Defines vs. const object

[roberts]

I'm procrastinating a new session of revision and debugging, but there is a nagging question regarding the transition from ANSI C to C++ I have.

I've seen programmers define a const int or floating point and then declare it. Example:

const double Planck = 6.626068e-34

Back when I was programming ANCI C, we would do

#define PLANCK 6.626068e-34

Essentially, neither be changed and represent the same value.

I may(more like probably) be needing to use such constants in the future, so what is the advantage of Plank vs. PLANK?

[hanst99]

Whether you put your constants in all uppercase or not is a matter of tastes, but there is literally no reason to use a define here. In this particular situation, you're unlikely to notice much of a difference, but there are other instances in which it matters (e.g. struct/class instances).

[roberts]

but there are other instances in which it matters (e.g. struct/class instances).

That's the kind of info I need to know. Why is there no reason to use a define here? Is it disadvantageous here?

What kind of instances call for a define vs. const object, and vice versa?

[hanst99]

Compare

#define MY_CONST ComplicatedClass();
const ComplicatedClass myConst;

"ComplicatedClass" needs to do something when constructed. myConst is created exactly once, while MY_CONST is inserting a default constructor call to ComplicatedClass every single time. That's quite a huge difference. I don't really know of any situation in which you'd prefer "define" - ing a constant over just using const.

[Moschops]

http://www.parashift.com/c++-faq-lite/new/const-vs-define.html

[roberts]

Ty Mos, that is what I was looking for.