C# / C++ Interop: C++ DLL returns char*

I have a C++ DLL wrapping other C++ code and am calling from C#.

   [DllImport("MyDll.dll")]
   static unsafe private extern IntPtr StatusToString();

The function being called is
   extern "C" {
      __declspec(dllexport)
      const char* StatusToString() {
         //return std::string("def").c_str();
         return "def";
      }
   }

The calling code is
   IntPtr abc = StatusToString();

As shown, with
   return "def";
and examining abc in the memory window, abc points to "abc".

However, with the C++ code modified to
   return std::string("def").c_str();
and examining abc in the memory window, abc points to garbage.

Why is that and what do I do?

Thanks,
Joe