Placement-new an STL container and destroying it safely afterwards

This code implements an unrestricted union which provides access by name and by index to any of its three members.

Since std::string is non-trivially constructed and destroyed, I need to provide special contructor and destructor for the union.

#include <iostream>
#include <string>

using namespace std ;

union MyUnion{
    string parts[3] ;
    struct{ string part1, part2, part3 ; } ;

    MyUnion(){
        new(parts+0) string ; //constructs the 3 strings in-place
        new(parts+1) string ;
        new(parts+2) string ;
    }
    ~MyUnion(){
        parts[0].~string() ; //calls string's destructor
        parts[1].~string() ;
        parts[2].~string() ;
    }
} ;

int main(){

    MyUnion u ;

    u.part1 = "one" ; //access by name
    u.part2 = "two" ;
    u.part3 = "three" ;

    cout << u.parts[0] << endl ; //access by index
    cout << u.parts[1] << endl ;
    cout << u.parts[2] << endl ;
}

This example compiles and works fine (seemingly), but my question is:

Is it safe to do this?
Can I be sure that there will be no memory leaks?
What if string's constructor throws an exception? Does that need to be caught so as to not try to destroy an object that was never constructed?