Does std::atomic

Lets say we have a structure as below-

struct test
{
    int a;
    int b;
};

I make a atomic variable of this structure and try to update a and b. Will these operations be atomic? What does it mean to make a structure/class atomic?

std::atomic<test> t;
t.a = 10;   // is this an atomic operation?
t.b = 20;   // is this an atomic operation?
// I understand a and b are not atomic in themselves but I am confused     
// as to when you say struct temp is atomic what does it mean?
// does it mean the t instance's this pointer is atomic?