c ++ 11无限制工会的例子

问题描述

我阅读了

但我无法编译给定的示例：

but I can't compile the given example:

union U1 {
    int m1;
    complex<double> m2; // ok
};

union U2 {
    int m1;
    string m3;  // ok
};

U1 u;       // ok
u.m2 = {1,2};   // ok: assign to the complex member

结果：

main.cpp:85:8: error: use of deleted function 'U1::U1()'
     U1 u;       // ok
        ^
main.cpp:75:11: note: 'U1::U1()' is implicitly deleted because the default definition would be ill-formed:
     union U1 {
           ^
main.cpp:77:25: error: union member 'U1::m2' with non-trivial 'constexpr std::complex<double>::complex(double, double)'
         complex<double> m2; // ok
                         ^
main.cpp:86:5: error: 'u' does not name a type
     u.m2 = {1,2};   // ok: assign to the complex member
     ^
make: *** [main.o] Error 1

问题：

我认为在非限制的联合中，如果没有手动给定默认构造函数，这是真的，如何写一个工作示例？

I thought that in a nonrestricted union the first element will be constructed if no default constructor is manually given. Is this true and how to write a working example?

以下一个也不会编译：

class Y
{
    public:
        constexpr Y() {}
};

union X
{
    int a;
    float b;
    Y x;
    //X(){}
};

X x;

int main(){}

同样的错误信息：

main.cpp:112:7: error: use of deleted function 'X::X()'
     X x;
       ^
main.cpp:104:11: note: 'X::X()' is implicitly deleted because the default definition would be ill-formed:
     union X
           ^
main.cpp:108:11: error: union member 'X::x' with non-trivial 'constexpr Y::Y()'
         Y x;
           ^
make: *** [main.o] Error 1

推荐答案

在标准中，[class.union]在第2段（在注释中）提到：

In the standard, [class.union] mentions in paragraph 2 (in the notes) tells this :

由于您的类没有默认构造函数，所以编译失败。

Since your class has not-default constructor, the compilation fails.

第3段甚至提供了一个例子：

Paragraph 3 even provides an example :

union U {
int i;
float f;
std::strings;
};

并说：

---

Bjarne写了同样的事情：

---

Bjarne wrote the same thing :

但示例错误。 std :: string 和 std :: complex 都有非默认构造函数。因此，具有这些要求的工会需要工会的构造函数。

but wrong examples. Both std::string and std::complex have non-default constructors. Therefore, unions with those require union's constructor.

这篇关于c ++ 11无限制工会的例子的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！