c++ - 您是否需要从所有派生类调用虚拟基类构造函数？即使它们不是最衍生的？

Question

我在多重继承和菱形问题上遇到了麻烦。

出现问题是因为我的基类构造函数需要一个参数。编译器尝试为我的两个抽象类生成默认构造函数，但失败了，因为默认构造函数无法确定基类的参数。

我不明白为什么我的抽象类要调用基本构造函数。我认为最派生的类是调用虚拟基类构造函数的类。

这是重现我所说内容的代码:

class VirtualBase
{

        public:

                VirtualBase(int initial) :
                        count(initial)
        {}

                int getCount() const
                {
                        return count;
                }

                void increment()
                {
                        count++;
                }

        private:

                int count;

};


class ContractA : public virtual VirtualBase
{

        public:

                virtual void doSomething() = 0;

};

class ContractB : public virtual VirtualBase
{

        public:

                virtual void doSomethingElse() = 0;

};

class Concrete : public ContractA, public ContractB
{

        public:

                Concrete() : 
                        VirtualBase(0)
        {}

                virtual void doSomething()
                {
                         increment();
                }

                virtual void doSomethingElse()
                {
                        // etc...       
                }

};

int main()
{

        Concrete concrete;
        concrete.doSomething();
        concrete.doSomethingElse();
        return 0;
}

我收到以下错误(针对每个契约(Contract)):

main.cpp: In constructor ‘ContractA::ContractA()’:
main.cpp:29:7: error: no matching function for call to ‘VirtualBase::VirtualBase()’
 class ContractA : public virtual VirtualBase
       ^
main.cpp:29:7: note: candidates are:
main.cpp:9:3: note: VirtualBase::VirtualBase(int)
   VirtualBase(int initial) :
   ^
main.cpp:9:3: note:   candidate expects 1 argument, 0 provided
main.cpp:4:7: note: VirtualBase::VirtualBase(const VirtualBase&)
 class VirtualBase
       ^
main.cpp:4:7: note:   candidate expects 1 argument, 0 provided
main.cpp: In constructor ‘Concrete::Concrete()’:
main.cpp:53:17: note: synthesized method ‘ContractA::ContractA()’ first required here 
    VirtualBase(0)
                 ^

Answer 1

您的示例使用 EDG 编译和 clang但它不能用 gcc 编译.我不确定代码是否应该按原样编译，因为抽象基类的构造函数似乎被声明为已删除:根据 12.1 [class.ctor] 第 4 段，第六项，默认的默认构造函数被声明为已删除，如果任何子对象都没有默认构造函数:

... A defaulted default constructor for class X is defined as deleted if: ...

• ...
• any potentially constructed subobject, except for a non-static data member with a brace-or-equalinitializer, has class type M (or array thereof) and either M has no default constructor or overload resolution (13.3) as applied to M’s default constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or
• ...

在创建virtual 基类方面，对于具有virtual 基类的类没有特殊豁免，即默认的默认构造函数将被删除。

对于抽象类，显然没有必要从构造函数成员初始化列表中调用虚基类。至少，这是 12.6.2 [class.base.init] 第 8 段根据其注释所说的:

In a non-delegating constructor, if a given potentially constructed subobject is not designated by a meminitializer-id (including the case where there is no mem-initializer-list because the constructor has no ctorinitializer), then

• if the entity is a non-static data member that has a brace-or-equal-initializer and either
• the constructor’s class is a union (9.5), and no other variant member of that union is designated by a mem-initializer-id or
• the constructor’s class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id, the entity is initialized as specified in 8.5;
• otherwise, if the entity is an anonymous union or a variant member (9.5), no initialization is performed;
• otherwise, the entity is default-initialized (8.5).

[ Note: An abstract class (10.4) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. — end note ] ...

most derived base的相关部分在12.6.2 paragraph 7, last sentence:

... A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.