C++11 std::array

I'd like to use std::array from C++11 as a field of my own class. It takes two template parameters (first defines type of data, second defines size of an array).

But I know the second parameter only in constructor. I'm not familiar with C++11 standard, but I suppose that it's impossible to set a template parameter during execution.

Are there any alternatives for std::array? std::vector is probably a little too much, cause I will never change the size of it.