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

From: Bhuyan, Jyotideep (JyotideepBhuyan_at_[hidden])
Date: 2023-09-29 18:42:49

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<T*>(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

Best Regards,
Jyotideep (JD)

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