問題描述

是在初始化變量之前調(diào)用父類的構(gòu)造函數(shù)，還是編譯器會先初始化類的變量?

Are parent class constructors called before initializing variables, or will the compiler initialize the variables of the class first?

例如:

class parent {
  int a;
public:
  parent() : a(123) {};
};

class child : public parent {
  int b;
public:
            // question: is parent constructor done before init b?
  child() : b(456), parent() {};
}

推薦答案

是的，基類在派生類成員和構(gòu)造函數(shù)體執(zhí)行之前初始化.

Yes, the base class is initialized before the members of the derived class and before the constructor body executes.

在非委托構(gòu)造函數(shù)中，初始化在以下順序:

In a non-delegating constructor, initialization proceeds in the following order:

——首先，并且只針對大多數(shù)的構(gòu)造函數(shù)派生類(1.8)，虛基類按順序初始化它們出現(xiàn)在有向的從左到右的深度優(yōu)先遍歷中基類的非循環(huán)圖，其中從左到右"是派生類中基類的外觀基本說明符列表.

— First, and only for the constructor of the most derived class (1.8), virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where "left-to-right" is the order of appearance of the base classes in the derived class base-specifier-list.

——然后??，直接基類在出現(xiàn)在基本說明符列表中的聲明順序(不管內(nèi)存初始化器的順序如何).

— Then, direct base classes are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

——然后??，非靜態(tài)數(shù)據(jù)成員按照它們在類定義(同樣不管內(nèi)存初始化器).

— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

——最后，復(fù)合語句構(gòu)造函數(shù)體被執(zhí)行.

— Finally, the compound-statement of the constructor body is executed.

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