英文:
C++ constructor inherited?
问题
在C++中，构造函数不会被继承。
然而，我在使用clang12时发现了这个奇怪的现象。
它在遵循C++17时编译，尽管它不应该。
如果我使用C++11或C++14，它就无法如我所预期地编译。
#include <iostream>
class Parent{
int x_;
public:
//Parent() = default;
Parent(int const &x) : x_{x} {}
void SayX(){ std::cout << x_ << std::endl; }
};
class Child : public Parent{
// works with C++17 ff.
};
int main(){
Child c {2};
c.SayX();
return 0;
}
--> 在C++17下输出2，而在C++11、14下无法编译。
英文:
In C++ constructors are not inherited.
However, I have this strange finding using clang12.
It compiles with C++17 following although it shouldn't.
If I use C++11 or C++14 it doesn't compile as I expected.
#include <iostream>
class Parent{
int x_;
public:
//Parent() = default;
Parent(int const &x) : x_{x} {}
void SayX(){ std::cout << x_ << std::endl; }
};
class Child : public Parent{
// works with C++17 ff.
};
int main(){
Child c {2};
c.SayX();
return 0;
}
--> Outputs 2 with C++17 ff., doesn't compile with C++11, 14
答案1
得分: 7
你没有看到一个继承的构造函数（你说得对，这些通过 using 是可选的），但是聚合初始化确实在C++17中得到了扩展，包括基类。根据cppreference（重点在于）：
聚合的元素包括：
[...]
对于类，声明顺序的直接基类，后跟声明顺序的直接非静态数据成员，既不是匿名位域，也不是匿名联合的成员。 <sub>(自C++17起)</sub>
所以 Child c {2}; 从 2 复制构造其 Parent 子对象。
英文:
You're not seeing an inherited constructor (you are right that these are opt-in through using), but aggregate initialization, which has indeed been extended in C++17 to cover base classes. From cppreference (emphasis mine):
> The elements of an aggregate are:
>
> * [...]
>
> * for a class, the direct base classes in declaration order, followed by the direct non-static data members that are neither anonymous bit-fields nor members of an anonymous union, in declaration order. <sub>(since C++17)</sub>
So Child c {2}; copy-constructs its Parent subobject from 2.
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