联合包含一个具有非平凡析构函数的成员( std :: string ).这意味着联合不能拥有默认的析构函数(它不知道要调用哪个成员的析构函数).因此，您需要提供一个自定义析构函数.

The union contains a member (std::string) with a non-trivial destructor. This means that the union can't have a defaulted destructor (it wouldn't know which member's destructor to call). So you need to provide a custom destructor.

在您的情况下，定义一个不执行任何操作的联合析构函数，然后在struct析构函数中进行工作:

In your case define a union destructor that does nothing and then do the work in the struct destructor:

struct Foo {
    int type;

    union U {
        int intValue;
        double doubleValue;
        std::wstring stringValue;

        
        ~U() noexcept {}

    } value;

    ~Foo()
    {
        using std::wstring;

        if (type == 3)
            value.stringValue.~wstring();
    }
};

请注意，复制/移动构造函数/分配也需要这样做.

Please note that you need to do this for copy/move constructor/assignments as well.

在C ++ 17中，您有 std :: variant ，这是一个安全的联合.

In C++17 you have std::variant which is a safe union.