const vs define

Hello!

I have a program that depends on some 11 values.
These values do not change or get defined/calculated during the program.
Instead they are a set of values that depend on something I will call mood.

What I am trying to understand is if I should find a way to have them as #define or as const.
This is a program for a robot so all optimization I can get is valuable.

Is it possible to pass at compile time an argument with mood?
What is the real gain in speed of using #define instead of cons?

Thanks for any info.

Consider this:

Code:

---

#include <stdio.h>

#define _Y_
#ifdef _X_
#define VAL1 1
#define VAL2 2
#elif defined _Y_
#define VAL1 3
#define VAL2 4
#else
#define VAL1 5
#define VAL2 6
#endif

int main () {
        printf("%d %d\n", VAL1,VAL2);
}

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You can have as many "elif's" as you want, then make your choice using the first #define before you compile. Alternately, you can leave that line out and give the compiler your choice, eg with gcc:

gcc myprog.c -D _X_

Thank you!

That was what I wanted to know! :)

The advantage of const is that it has a type, whereas #define values don't. Consider these two similar lines:

Code:

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const unsigned SOMEVAL = 100;
#define SOMEVAL 100

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With const, and appropriate compiler warnings turned on, the compiler will warn you that this assignment truncates the value:

Code:

---

char x = SOMEVAL;

---

Personally, I find the use of const in those situations to be of no value. I just #define macro values.

The value of const, in my view, is to let the compiler know you don't plan on modifying a value once it's calculated. That could possibly result in slightly more efficient code. And it prevents you passing the variable to a deviant function that modifies variables secretly passed by reference (i.e. declared with the & operator).

A const will also create an object that will take memory. A #define won't. So space-wise I believe it would be better to have a #define. For simple occassion a compiler can optimize the const and don't create an object and just replace the values. Speed-wise there should be no difference.

Quote:

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Originally Posted by C_ntua

Theoritically, a const will also create an object that will take memory. A #define won't. So space-wise I believe it would be better to have a #define.

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Without optimisation, it should actually be the opposite: there would be numerous copies of the value throughout the code if you use #define, whereas there would be only one copy of that object if you use const.

Quote:

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Originally Posted by C_ntua

For simple occassion a compiler can optimize the const and don't create an object and just replace the values.

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Rather, the compiler may be able to recognise that the same literal constant is used several times, and thus make it such that only one copy is loaded in the end.