Cannot initialize union in C++ 11

I can't initialize union in std::pair.

I have this kind of typedef

enum EValueType
{
    Int,
    Double,
    Variable,
    Unknown
};

union UValueContainer
{
    int __int;
    double __double;
    double* __variable;
};

typedef std::pair<EValueType, UValueContainer> variant;

and I have a base class with constructor, which gets a attribute of variant type

class fooBase
{
    fooBase(variant attribute);
    ...
}

and a derived class with constructor, which have to initialize argument of base class constructor.

class fooDerived : public fooBase
{
    fooDerived(double* value) : fooBase({Variable, UValueContainer{.__variable = value}}
}

How can I do it?