Question

EDIT: I’m planing to refactor some code, and replace the define with a namespace alias. I can’t do this though just because “macros are evil”. I need to explain why I want to make the change and what can go wrong if I don’t.

Leaving aside the stance that “macros are evil”, what are the downfalls of #define over a namespace alias?

Take the code

#define MY_NAMESPACE my_namespace

versus

namespace MY_NAMESPACE = my_namespace;

The reason for having aliases is not in the scope of the question. You can also assume the name of the namespace is unique enough that it doesn’t appear anywhere else (i.e. it just refers to that namespace, it can’t – not now, not in the future – refer to a variable or a class or whatever), so there can be no ambiguity there.

Answer 1

In this particular case, it depends. If using a namespace alias
does the trick, by all means prefer it to macros, for all of the
usual reasons. But the two do radically different things. You
cannot open a namespace using its alias, i.e.:

namespace XYZ_ver1 {}
namespace XYZ = XYZ_ver1;

namespace XYZ {     //  Illegal!
}

This works with a macro; in fact, you can define the macro
before the namespace has ever appeared. If you need this, then
you need to use a macro.