When I use new[] to create an array of my classes:
int count = 10; A *arr = new A[count];
I see that it calls a default constructor of A
count
times. As a result arr
has count
initialized objects of type A
. But if I use the same thing to construct an int array:
int *arr2 = new int[count];
it is not initialized. All values are something like -842150451
though default constructor of int assignes its value to 0
.
Why is there so different behavior? Does a default constructor not called only for built-in types?
It is a special type of method which is used to initialize the object. Every time an object is created using the new() keyword, at least one constructor is called. It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.
If no constructors are explicitly declared in the class, a default constructor is provided automatically by the compiler.
This constructor is called the default constructor because it is run "by default;" if there is no initializer, then this constructor is used. The default constructor is used regardless of where a variable is defined.
Deleting the default constructor of a class is a good idea when there are multiple choices for the default or uninitialised state. For example, suppose I have a class, template<typename F> class Polynomial; which represents a polynomial over a field, F .
See the accepted answer to a very similar question. When you use new[]
each element is initialized by the default constructor except when the type is a built-in type. Built-in types are left unitialized by default.
To have built-in type array default-initialized use
new int[size]();
Built-in types don't have a default constructor even though they can in some cases receive a default value.
But in your case, new
just allocates enough space in memory to store count
int
objects, ie. it allocates sizeof<int>*count
.
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