new[] 是否调用 C++ 中的默认构造函数？

Question

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?

Answer 1

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]();

Answer 2

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 .

Answer 3

Primitive type default initialization could be done by below forms:

    int* x = new int[5];          // gv gv gv gv gv (gv - garbage value)
    int* x = new int[5]();        // 0  0  0  0  0 
    int* x = new int[5]{};        // 0  0  0  0  0  (Modern C++)
    int* x = new int[5]{1,2,3};   // 1  2  3  0  0  (Modern C++)

Answer 4

int不是类，它是内置数据类型，因此没有为其构造函数的调用。