c++ - 继承虚基类的构造函数

Question

虚基类在最派生类中初始化，所以我的猜测是继承基类的构造函数应该也能工作:

struct base {
    base(int) {}
};

struct derived: virtual base {
    using base::base;
};

derived d(0);

然而，这无法用 GCC 5.2.0 编译，它试图找到 base::base()，但在 Clang 3.6.2 上工作正常。这是 GCC 中的错误吗？

Answer 1

这是 gcc 错误 58751“[C++11] 继承构造函数不能与虚拟继承一起正常工作”(又名:63339“使用来自虚拟基的构造函数”被隐式删除”):

来自 58751 的描述:

In the document N2540 it states that:

Typically, inheriting constructor definitions for classes with virtual bases will be ill-formed, unless the virtual base supports default initialization, or the virtual base is a direct base, and named as the base forwarded-to. Likewise, all data members and other direct bases must support default initialization, or any attempt to use a inheriting constructor will be ill-formed. Note: ill-formed when used, not declared.

Hence, the case of virtual bases is explicitly considered by the committee and thus should be implemented.

从错误报告中借用的解决方法:

struct base {
    base() = default;  // <--- add this
    base(int) {}
};

根据错误报告，在这种情况下，构造函数 base::base(int) 由隐式生成的构造函数 derived::derived(int) 调用。

我检查过 your code不编译。但是this会调用 base::base(int) 构造函数。