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I grew some doubts after discussing this with colleagues…
As the title asks, when can it be assumed that built-in types will be initialized to 0 instead of an unknown value?
Do the rules vary among the c++ standards?
The full rules are in [dcl.init] (C++11). To summarize: when no initialiser is provided in a declaration, the entity is so-called default-initialised. For class types, this means the default constructor is called. For non-class types, this means no initialisation is performed.
However, [dcl.init] §9 states: “Every object of static storage duration is zero-initialized at program startup before any other initialization takes place.”
This means that static-duration variables (such as namespace-scope variables) of non-class type are zero-initialised. Other objects of non-class types (such as local variables) are not initialised.