id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
11602	boost.variant constructor accepts any type as parameter	piotrwn1@…	Antony Polukhin	"There is one ""converting constructor"" in boost.variant which accepts any type - even those which are not convertible to instantiation types of boost.variant. This causes that writing function accepting some specific variant instantiation cause that this function is on candidate list of any type in your program.

The problem is with this constructor:


{{{
    template <typename T>
    variant(const T& operand)
    {
        convert_construct(operand, 1L);
    }

}}}

And the small code presenting the issue:

{{{
#include <iostream>
#include <boost/variant.hpp>

// types not related in any way
class A {};
class B {};
class C {};
class D {};

using ABC_variant = boost::variant<A,B,C>;

std::ostream& operator << (std::ostream& os, const ABC_variant&)
{
	return os << ""ABC"";
}

int main() {
	D d;
	std::cout << d;
}
}}}

As described in [http://stackoverflow.com/questions/32275725/boostvariant-construction-weirdness-its-ctor-accepts-everything] 

compiler (gcc4.9) tries to use ostream operator with this diagnostic:

/usr/include/boost/variant/variant.hpp:1591:38: error: no matching function for call to 'boost::variant<A, B, C>::initializer::initialize(void*, D&)'
               initializer::initialize(

However it should simple report that ostream operator for D is missing.

One simple way is to declare this boost.variant constructor as explicit (what probably would impact to many client code) or to make some restriction in this constructor like this C++11 way:


{{{
template <typename T, typename ...C>
struct IsAnyOf;

template <typename T, typename ...C>
struct IsAnyOf<T,T,C...> : std::true_type {};

template <typename T>
struct IsAnyOf<T> : std::false_type {};

template <typename T, typename C1, typename ...C>
struct IsAnyOf<T,C1,C...> : IsAnyOf<T, C...> {};

template <typename T, 
	  typename EnableIf = typename std::enable_if<IsAnyOf<VariantType...>::value>::type>
	variant(const T& operand)


}}}

BR,
Piotr Nycz (piotrwn1 @ gmail com)
"	Bugs	closed	Boost 1.62.0	variant	Boost 1.58.0	Problem	fixed		raad@…