如何在Union中初始化非POD成员

问题描述

在c ++ 11中，Union支持非POD成员。我想在构造函数中初始化一个非POD成员。

In c++11, Union supports non-POD member. I want to initialize a non-POD member in the constructor.

在wikipedia c ++ 11上，它使用展示位置new初始化非POD成员。

On wikipedia c++11 page, it uses a placement 'new' to initialize a non-POD member.

#include <new> // Required for placement 'new'.

struct Point {
    Point() {}
    Point(int x, int y): x_(x), y_(y) {}
    int x_, y_;
};

union U {
    int z;
    double w;
    Point p; // Illegal in C++03; legal in C++11.
    U() {new(&p) Point();} // Due to the Point member, a constructor definition is now required.
};

我想知道如果我使用ctor-initializer-list而不是placement'new' ？

I am wondering is there any difference if I use ctor-initializer-list instead of placement 'new'?

U() : p() {}

推荐答案

您可以在 ctor-initializer-list 中最多一个成员初始化。 union是一个类，所以[class.base.init]（C ++ 11§12.6.2）中的成员初始化器的规则适用于具有类键 struct 或 class 。一个明显的例外情况在12.6.2 / 8中说明：试图初始化一个联合的多个非静态数据成员会使程序生成错误。

You are free to initialize at most one member of a union in the ctor-initializer-list. A union is a class, so the rules for member initializers in [class.base.init] (C++11 §12.6.2) apply just as they do to classes with the class-key struct or class. One obvious exception is stated in 12.6.2/8: "An attempt to initialize more than one non-static data member of a union renders the program ill-formed."

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