使用联盟作为参考 [英] Use of Union with reference

问题描述

在工作中，我一直在使用Linux和C ++ 11和C ++ 14的GCC编译器.在工作中的某些代码中，我使用联合来存储引用和指针，如下所示:(简化为重要部分)

At work I've been using linux and the GCC compiler for C++11 and C++14. In some of the code at work, I've used a union to store both a reference and a pointer, as so: (Simplified to just the important parts)

struct MyStruct
{
    //Stuff
    union { double& x; double* x_ptr; };
    MyStruct(double& value) : x(value) {}
    //More stuff
};

我相信这段代码清晰，可读，明确，并提供了一种方便的方式来存储可以转移到其他地方的引用.它提供易于理解的语法糖，而不会提高性能，同时又提高了可读性.但是，当我尝试在Visual Studio 15中使用这样的代码时，由于"double&类型的非法联合成员"，代码无法编译.

I believe this code is clear, readable, unambiguous, and provides a convenient way to store references which can be shifted to something else. It provides easily understandable syntactic sugar without costing performance while improving readability. When I attempted to use code like this in visual studio 15, however, the code failed to compile due to "an illegal union member of type double&".

1. 此代码在标准下还是在Visual Studio 2015下是非法的?
2. 我可以在Visual Studio 2015中对其进行编译，还是可以提交错误报告/更改请求/内容?
3. 以这种方式使用工会是不好的做法吗?

注意:在我的工作中，几乎所有代码都是为Linux编写并使用GCC编译的，对于我的特定项目，可以保证C ++ 11，而GCC是唯一将要使用的编译器.

Note: At my work, pretty much all code is written for Linux and compiled with GCC, and for my specific project, C++11 is guaranteed and GCC is the only compiler that's going to be used.

编辑:请不要告诉我，将引用放在联合体内没有任何意义".当引用存储在结构内部时，它占用的空间与指针相同.此外，以下使用clang进行编译:

Please don't tell me that putting a reference inside a union "has no meaning". When a reference is stored inside a struct, it takes up the same amount of space as a pointer. In addition, the following compiles with clang:

struct MyStruct
{
    //This will compile
    union 
    { 
        struct { double& x; }; 
        double* x_ptr; 
    };
    //This won't compile; WHY?
    /*union 
    { 
        double& x;
        double* x_ptr; 
    };*/
    MyStruct(double& val) : x(val){}
    void Repoint(double& new_value) 
    { 
        x_ptr = &new_value; 
    }
};

为什么将引用包装在匿名结构中而不是仅在联合中时编译?

Why does it compile when the reference is wrapped in an anonymous struct, but not when it's just in the union?

实时示例

推荐答案

除了@Brian:您可以使用例如 std :: reference_wrapper 而不是简单的引用:

In addition to @Brian: You can make it compile by using e.g. std::reference_wrapper instead of a plain reference:

#include <functional>

struct MyStruct
{
    //Stuff
    union { std::reference_wrapper<double> x; double* x_ptr; };
    MyStruct(double& value) : x(value) {}
    //More stuff
};

int main()
{
    double value = 123;
    MyStruct myStruct(value);
}

实时示例

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