CODEWITHSUNDEEP
c++constructor
poy
poy

Reputation: 10507

c++ constructors differences

If I have the following class:

class A{
private:
  int x;
public:
  A(){
    x = 5;
  }
};

Whats the difference between these 2 declarations?

A a;

vs.

A a();

Thanks.

Upvotes: 0

Views: 120

Answers (4)

Joseph Mansfield
Joseph Mansfield

Reputation: 110658

Perhaps it is interesting to note, in addition to what others have said, that there is a difference between the following two lines:

A a;
A a{}; // Using uniform initialization from C++11 to avoid the ambiguity

And also between the following two lines:

A* a = new A;
A* a = new A(); // or new A{}

In the first line of each example, the object is default-initialized. In the second lines, the object is value-initialized. The difference is that while default-initialization will call the default constructor of A, value-initialization will zero-initialize the object first and then call the default constructor (if there are no user-provided constructors).

For anything that is not a class type, default-initialization will perform no initialization. For anything that is not a class type or is a union without a user-provided constructor, value-initialization will zero-initialize the object.

Upvotes: 3

K-ballo
K-ballo

Reputation: 81349

A a;

This declares an element of class A and constructs it using the default constructor.

A a();

This declares a function called a taking no parameters and returning an object of type A.

Upvotes: 2

Konrad Rudolph
Konrad Rudolph

Reputation: 545528

The second code does not define an object called a, it declares a function a with return type A without arguments. This property of the C++ compiler is commonly known as the most vexing parse.

Upvotes: 2

interjay
interjay

Reputation: 110069

A a;

This creates an object of type A and calls the default constructor.

A a();

This declares a function called a that returns an object of type A.

Upvotes: 8

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