为什么在 C++ 中未初始化数组中有随机数，而在半初始化数组的未初始化成员中没有？

Question

Why in example 1 code it assigns 0s in non initialized items of array, but in example 2 assigns random numbers in completely non initialized array? Why it dont assign 0s to completely non initialized array as well?

Example 1:

int ar[5] ={0,1};
for (int i =0; i< 5; i++){
   cout << ar[i] << " ";
}
// output: 0 1 0 0 0 

Example 2:

int ar[5] ;
for (int i =0; i< 5; i++){
   cout << ar[i] << " ";
}
// output: 1875998720 0 1875947552 0 1876031856 

Answer 1

They're two different initializations.

The case 2, int ar[5]; performs default initialization , as the effect all the elements are initialized to indeterminate values.

The case 1, int ar[5] ={0,1}; performs aggregate initialization , as the effect the 1st and 2nd element are initialized as 0 and 1 , the remaining elements are value-initialized (zero-initialized) as 0 .

Answer 2

In first Example ! You are initializing the array of size 5 with default values.

int arr[5] = {0,1}

values in curly braces will be assigned to the corresponding index in the array. For example 0 from at the first index of array, 1 at the second index and then default 0 values to the rest of the indexes in array.

In Second Example You are just declaring the array but not initializing. Therefor, at every index of the array, there is garbage value and when you iterate the array you receive unexcepted (garbage) value at each index of the array.

If you declare a variable but don't initialize with any value, it assign the random garbage value that we can't predict.