Why does not my code in C++ that declares an (local) int array without doing anything initialize by zeros?

Question

I have two C++ codes in which the one has an global int array while the other code has a local array whose length is determined by user input (so at runtime). Both arrays are not explicitly initialized.

#include <iostream>
using namespace std;
int M = 1000;
int a[M];
int main() {
  for(int i = 0; i < M; i++)
    cout << a[i];
  cout << endl; 
}

#include <iostream>
using namespace std;
int main() {
  int M;
  cin >> M;
  int a[M];
  for(int i = 0; i < M; i++)
    cout << a[i];
  cout << endl; 
}

I observed that the global array is filled with zeros, while the local array (whose length is determined at runtime) is not filled with zeros and instead filled with a random numbers (but same at a time). I used the g++ compiler.

What is this behavior? Does C++ standard define this behavior?

Does this answer your question? [Why are global and static variables initialized to their default values?](https://stackoverflow.com/questions/2091499/why-are-global-and-static-variables-initialized-to-their-default-values) — Tarek Dakhran, Feb 17 '20 at 18:00
Yes it is defined in C++ standard, it is also valid for static variables. — calynr, Feb 17 '20 at 18:00
Also your second code is not standard. VLA are compiler extension and not standard. Use std::vector. — Rosme, Feb 17 '20 at 18:04
Note a declaration `int a[M];` where `M` is not a compile-time constant is not Standard C++, but a compiler extension. It may interact in bizarre ways with templates and other language features. If you want a local array to be initialized with zeros, do `int a[M] = {};`. — aschepler, Feb 17 '20 at 18:05

eerorika · Answer 1 · 2020-02-17T18:14:58.547

What is this behavior?

The behaviour is that objects with static storage duration are zero-initialised before any other initialisation (if any).

The behaviour for all other storage duration is that there is no additional zero initialisation.

Does C++ standard define this behavior?

Yes, the zero initialisation of static objects is defined in the standard. The behaviour of reading indeterminate values is specified to be undefined.

Both programs are ill-formed because the size of an (non dynamic) array must be compile time constant, which M is not.

Vlad from Moscow · Answer 2 · 2020-02-17T20:18:14.453

From the C++ 17 Standard (11.6 Initializers)

12 If no initializer is specified for an object, the object is default-initialized. When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced (8.18). [ Note: Objects with static or thread storage duration are zero-initialized, see 6.6.2. — end note ]

So the array declared in the global namespace has the static storage duration and zero-initialized

While the array declared in main has the automatic storage duration and has indeterminate values of its elements.

Pay attention to that the variable length arrays is not a standard C++ feature.

int M = 1000;
int a[M];

or

cin >> M;
int a[M];

That is the both programs are not standard compliant.

And moreover variable length arrays may not be explicitly initialized in a declaration.

They can be supported by some compilers as their own language extensions.

Why does not my code in C++ that declares an (local) int array without doing anything initialize by zeros?

2 Answers2