Compile-time warning

[dmacks]

Compiling libGDSII-0.21 on OS X 10.13 with Apple's compiler toolchain...

libtool: compile:  g++ -DHAVE_CONFIG_H -I. -I.. -g -O2 -MT ReadGDSIIFile.lo -MD -MP -MF .deps/ReadGDSIIFile.Tpo -c ReadGDSIIFile.cc  -fno-common -DPIC -o .libs/ReadGDSIIFile.o
ReadGDSIIFile.cc:654:50: warning: adding 'libGDSII::BYTE' (aka 'unsigned char')
      to a string does not append to the string [-Wstring-plus-int]
       *ErrMsg = new string("unknown data type " + DType);
                            ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
ReadGDSIIFile.cc:654:50: note: use array indexing to silence this warning
       *ErrMsg = new string("unknown data type " + DType);
                                                 ^
                            &                    [      ]
1 warning generated.

Should it be some sort of ostringstream (like around line 579) or other type of concatenation or sprintf?