicit cast. This causes an error if an implicit conversion of the underlying types is not possible.

The following code can be used to test the class above.

class X {};
class Y : public X {};

int main() {
    Y obj;
    Foo<Y> y(&obj);
    Foo<X> x = y; // implicit cast works as expected.
    // y = x; // implicit conversion fails (as expected).
    // y = static_cast<Foo<Y>>(x); // conversion fails because constructor is
                                   // called instead of conversion operator.
}

Is there a way to cause the compiler to use the conversion operator for explicit conversions?

I wouldn't go with a mere explicit conversion/cast to cause a static cast on pointers; that is too easy to do accidentally (you mean to explicitly construct a Foo<Y> to pass to a template function; you accidentally convert a Foo<X> in an invalid way). Instead, I'd have a function that clearly is a static foo cast, so you can see the points of danger, and it only happens when you mean it to. — Yakk - Adam Nevraumont

songyuanyao songyuanyao · Accepted Answer · 2017-07-16T15:38:35

For static_cast<Foo<Y>>(x);, you're trying to construct a Foo<Y> from x (which is a Foo<X>) directly, for such context the converting constructor is preferred to conversion function.

(emphasis mine)

If both conversion functions and converting constructors can be used to perform some user-defined conversion, the conversion functions and constructors are both considered by overload resolution in copy-initialization and reference-initialization contexts, but only the constructors are considered in direct-initialization contexts.
struct To {
    To() = default;
    To(const struct From&) {} // converting constructor
};

struct From {
    operator To() const {return To();} // conversion function
};

int main()
{
    From f;
    To t1(f); // direct-initialization: calls the constructor
// (note, if converting constructor is not available, implicit copy constructor
//  will be selected, and conversion function will be called to prepare its argument)
    To t2 = f; // copy-initialization: ambiguous
// (note, if conversion function is from a non-const type, e.g.
//  From::operator To();, it will be selected instead of the ctor in this case)
    To t3 = static_cast<To>(f); // direct-initialization: calls the constructor
    const To& r = f; // reference-initialization: ambiguous
}

You can make the conversion constructor to be discarded from the overload set for this case by SFINAE; i.e. make it valid only when the implicit conversion of the underlying pointers is allowed.

template <typename T2, typename = std::enable_if_t<std::is_convertible<T2*, T1*>::value>>
Foo(const Foo<T2>& other) : obj(other.obj) {}

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Constructor is always used instead of explicit conversion operator

2 Answers