Hi,
This is related to how C++ new works and how C++ object works.
Unlike C, C++ objects have constructors. The constructor is called when initializing the object to setup required values for the object to operate properly. With new, it allocates the required memory and calls the constructor to setup the TString. But with malloc it just allocate memory and does not initialize the object; leaving an empty shell of the object behind. When you use the empty shell - it crashes.