What is the difference between object creating types in c++?

Question

Assume that I have "A" class. I want to create an "a" object using that class with 2 different ways, i.e:

A a();
A a = A();

What is the difference between them?

Answer 1

A a(); is not a variable declaration, it is a function declaration. It declares a function a that takes no parameters and returns an A.

A a = A(); is a variable declaration and initialization. It declares a variable a of type A and initializes it from the temp object that is created by explicitly calling A's default constructor. The simpler way to do this is A a; (note the lack of parenthesis!), which a smart compiler is likely to optimize A a = A(); into anyway. If you want to explicitly call the default constructor (if one is present), use curly brackets instead of parenthesis: A a{};