抽象类的虚拟基类的初始化 [英] Initialization of virtual base class of abstract class

问题描述

考虑代码：

#include <iostream>

using std::cout;
using std::endl;

struct A
{
    virtual void foo(){ };

    A()
    {
        cout << "A()" << endl;
    }
};

struct B : virtual A
{
    virtual void bar() = 0;

    B() : A() //mem-initializer of virtual base class
    {
        cout << "B()" << endl;
    }
};

struct C : B
{
    void bar(){ };
    C() : B()
    {
        cout << "C()" << endl;
    }
};

C c;

int main()
{
    //A()
    //B()
    //C()
    //Is output
}

演示

我写了代码以了解 12.6的规则注释。 2/8 N37973 ：

[注意：抽象类（10.4）绝不是大多数派生类，因此
它的构造函数从不初始化虚拟基类，因此
对应的mem初始化器可以省略。 - end note]

[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]

如果我们在 B（）并使基类 A 作为非虚拟，我们将有相同的结果。

If we omit mem-initializer in the B() and make the base class A as non-virtual we'll have the same result.

实际 。

那么我引用的注释的目的是什么？

So what purpose of the note that I cited?

推荐答案

它说的是这样的：

#include <iostream>

using std::cout;
using std::endl;

struct A
{
    virtual void foo(){ };

    // N.B.: no default ctor
    A(int)
    {
        cout << "A(int)" << endl;
    }
};

struct B : virtual A
{
    virtual void bar() = 0;

    B()
    {
        cout << "B()" << endl;
    }
};

struct C : B
{
    void bar(){ };
    C() : A(10), B()
    {
        cout << "C()" << endl;
    }
};

C c;

int main()
{

}

如果 A 不是 B 或 B的虚拟基础不是抽象的，则 B :: B（）必须有适当的 mem-initializer $ c> A ;然而，由于 B 是抽象的， A 是虚拟基础， B 的构造函数不会实际构造 A ，可以省略 mem-initializer 。注意，g ++目前不实现这个规则，并且仍然需要 B （它将永远不会实际使用）的 mem-initializer 。 Cl，但是。 演示。

If A weren't a virtual base of B, or B weren't abstract, then B::B() must have an appropriate mem-initializer for A; however, since B is abstract and A is a virtual base, and B's constructor will never actually construct A, the mem-initializer can be omitted. Note that g++ currently does not implement this rule and still requires a mem-initializer for B (which it will never actually use). Clang does, however. Demo.

另请参见 CWG问题257 ，介绍了此规则，其后的 CWG问题1658 。相关的GCC错误报告是错误19249 和 53878 。

See also CWG issue 257, which introduced this rule, with the phrasing further altered later by CWG issue 1658. The relevant GCC bug reports are bug 19249 and 53878.

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