C++ Single value enums versus preprocessor directives

Question

Very simple question about best practices and performance. I know that it's a bad idea to use bare constants in your code directly (e.g. -1 meaning "unassigned"). I generally don't like using the preprocessor for such things if I can help it, since I don't like shouting (e.g. #define UNASSIGNED -1) and don't like breaking with the convention of putting preprocessor names in all caps. So I've taken to using anonymous enums:

enum { Unassigned = -1 };

Question: Is there any runtime performance penalty for this compared to the preprocessor approach? Is this a bad idea?

Have you considered static const variables? I'm not suggesting it, just questioning. — Joseph Mansfield
– Joseph Mansfield, Commented Jan 15, 2014 at 15:05
likewise, have you considered if in case you need multiple such constants, that somehow belong together, an enum class is a possible solution? — PlasmaHH
– PlasmaHH, Commented Jan 15, 2014 at 15:09

Angew is no longer proud of SO · Accepted Answer · 2014-01-15 15:06:48Z

There is no runtime performance impact - the compiler will know they're just the value -1 in both cases.

However, I believe the best approach is to use a correctly typed constant for this. For example:

const int Unassigned = -1;

(Substitute your actual type for int, of course).

Agreed. Now that you mention it, in C++11 it's probably even better to use constexpr int Unassigned = -1;

Wojtek Surowka · Accepted Answer · 2014-01-15 15:06:15Z

3

There should be no performance impact. However, why not just const int Unassigned = -1?

answered Jan 15, 2014 at 15:06

Wojtek Surowka

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