What are the differences between test t; and test t () ;? if Test is a class

Question

Possible duplicate:
Why is there no constructor call?

I am using Visual studio 2012, suppose Test is a class

class Test { }; 

When I create a new instance of Test, what's the difference between the two methods?

method 1

 Test t; 

way 2

 Test t(); 

I got this question in the code below, initially, I defined an instance of A in path 2, I have only one error, because B does not provide a default constructor, but when I define it in order 1, I got an additional error.

 class B { B(int i){} }; class A { A(){} B b; }; int main(void) { A a(); // define object a in way 2 getchar() ; return 0 ; } 

if I define a so that 1

 A a; 

I get another error:

error C2248: "A :: A": cannot access the private member declared in class 'A'

Therefore, I suggest that there should be some differences between the two methods.

Answer 1

Test t; defines a variable of type t type Test .

Test t(); declares a function named t that takes no parameters and returns Test .

Answer 2

What is the difference between the two ads?

 A a(); 

Declares a function, not an object. This is one of the most annoying parsing in C ++.
It declares a function called a , which takes no parameters and returns a type a .

 A a; 

Creates an object named a type a , invoking its default constructor.

Why are you getting a compilation error?

For the default access specifier of the private class, that’s why you get an error because your class constructor is private and cannot be called when creating an object with the above syntax.