Declaring an Array of Objects: instantiates them

Question

Why is C++ created in such a way, that if you have a class A and you declare an array of type A, then the whole array gets filled with objects instantiated with the default constructor of the class?

To turn the question around, what would be an alternative? If declaring an array of objects didn't instantiate them then how would you instantiate them manually? Would you need to use placement new to do it? Doesn't it seem like that would be horribly error prone? Given that the objects need to be instantiated before you use them then doing it automatically seems like the best solution to me. As recommended above, if you prefer to do it as you need them then use a container like vector rather than a static array. — Retired Ninja
– Retired Ninja, Commented Apr 26, 2012 at 22:07

Mark Ransom · Accepted Answer · 2012-04-26 22:00:50Z

2

Because when you create an array of a given size, each element of the array has to be valid as soon as it's created.

If you want a different behavior you can use vector and push_back. A vector is created empty; when you want to add a new element, push_back will take an object that is created any way you want and make a copy of it in the vector.

answered Apr 26, 2012 at 22:00

Mark Ransom

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2 Comments

Matthew Flaschen Over a year ago

And you can use vector::reserve to reserve capacity without constructing anything.

Jerry Coffin Over a year ago

Though, of course, you don't have to create a vector empty -- you can pass an initial size to the constructor if you want.

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