c++ constructors differences

Question

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.

score 8 · Accepted Answer · answered Mar 07 '12 at 22:10

8

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.

answered Mar 07 '12 at 22:10

interjay

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2

Also known as most vexing parse. – Cat Plus Plus Mar 07 '12 at 22:12
1

I've seen the phrase "most vexing parse" used to describe the line `A a(B());`. I haven't seen it used to describe `A a();`, but maybe that counts as well. – interjay Mar 07 '12 at 22:14
It's disambiguation rule that prefers function declarations to anything else. – Cat Plus Plus Mar 07 '12 at 22:15

score 3 · Answer 2 · answered Mar 07 '12 at 23:12

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.

score 2 · Answer 3 · answered Mar 07 '12 at 22:11

2

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.

answered Mar 07 '12 at 22:11

Konrad Rudolph

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score 2 · Answer 4 · answered Mar 07 '12 at 22:11

2

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.

answered Mar 07 '12 at 22:11

K-ballo

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c++ constructors differences

4 Answers4