Re: [boost] boost::any_cast gives build errors with VS2022, boost v1.82 and C++17

On Fri, Sep 29, 2023, 23:41 Bhuyan, Jyotideep via Boost < boost@lists.boost.org> wrote:

Hi,

I was using VS2019, boost v1.78, and C++14 version and the below code was working well. Same code gives build error with VS2022, boost v1.82, and C++17 due to "o << *boost::any_cast<T>(&a); " Instruction. I wanted to know why it behaves differently.

Code :

class any_out { struct streamer { virtual void print(std::ostream& o, const boost::any& a) = 0; virtual streamer* clone() = 0; virtual ~streamer() {} };

public: any_out() : m_streamer(nullptr) { }

~any_out() { delete m_streamer; }

template <typename T> any_out(const T& value) : m_streamer(new streamer_imp<T>), m_object(value) { }

any_out(const any_out& a) : m_streamer(a.m_streamer ? a.m_streamer->clone() : nullptr), m_object(a.m_object) { }

template <typename T> any_out& operator=(const T& r) { any_out(r).swap(*this); return *this; }

any_out& operator=(const any_out& r) { any_out(r).swap(*this); return *this; }

any_out& swap(any_out& r) noexcept { std::swap(m_streamer, r.m_streamer); std::swap(m_object, r.m_object); return *this; }

friend std::ostream& operator<<(std::ostream& o, const any_out& a) { if(a.m_streamer) { a.m_streamer->print(o, a.m_object); } return o; }

boost::any& get() { return m_object; } const boost::any get()const { return m_object; } bool empty() const { return m_object.empty(); }

private:

template <typename T> struct streamer_imp : public streamer { virtual void print(std::ostream& o, const boost::any& a) { try { if (a.empty()) { o << "<empty>"; } else { o << *boost::any_cast<T>(&a); }

} } virtual streamer* clone() { return new streamer_imp<T>(); } };

// Members streamer* m_streamer; boost::any m_object;

};

I have migrated to VS2022, boost v1.82, and C++17, and due to a build error, I have changed the code below.

virtual void print(std::ostream& o, const boost::any& a) { try { if (a.empty()) { o << "<empty>"; } else { o << boost::any_cast(a); }

} }

any_out anyTest; anyTest = 100; std::string actualStr; actualStr = make_string() << anyTest;

actualStr prints nullptr (0000000) instated of value 100 for VS2022, boost v1.82 and C++17

It looks like you are using boost::any incorrectly. You cannot know ahead of time that an any is golding a T. If you want to use a T, you have to check it. An any my be non-empty, but may be storing some other type U. any_cast allows you to chech it. If it retuns a null pointer, it means you are storing some other type than T. Maybe you need boost:optional<T> rather than boost::any? Regards, &rzej;

Best Regards, Jyotideep (JD)

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