CODEWITHSUNDEEP
c
Shaarib
Shaarib

Reputation: 71

Defining a constant in terms of other constants

sorry if this question seems naive, but I haven't been able to find a clear answer to it anywhere. I must define a constant in terms of previously defined constants, like

#define CONST_A 2
#define CONST_B 3
#define CONST_C CONST_A*CONST_B

The actual values of CONST_A and CONST_B are fed as defineflags to gcc, so I can't just write #define CONST_C 6.

If I understand correctly, this will tell the preprocessor to replace any appearance of CONST_C by 2*3 and not 6, right? I'm mainly worried about performance, so I would prefer the latter. I'm guessing this could be done by using static const instead of preprocessor #define. Is this the best option?

Thanks in advance!

Upvotes: 3

Views: 496

Answers (2)

ouah
ouah

Reputation: 145829

C say that constant expressions can be evaluated at compile time and any today's decent compiler will evaluate constant expressions at compile time. This compiler operation is known as constant folding.

(C99, 6.6p2) "A constant expression can be evaluated during translation rather than runtime, and accordingly may be used in any place that a constant may be."

Upvotes: 0

user4815162342
user4815162342

Reputation: 154911

Don't worry about performance of constant expressions like 2 * 3 in C. C compilers have been able to eliminate such expressions by evaluating them at compile-time for at least 20 years.

static const can be preferred for other reasons, such as type-safety or not having to worry about precedence (think what happens if CONST_A is defined as 2+2), but not for performance reasons.

Upvotes: 9

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