C++ "cannot add two pointers", adding a hardcoded string to a CString

Question

I get this error quite often when I try to do something like this

CString filePath = theApp->GetSystemPath() + "test.bmp";

The compiler tells me

error C2110: '+' : cannot add two pointers

But if I change it to this below it works fine?

CString filePath = theApp->GetSystemPath();
filePath += "test.bmp";

The function GetSystemPath returns a LPCTSTR if that has anything to do with it

Answer 1

This has to do with the types of objects that you are dealing with.

CString filePath = theApp->GetSystemPath() + "test.bmp";

The line above is attempting to add the type of GetSystemPath() with "test.bmp" or an LPCTSTR + char[]; The compiler does not know how to do this because their is no + operator for these two types.

The reason this works:

filePath += "test.bmp";

Is because you are doing CString + char[] (char*); The CString class has the + operator overloaded to support adding CString + char*. Or alternatively which is constructing a CString from a char* prior to applying the addition operator on two CString objects. LPCTSTR does not have this operator overloaded or the proper constructors defined.

Answer 2

Well you can't add two pointers. The reason filePath += "test.bmp"; works is that the left hand side is a CString not a pointer. This would also work

CString(theApp->GetSystemPath()) + "test.bmp";

and so would this

theApp->GetSystemPath() + CString("test.bmp");

The rules of C++ prevent you overloading operators unless at least one of the argument is of class type. So it's not possible for anyone to overload operator+ for pointers only.