Different ways of initializing an object with default constructor

Question

class A {};

A a;// 1
A a{};// 2
A a = {};// 3
A a = A();// 4

There seems to be all options. Are 1, 2 and 3 are same and just the matter of style or there is some difference? 4 supposed to first create a temporary object and then assign it to an a, but it will happen only if I will turn off comliler's optimization completely, right?

Answer 1

1. Is different because it does default initialisation. This does not matter in case of A however, because there are no members to initialise.