Constructors comparison

Question

Possible Duplicate:
Initializing in constructors, best practice?

I'm new to C++.

Suppose we have this class definition:

Class MyClass {
  int a;
  int b;

  //....
}

I would like to know which is the difference between the two class contructors:

public: 
    MyClass(int a, int b) : a(a), b(b) {}

and (i would say Java-style):

MyClass(int a, int b) {

this->a = a;
this->b = b;
}

I suppose the first is better in C++; right? why?

Yes, you are right. I'm sorry. – Aslan986 May 02 '12 at 16:32 — Aslan986, May 02 '12 at 16:32

score 8 · Accepted Answer · answered May 02 '12 at 16:26

8

The first one (using initializer list) initializes the data members to the given values. The second one initializes them first, then assigns them the values. That is the reason the first is prefered. There is no unnecessary assignment operation there.

This is particularly important when your data members are expensive to construct and/or assign to. Also bear in mind that some types are not default constructable, making it mandatory to use the initializer list.

answered May 02 '12 at 16:26

juanchopanza

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...and the first way is called "ctor-initializer", which you can look up on the Net – Alexander Pavlov May 02 '12 at 16:27
...unless `a` and `b` are primitives (as in this example), when it won't really matter what you do. – Jon May 02 '12 at 16:27
Alexander: I've always heard them called initializer lists? – Peter May 02 '12 at 16:28
@Jon: isn't this the case only with a good optimizing compiler? – Vlad May 02 '12 at 16:29
@Vlad: In 2012 I 'd say with any compiler really. – Jon May 02 '12 at 16:30
@jon if a and b are primitives, there is still semantically and potentially an unnecessary assignment (ignoring compiler optimizations). – juanchopanza May 02 '12 at 16:31
1

@Jon: for embedded environments I wouldn't be so optimistic. – Vlad May 02 '12 at 16:31

Constructors comparison

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