What's the use of const variable members in C++?

Question

Are there any scenarios where a const variable member is useful in C++?

If you want to make an immutable class, the usual approach is to declare private members with get-only const functions to access their values. This has the advantage that the class can be copy assigned and so on. So in this case you don't need const variable members.

On the other hand, if the class has a const member variable, it won't get an automatic copy assignment operator. I don't see an scenario where this would be useful.

"If you want to make an immutable object, the usual approach is to declare private member with get-only const functions to access their values." Where did you hear that? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Jun 7, 2015 at 18:46
"This has the advantage that the class can be copy assigned" if you want an object to remain constant, that's a disadvantage. It's a point of failure. — Drew Dormann
– Drew Dormann, Commented Jun 7, 2015 at 18:49
If you can assign a new value to an object, it's not immutable. — molbdnilo
– molbdnilo, Commented Jun 7, 2015 at 19:51
@LightnessRacesinOrbit I think the confussion was because I used "object" instead of "class". I fixed that. — a06e
– a06e, Commented Jun 7, 2015 at 20:06

Cheers and hth. - Alf · Accepted Answer · 2015-06-07 18:59:24Z

2

A main advantage of a const data member is the same as with a reference member (indeed a reference can be usefully thought of as a const pointer), namely that it forces initialization, unless the member is of a type with a user-defined default constructor. The compiler will insist on initialization. Still, I've never found that so useful that I've started doing it.

An alternative, if guaranteed initialization is what one desires, is to wrap the data member in a class that does not provide default construction. With this approach the data member can be assigned to, if it supports assignment.

Another advantage (of a const data member) is that it expresses an intended constraint, with compiler checking, and that's almost always good. The more constraints on how values can change, the less there is to consider to understand or debug the code.

answered Jun 7, 2015 at 18:59

Cheers and hth. - Alf

146k15 gold badges218 silver badges342 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Razib · Accepted Answer · 2015-06-07 18:46:57Z

0

Once you initialize a variable const then you never can re-initialized it. Each later attempt to re-initialize the const variable will produce a compilation error.

It is helpful when you want to prevent the accidental modification of some variable which you never want to be change. Like in mathematics we need PI, we can declared it as a constant -

private const double PI = 3.1416;

answered Jun 7, 2015 at 18:46

Razib

11.2k12 gold badges59 silver badges86 bronze badges

1 Comment

ChrisF Over a year ago

Comments purged as they were not constructive.

Collectives™ on Stack Overflow

What's the use of const variable members in C++?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related