Reputation: 149

a question about constructor in c++

I am newbie to c++, I have not yet seen this kind of constructor, what does it do?

class A {
    int x;
public:
    A(int xx):x(xx) {}
};

int main() {
    A a(10);
    A b(5);
    return 0;
}

Is the code above valid?
What does this constructor do? A(int xx):x(xx) means what? A cast?

Upvotes: 1

Answers (7)

Kate Gregory

Reputation: 18974

The string :x(xx) is called an initializer. As you can see it's valid on only a constructor. The effect is to initialize x with the value xx. So your code makes two A objects - one has an x of 10 and the other of 5.

This is more efficient than letting it be initialized and then changing its value in the body of the constructor by writing x=xx;

Upvotes: 1

charley

Reputation: 5949

The code is valid: The member variable "x" is being set a value in the "base/member initializer list".

This type of initialization is required when you are initializing a value for a reference member, constant member, or to forward arguments to the base constructor.

It is optional in other cases, like this one, where the value could have been explicitly set in the constructor body (but this is arguably faster, since it is initialized as memory is allocated).

Upvotes: 0