C/C++ - Is it Recommended to use const static or #define

Question

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“static const” vs “#define” in C

My first thought is that this is implied, but is there ever a reason why you would use const instead of #define?

If you set a global variable, why would you ever want to change it, and wouldn't you want to protect it globally as well?

Can you give an example of a definition you're asking about? — Michael Mior, Jan 16 '13 at 23:56
"why you would use const instead of #define" is the wrong question. It should be "why would you use #define instead of const?" and the answer is "for no reason at all". — R. Martinho Fernandes, Jan 17 '13 at 00:02
Also, for this question, you should differentiate between C or C++. C++ would *always* prefer static const, or const to #define. C++ will only use the preprocessor when it is absolutely necessary. C on the other hand will use the preprocessor more, so an example is needed to give a "correct" answer. — Josh Petitt, Jan 17 '13 at 00:05
@JoshPetitt, C _has_ to use the preprocessor more, because a `const int` is not a valid constant expression so can't be used in as many places as in C++. — Jonathan Wakely, Jan 17 '13 at 00:06

score 5 · Accepted Answer · edited Jan 17 '13 at 00:01

Const usually replaces #define

#define is a pre-processor macro that can do textual replacement. You can use it to define a constant or a macro or do all sorts of other things.

const is a type-safe way to define a compile-time constant

These two mechanisms occur at different times in the compilation process, but in general, const was created to rectify the problems of #define.

I've rarely seen people do something like

#define CONSTINT  const int

but it is legal.

score 4 · Answer 2 · answered Jan 16 '13 at 23:59

const is only relevant for variables that are passed around at runtime that ensures that subroutines cannot change them. #define is a preprocessor compiletime directive that replaces whatever you define with what you have defined it as. Therefore, they are for different purposes.

score 2 · Answer 3 · answered Jan 17 '13 at 00:04

Edit this is an answer to your original question, of whether you'd use const with a define ... it doesn't really make sense now you've edited the question to ask something different.

A #define does not define a variable, so you can't change it anyway, so the question doesn't make sense.

This isn't even possible:

#define FOO 99

int main()
{
    FOO = 98;
}

Because the preprocessor substitutes the macro FOO for the replacement 99, so the compiler sees this code:

int main()
{
    99 = 98;
}

And obviously that's nonsense. You can't assign to a literal, it's not a variable (const or not) it's just a value.

C/C++ - Is it Recommended to use const static or #define

3 Answers3