继承和重载默认构造函数 [英] Inherit and overload default constructor

问题描述

我一直在寻找这个，我很惊讶，我没有找到任何东西。为什么不能使用使用声明继承基类构造函数并在派生类中添加重载？我使用Visual C ++ 2013，当默认构造 b 时，基类构造函数被忽略：

I've been searching for this and I'm amazed I haven't found anything. Why can't I inherit a base class constructor using using declaration and add an overload in the derived class? I'm using Visual C++ 2013, the base class constructor is ignored when default-constructing b:

error C2512: 'B' : no appropriate default constructor available

< ve通过重新定义构造函数来处理这个问题，但是我不喜欢这样。这只是一个最小的例子，如果我只有一个基类构造函数，它不会打扰我。

I've dealt with this by re-defining the constructors, but I don't like that. This is just a minimal example, it wouldn't bother me if I had only one base class constructor.

struct A
{
    A() : a(10) {}

    int a;
};

struct B : A
{
    using A::A;

    explicit B(int a) { this->a = a; }
};

int main()
{
    B b;
}

推荐答案

构造函数不被继承。从[class.inhctor] / p3：

The problem is that default-constructors are not inherited. From [class.inhctor]/p3:

对于候选的继承构造函数[..]中的每个非模板构造函数，构造函数隐式声明为具有相同的构造函数特性，除非有一个用户声明的构造函数，在完成类中使用相同的签名，其中使用声明出现或构造函数将是默认的副本，或移动该类的构造函数。

For each non-template constructor in the candidate set of inherited constructors [..], a constructor is implicitly declared with the same constructor characteristics unless there is a user-declared constructor with the same signature in the complete class where the using-declaration appears or the constructor would be a default, copy, or move constructor for that class.

您还有一个用户声明的构造函数，默认构造函数。只需添加默认值即可使其有效：

You also have a user-declared constructor that suppresses the creation of an implicit default-constructor. Just add a defaulted one to make it work:

B() = default;

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