Question

I have recently fixed a bug in an application of mine: the problem was that an object that resides on the stack had a field left uninitialized.

The object had a class declaration of this type:

struct A{
    int somefield, someotherfield;
    A(): someotherfield(0) {}
}

and when declaring a local variable (like A var; in a function), somefield was left uninitialized, and so a read of it would return a randomish value.

I was certain that fields of a class, which don’t appear in the constructor initialization list, would always get initialized by a synthesized trivial constructor (in the case of an int, a zero value). Evidently I am wrong.

So what are the general rules about implicit field initialization?

Answer 1

• classes and structs are initialized by contructor
• Basic types int double char short … are not initialized and contain random numbers
• Pointers are not initialized and point to random positions
• arrays of classes or structs cause each element to be initialized by its constructor
• arrays of basic types or pointers are random.