Assign a dynamically allocated array in a constructor to a unique smart pointer member variable

In the example below, I have a smart pointer sp as a member variable and I want to assign a dynamically allocated array to it in the constructor, however I get a compiler error no match for ‘operator=’, what is the correct way to do this?

In the example below, I have a smart pointer sp as a member variable and I want to assign a dynamically allocated array to it in the constructor, is using the reset() method of smart pointer the correct way to do this or should I be using a shared smart pointer?

struct SampleStructure
{
std::unique_ptr<idx_t[]> sp;

SampleStructure(int a, int b){

    sp.reset(new idx_t[a + 1]); 
 }
};