c++ - 是否多次调用多个继承的构造函数？

Question

多重继承的构造函数会被多次调用吗？构造函数的调用顺序是什么？这是否取决于继承列表中的顺序？

这是一个例子(只是为了说明情况，没有现实生活中的例子)。

class Base {};
class DerivedBaseOne : public Base {};
class DerivedBaseTwo : public Base {};
class Derived : public DerivedBaseTwo, public DerivedBaseOne 
{};

//somewhere in the code, is Base() called two times here?
Derived * foo = new Derived();

Base() 构造函数是否被调用了两次？构造函数的调用顺序是什么？先打基础？还是先 DerivedBaseOne() 或 DerivedBaseTwo()？

Answer 1

继承层次结构的构造函数调用顺序为:

Base()  
DerivedBaseTwo()  
Base()
DerivedBaseOne()  
Derived()

顺序确实定义明确，取决于您提到基类派生的顺序以及您在类中为成员声明成员的顺序。 (请参阅下面的 C++ 标准引用。)

Base() 构造函数会被调用两次吗？
是

Base() 类构造函数在这里被调用了两次，因为两个类 DerivedBaseTwo() 和 DerivedBaseOne() 派生自它，所以基类构造函数为它们中的每一个调用一次。您的 Derived 类通过多个路径具有两个不同的 Base 子对象(一个通过 DerivedBaseOne() 另一个通过 DerivedBaseTwo())。

多重继承的类层次结构是不寻常的，它会导致一个称为 Diamond Shaped Inheritance Problem 的问题。为了避免这个问题，C++ 引入了 Virtual base class 的概念。 。

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引用:

C++03 标准:12.6.2/5，初始化基和成员

Initialization shall proceed in the following order:

— First, and only for the constructor of the most derived class as described below, virtual base classes shall be 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 class names in the derived class base-specifier-list.

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

— Then, nonstatic data members shall be initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

— Finally, the body of the constructor is executed.