Difference between these two constructor calls

Question

Possible Duplicate:
Do the parentheses after the type name make a difference with new?

I believe this question was already asked, but I cannot find it with a quick search.

Foo ob* = new Foo; 

Foo ob* = new Foo();

Is there a difference between these two ways of creating an object in C++? If not then is one of these considered a bad practice? Does every compiler treats it the same?

There is a subtle difference between using parentheses and not using parentheses, and a really detailed explanation is offered in this question: http://stackoverflow.com/questions/620137/do-the-parentheses-after-the-type-name-make-a-difference-with-new — 逆さま, Nov 14 '11 at 14:25
http://stackoverflow.com/questions/2417065/c-does-the-default-constructor-initialize-built-in-types — Ken Brittain, Nov 14 '11 at 14:27

score 7 · Answer 1 · answered Nov 14 '11 at 14:24

7

The first is default initialization, the second is value initialization. If Foo is of class type, they both invoke the default constructor. If Foo is fundamental (e.g. typedef int Foo;), default initialization performs no initialization, while value-initialization performs zero-initialization.

For class types and arrays, the initialization proceeds recursively to the members/elements in the expected way.

answered Nov 14 '11 at 14:24

Kerrek SB

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Your answer is so much more accurate than mine, dammit – Lightness Races in Orbit Nov 14 '11 at 14:25

score 3 · Answer 2 · answered Nov 14 '11 at 14:22

3

There is no difference, other than the fact that if Foo is a built-in type then the former does not value-initialise it.

So:

new int;   // unspecified value
new int(); // 0

This matches up nicely with "normal" allocation for built-ins, too:

int x;     // unspecified value
int x = 0; // well, you can't do `int x()`, but, if you could...

answered Nov 14 '11 at 14:22

Lightness Races in Orbit

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You can do `int x{};` nowadays :-) – Kerrek SB Nov 14 '11 at 14:24
@KerrekSB: Good point ;) Doesn't really help me make a comparison with something "familiar" though! – Lightness Races in Orbit Nov 14 '11 at 14:24
Hehe, no. I think last time we concluded that the best you can do for automatic objects is `int x((int());` or so. – Kerrek SB Nov 14 '11 at 14:25
@KerrekSB: Lol yes, was just playing with that and decided not to bother any further ;) – Lightness Races in Orbit Nov 14 '11 at 14:26

Difference between these two constructor calls

2 Answers2