mjhjmtu

My question is about how the member data with initializer get initialized where there is also a default argument in the constructor.

class InputPlay 

 public:
 InputPlay(std::string s = "test" ) : _s(s) ;

 private:
 std::string _s = "default";
;

Question:

Is there are going to be 2 times initialization for the variable _s when the construct is called? aka the _s will be initialized by the string literal default and then replace by the default argument "test" in the constructor?

No, _s will only be initialized once. The in class initialization is syntactic sugar for synthesizing a member initializer. If you provide your own member initializer then the compiler will use that instead of synthesizing one for you from the in class initialization.

No. For non-static data member, when both default member initializer and member initializer list are provided, the default member initializer will be ignored. That means _s will be initialized by the argument s of the constructor directly.

If a member has a default member initializer and also appears in the member initialization list in a constructor, the default member initializer is ignored.

= "default"; is a default member initializer. It's the initializer to use for _s if you don't provide one. But since you do provide one (_s(s)) the default initializer never comes into player and _s is simply initialized to s.

From cppreference.com :

If a non-static data member has an default member initializer and also appears in a member initializer list, then member initializer list is executed and the default member initializer is ignored:

Note that by definition something can only ever be initialized once. If something looks like it's initialized twice, it's usually being initialized and then assigned another value.