Difference between two ways of using default constructor

Question

What is the difference between defining an object of class "Person" by using its default constructor following two different ways:

Method 1:

Person person = Person();

Method 2:

Person person();

When I initialized some variables inside the default constructor and tried to access those variables or set those variables by get/set methods in the main routine, I got compilation error in Method 2, but Method 1 works.

Thanks.

"I got compilation"? what does that even mean? BTW the second one is not invoking the default consturctor, it's a function declaration. — The Paramagnetic Croissant, Jul 27 '14 at 16:51
Your question is based on the false assumption that both of those use the default constructor. — chris, Jul 27 '14 at 16:51
possible duplicate of [About Pointers To Functions in function declarations](http://stackoverflow.com/questions/9327505/about-pointers-to-functions-in-function-declarations) — The Paramagnetic Croissant, Jul 27 '14 at 16:52
possible duplicate of [Most vexing parse: why doesn't A a(()); work?](http://stackoverflow.com/questions/1424510/most-vexing-parse-why-doesnt-a-a-work) — πάντα ῥεῖ, Jul 27 '14 at 16:59
@πάνταῥεῖ: You're awfully dupe-happy nowadays. That's obviously not a duplicate. — Lightness Races in Orbit, Jul 27 '14 at 17:08

Joseph Mansfield · Accepted Answer · 2014-07-28T13:39:27.390

11

Person person = Person();

This declares a Person object called person. It initialises this object with a temporary object created by Person(). That means it'll invoke the copy/move constructor of Person (which will probably be elided).

Person person();

This declares a function called person that returns a Person object. This is not a declaration of an object.

You most likely want this instead:

Person person;

This declares a Person object called person which is default constructed.

You may want to use the new "uniform" initialisation syntax introduced in C++11, which avoids the ambiguities between variable and function declarations:

Person person{};

And finally, if you're AAA-minded, you may want to use the style proposed by Herb Sutter:

auto person = Person{};

edited Jul 28 '14 at 13:39

answered Jul 27 '14 at 16:52

Joseph Mansfield

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Worth pointing out that a compiler that doesn't elide that copy/move is probably worthless these days. – chris Jul 27 '14 at 16:54
@chris You know when you're thinking faster than your writing and end up missing something out? That happened. – Joseph Mansfield Jul 27 '14 at 16:55
2

Maybe mention `Person person{}`? – Deduplicator Jul 27 '14 at 17:17
1

@Deduplicator or `auto person = Person{};` ;-) Just kidding, the [author of this answer is on record](http://josephmansfield.uk/articles/dont-use-auto-unless-you-mean-it.html) for not liking it. – TemplateRex Jul 28 '14 at 13:22
@TemplateRex Added it anyway. ;) – Joseph Mansfield Jul 28 '14 at 13:39

Difference between two ways of using default constructor

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