C++ Segmentation Fault at Union Member Assignment

The following code produces a segmentation fault upon assignment to a union member. This must be a result of misunderstanding unions, but I cannot pinpoint my error.

Q1: Is it necessary to include some initialization for the appropriate union member variables in T1's constructor?

I have tried something similar to the suggested solution (found in code block below) to initialize the appropriate member, but no success.

Q2: If T1 initialization is OK, why does assigning to this location produce a segmentation fault?

Relevant Code:

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 Output

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

Obviously, this code is shaved down for readability reasons. Map entries have been thoroughly validated, as well.