C ++：数组元素设置为0

Question

I have been using arrays for a while now.

I have a few questions that i need to ask.

Now I know i cant use an array without giving a value its element(s).

For example this would give me an error

int fly[5];
cout << fly[4] << endl;

And if i print an element that doesn't have a set value it would give an error:

int fly[5];
fly[2] = 1;
cout << fly[4] << endl;

Now I found if I give 1 element of the array any number in the array intialization. Then the rest of the elements are set to 0.

So this code prints 0

int fly[5] = {15};
cout << fly[4] << endl;

Why does this happen. Can anyone explain?

Answer 1

It happens because the C++ language standard says it should happen. The standard says it should happen because it's sensible behaviour and saves a lot of typing in cases where you want a large array initialized.

Your first couple of examples produce undefined behaviour, in that the value of those array elements can be anything, but they shouldn't "give an error". (A sufficiently clever compiler might issue a warning.)

Answer 2

First of all, your first two examples are not guaranteed to give an error. If they do not, they will compile and run with undefined behavior. The last one has well-defined behavior. If you do not specify values for every element of an array, the remainder are initialized with the default value, in this case 0.

Answer 3

When you brace-initialize an array and the initializer is too short, the missing elements are value-initialized. For primitive types such as int , this means zero-initialization (ie the initial value is zero). For example, you can say int a[100] = {}; to get an all-zero array. Integers aren't special, though, and this works for default-constructible class types just as well.

Your first two examples constitute undefined behaviour, since you are reading an uninitialized variable.