How are local and global variables initialized by default?

Question

Based on below, am i right?

• global_A reference is initialized to null.
• global_int is 0
• local_A reference is null
• local_int is uninit

Answer 1

Building up on Andrey's response.

$3.6.2- "Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place.". In OP, "global_A" and "global_int" have static storage duration. "local_int" and "local_A" have no linkage as these are local objects.

$8.5/5- To zero-initialize an object of type T means:

— if T is a scalar type (3.9), the object is set to the value of 0 (zero) converted to T;

— if T is a non-union class type, each nonstatic data member and each base-class subobject is zeroinitialized;

— if T is a union type, the object’s first named data member89) is zero-initialized;

— if T is an array type, each element is zero-initialized;

— if T is a reference type, no initialization is performed.

$6.7.4/4- "The zero-initialization (8.5) of all local objects with static storage duration (3.7.1) is performed before any other initialization takes place. A local object of POD type (3.9) with static storage duration initialized with constant-expressions is initialized before its block is first entered. An implementation is permitted to perform early initialization of other local objects with static storage duration under the same conditions that an implementation is permitted to statically initialize an object with static storage duration in namespace scope(3.6.2). Otherwise such an object is initialized the first time control passes through its declaration; such an object is considered initialized upon the completion of its initialization. If the initialization exits by throwing an exception, the initialization is not complete, so it will be tried again the next time control enters the declaration. If control re-enters the declaration (recursively) while the object is being initialized, the behavior is undefined."

EDIT 2:

$8.5/9- "If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a nonstatic object, the object and its subobjects, if any, have an indeterminate initial value90); if the object or any of its subobjects are of const-qualified type, the program is ill-formed."

In general, you want to read up these sections along with $8.5 for good hold on this aspect.

Answer 2

This code won't compile unless you forward declare A.

global_A reference is initialized to null - No, it will reference an A object. global_int is 0 - Think so, need to check. local_A reference is null - No, same as with global_A. local_int is uninitialized - Yes, it will get some garbage value. Both global_A.x and local_A.x is uninitialized - Yes.

You can always debug and see for yourself.

Answer 3

Just for the sake of completeness if you had references:

References must be initialized on declaration, otherwise you get punished by a compiler error. This means a reference always needs another value or reference it refers to (like the says), this is ensured by the compiler so you cannot forget it. This also implies that references can never be null pointers. However the object they refer to might become invalid.

Answer 4

well guys .. i am more confused as i see responses from here. Anyway i did a test as shown below:

1 #include

  2 using namespace std;
  3 
  4 class A {
  5 
  6 public :
  7         A() : x(9) {};
  8         int x;
  9 
 10 };
 11 
 12 A global_a;
 13 int global_b;
 14 
 15 int main() {
 16 
 17         A local_a;
 18         int local_b;
 19         cout << "global_a.x = " << global_a.x << '\n';
 20         cout << "local_a.x = " << local_a.x << '\n';
 21 
 22         cout << "global_b = " << global_b << '\n';
 23         cout << "local_b = " << local_b << '\n';
 24 
 25 }

Results using my g++ compiler on ubuntu linux:

global_a.x = 9

local_a.x = 9

global_b = 0

local_b = 0

I do think local_b should be undefined but somehow compiler initialized it by default. However local_a .. i am not sure if that should be initialized by default. From the testing here .. local_a seem to be initialized. Not sure if that complies with standard c++ specification (eg C++ PRimer 4th edition says default constructor is used regardless where a class variable is declared - does that means variable of class type is initialized whether it is global or local?).

Whatever it is .. it is one big hell of confusion. Maybe i should quit learning C++. Java is so much more straight forward. Hell yeahhh!!