The usage of anonymous enums

What is the purpose of anonymous enum declarations such as:

enum { color = 1 };

Why not just declare int color = 1?

Solution 1:

That's a so-called enum trick for declaring a compile-time integer constant. It's advantage is it guarantees that no variable is instantiated and therefore there's no runtime overhead. Most compilers introduce no overhead with integer constants anyway.

Solution 2:

Enums don't take up any space and are immutable.

If you used const int color = 1; then you would solve the mutability issue but if someone took the address of color (const int* p = &color;) then space for it would have to be allocated. This may not be a big deal but unless you explicitly want people to be able to take the address of color you might as well prevent it.

Also when declaring a constant field in a class then it will have to be static const ^{(not true for modern C++)} and not all compilers support inline initialization of static const members.

Disclaimer: This answer should not be taken as advice to use enum for all numeric constants. You should do what you (or your co-workers) think is more readable. The answer just lists some reasons one might prefer to use an enum.