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

[Bhuyan, Jyotideep via Boost]

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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[Andrzej Krzemienski via Boost]

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;

[Peter Dimov via Boost]

Bhuyan, Jyotideep 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.

Your code compiles and works for me with VS2022 and C++17 (after adding
a `catch` clause after the `try`, e.g. https://godbolt.org/z/eWv9sjYzz.)

What errors do you get?

[Bhuyan, Jyotideep via Boost]

This is strange ☹
By mistake, I removed the catch block from the example code. This was present in the original piece of code.

I am getting the compilation error with the new boost 1.82 and the VS2022 (v143 ) + C++17 is as follows at this line.
o << *boost::any_cast<T>(&a); //which was running with boost version 1.78 + VS2019 (v142) + C++14



Severity
Code
Description
Error
C2679
binary '<<': no operator found which takes a right-hand operand of type 'const ValueType' (or there is no acceptable conversion)



Best Regards,
Jyotideep (JD)



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From: Boost <boost-...@lists.boost.org> On Behalf Of Peter Dimov via Boost
Sent: Saturday, September 30, 2023 12:55 PM
To: bo...@lists.boost.org
Cc: Peter Dimov <pdi...@gmail.com>
Subject: Re: [boost] boost::any_cast gives build errors with VS2022, boost v1.82 and C++17

Bhuyan, Jyotideep 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);
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Bhuyan, Jyotideep 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.



Your code compiles and works for me with VS2022 and C++17 (after adding

a `catch` clause after the `try`, e.g. https://urldefense.proofpoint.com/v2/url?u=https-3A__godbolt.org_z_eWv9sjYzz&d=DwICAg&c=9mghv0deYPYDGP-W745IEdQLV1kHpn4XJRvR6xMRXtA&r=nKjmvhl-FVZt-1iD5Ea7mULt--dNphU4qGuhbCJsO90&m=phs2270IlMJnrkY8KPgf3575O_9O78a6bb347nW4R28oRm_nFrGGqsw3BpqiTWZ4&s=hynNTB7tJ605xXLfaD7lhvL1xAyZ7JlB7RRwzxtQq0I&e=.)

> Unsubscribe & other changes: https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.boost.org_mailman_listinfo.cgi_boost&d=DwICAg&c=9mghv0deYPYDGP-W745IEdQLV1kHpn4XJRvR6xMRXtA&r=nKjmvhl-FVZt-1iD5Ea7mULt--dNphU4qGuhbCJsO90&m=phs2270IlMJnrkY8KPgf3575O_9O78a6bb347nW4R28oRm_nFrGGqsw3BpqiTWZ4&s=PmpOkIwy-jd3O0wS2fcKgI6GRM5c7a6g75F3Fo_jPhA&e=





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