Question

There is a structure TA

template <typename T>
struct TA
{
    typedef std::vector <T> Type;
};

and test() function having a default parameter of the type TA.

template <typename T>
void test ( typename TA<T>::Type a1, 
            typename TA<T>::Type a2 = typename TA<T>::Type(a1.size()) )
{}

Is it posssible to use a1.size() in default parameter a2 definition?

int main()
{
    TA <double> ::Type a1;
    test<double>(a1);
}

Answer 1

Is it posssible to use a1.size() in default parameter a2 definition?

No. It is forbidden by the Standard. You cannot use function parameter to set the default value of other parameter.

§8.3.6/9 (C++03) explicitly says,

Default arguments are evaluated each
time the function is called. The order
of evaluation of function arguments is
unspecified. Consequently, parameters
of a function shall not be used in
default argument expressions, even if
they are not evaluated.

So the solution is: use overload:

template <typename T>
void test(typename TAs<T>::Type a)
{
    test(a, typename TA<T>::Type(a.size()));
}