What is the default values for members of a struct and members of a class in c++, and how do these rules differ (e.g. between classes/structs/primitives/etc) ? Are there circumstances where the rules about the default values differs ?
There are no differences between structs and classes in this regard in C++. They all are called just class types.
Members of class types have no default values in general case. In order to for a class member to get a deterministic value it has to be initialized, which can be done by
- Default constructor of the member itself
- Constructor initializer list of the enclosing class
- Explicitly specified initializer for object of the enclosing class (that includes value-initialization and initialization with aggregate initializer).
Additionally, all objects with static storage duration are zero-initialized at the program startup.
Aside from the above cases, class members, once again, have no default values and will initially contain unpredictable garbage values.
Yeah, there is one. If you initialize an object with the default constructor and use parentheses then the POD members will be zero initialized:
someClass * p = new someClass();
来源：https://stackoverflow.com/questions/2614809/what-is-the-default-value-for-c-class-members