联合成员分配时的C++分段错误

以下代码在分配给union成员时产生分段错误。这一定是误解工会的结果，但我无法指出我的错误。

Q1：是否有必要在 T1 的构造函数中为适当的联合成员变量包含一些初始化？

我尝试了类似于建议的解决方案（在下面的代码块中找到）来初始化适当的成员，但没有成功。

Q2：如果T1初始化没问题，为什么分配到这个位置会产生segmentation fault？

相关代码：

struct T1
{
    T1(int type)
    {
        this->type = type;
        //Q1 : need to init appropriate union struct here also? Something like:
        /*
        *if (type == 1)  
        *{
        *    union_member1.str = "";
        *}
        *else
        *{
        *    union_member2.value = 0;
        *}
        */
        //If so, better alternative to this?
    }

    int type;           //tells which union struct is used
    
    union               
    {
        struct              //type 1
        {
            std::string str;
        } union_member1;

        struct              //type 2
        {
            int value;
        } union_member2;
    };
};

struct T2
{
    bool used;
    /*
    * other variables here
    */
}; 

std::map<std::string, T2> mymap;
T1* global_ptr;

int main()
{
    for (auto& map_entry : mymap)
    {
        if (!(map_entry.second.used))
        {
            global_ptr = new T1(1);                                 
            global_ptr->union_member1.str = map_entry.first;        //Q2: SEG fault occurs with this assignment
            std::string s = map_entry.first;                        //    but not with this assignment
        }
    }
}

GDB 输出

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b70b03 in std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) ()
   from /usr/lib/x86_64-linux-gnu/libstdc++.so.6

显然，出于可读性原因，这段代码被精简了。地图条目也经过彻底验证。

回答

    if (type == 1)  
    {
        new (&union_member_str) std::string();
    }

    if (type == 1)  
    {
        using UMT = decltype(union_member1);
        // or
        // typedef decltype(union_member1) UMT;
        new (&union_member1) UMT();
    }

    if (type == 1)  
    {
        union_member_str.string::~string();
    }

    if (type == 1)  
    {
        using UMT = decltype(union_member1);
        union_member1.UMT::~UMT();
    }