Is there a difference between these 2 methods of initialising member variables?

Question

Note::Note(Traymenu *trayMenuIn, QWidget *parent) :
    ui(new Ui::Note){
    ui->setupUi(this);

Note::Note(Traymenu *trayMenuIn, QWidget *parent){
    ui = new Ui::Note;
    ui->setupUi(this);

Both kinds are working. The above code is suggested by QtCreator, the lower code is what I would do if I had to write it on my own.

Note's private member is

Ui::Note *ui;

The former does initialization, the latter does assignment (of `ui`). — jrok, Feb 28 '14 at 18:43
But isn't initialising the exact same thing as assigning a value to a declared pointer variable? — user2366975, Feb 28 '14 at 18:45
@user2366975 no. the former is at *construction* time, the latter undergoes default construction, *then* assignment. — WhozCraig, Feb 28 '14 at 18:46
Does it affect practical use in terms of stability, say deleting that object later on? — user2366975, Feb 28 '14 at 18:47
@user2366975: No, but it makes your code more robust. For example, you cannot accidentally access ui before it has the correct value. — Christian Hackl, Feb 28 '14 at 18:48
Yes, the one with the initialization list. But please read the FAQ entry in my answer for more details and some exceptions :) — Christian Hackl, Feb 28 '14 at 18:50
@user2366975 The language's initializer list mantra should be utilized where at all possible. There are times, rarely, where it simply isn't feasible; this isn't one of them. And its a good habit to form. — WhozCraig, Feb 28 '14 at 18:52
And it's the only *possible* way for const or reference members. — Christian Hackl, Feb 28 '14 at 18:53
"Does it affect practical use in terms of stability" Yes. Because you rely on an anti-idiom, and eventually you'll use it in presence of exceptions, and you'll [be sorry](http://stackoverflow.com/a/21971907/1329652). — Kuba hasn't forgotten Monica, Feb 28 '14 at 21:48

score 4 · Accepted Answer · answered Feb 28 '14 at 18:46

4

Only the first form is initialization. The second form initializes ui with an undefined value, then assigns a value to it.

You should prefer the first form (initialization). See the related C++ FAQ entry.

answered Feb 28 '14 at 18:46

Christian Hackl

26,144
3
29
57

score 2 · Answer 2 · answered Feb 28 '14 at 18:48

Although the difference is tiny, I would prefer the code snippet suggested by QtCreator: it uses the initialization syntax rather than the assignment syntax for the code that logically represents initialization.

Since the member being initialized is a pointer, there is no performance penalty even with the optimization turned off. However, it is a good idea to get into a habit of initializing as much as possible with the initialization list, because this prevents potential coding issues inside the constructor itself.

Is there a difference between these 2 methods of initialising member variables?

2 Answers2