Why do uninitialized objects of built-in type defined inside a function body have undefined value?

Question

Question:
Why do uninitialized objects of built-in type defined inside a function body have undefined value, while objects of built-in type defined outside of any function are initialized to 0 or ''?

Take this example:

#include <iostream>
using std::cout; using std::endl;

int ia[10];                   /* ia has global scope */

int main() 
{
    int ia2[10];              /* ia2 has block scope */

    for (const auto& i : ia)
        cout << i << " ";     /* Result: 0 0 0 0 0 0 0 0 0 0 */

    cout << endl;
    for (const auto& i : ia2)
        cout << i << " ";     /* Result: 1972896424 2686716 1972303058 8
                                 1972310414 1972310370 1076588592 0 0 0 */
    return 0;
}

Answer 1

Because one of general rules of C++ is that you don't pay for what you don't use.

Initializing global objects is relatively cheap because it happens only once at program startup. Initializing local variables would add overhead to every function call, which not everybody would like. So it was decided to make initialization of locals optional, the same way as in C language.

BTW If you want to initialize your array inside a function, you can write:

int ia2[10] = {0};

or in C++11:

int ia2[10]{};