Storing containers in a boost::variant - c++

Question

The last line here:
typedef boost::variant<std::vector<int>, std::vector<float>> C;
class A: public boost::static_visitor<>
{
public:
void operator()(const std::vector<int>& value) const
{
}
void operator()(const std::vector<float>& value) const
{
}
};
C container(std::vector<float>());
boost::apply_visitor(A(), container );
Is giving me the error:
c:\boost_1_49_0\boost\variant\detail\apply_visitor_unary.hpp(60): error C2228: left of '.apply_visitor' must have class/struct/union
1> type is 'boost::variant<T0_,T1> (__cdecl &)'
1> with
1> [
1> T0_=std::vector<int>,
1> T1=std::vector<float>
1> ]
1> c:\visual studio 2010\projects\db\xxx\main.cpp(255) : see reference to function template instantiation 'void boost::apply_visitor<A,C(std::vector<_Ty> (__cdecl *)(void))>(Visitor &,Visitable (__cdecl &))' being compiled
1> with
1> [
1> _Ty=float,
1> Visitor=A,
1> Visitable=C (std::vector<float> (__cdecl *)(void))
What is the problem here? Is it sensible in you opinion to have a container type C which such a definition?
I am using the following type throughout my code:
typedef boost::variant<int, float, ...> Type;
Do you think it would be wiser to use this container definition instead:
typedef std::vector<Type> C; // mixed container
Why?

Answer 1

Your problem is that this
C container(std::vector<float>());
is a function declaration (that’s the most vexing parse) (a function container which takes a function returning std::vector<float> as its sole argument, and returns C). Easy fix: extra parentheses:
C container((std::vector<float>()));
The fact that you’re using containers in the variant is irrelevant to the problem. The same would have happened with boost::variant<int, float>.