C++ const in getter

Question

I'm still learning about C++ and I'm reading everywhere that I have to use const everywhere I can (for speed reason I think).

I'm usually write my getter method like this:

const bool isReady() {
    return ready;
}

But I've seen that some IDE autogenerate getter in this way:

bool getReady() const {
    return ready;
}

But, writing delegates, it happened to me to find this error if the const is after the function:

member function 'isReady' not viable: 'this' argument has type 'const VideoReader', but function is not marked const

So that, what is the better way to write a const getter? Do I really have to care about?

In reference to booleans this question seemed a little silly. I found a great explanation of const at this question though: http://stackoverflow.com/questions/5503352/const-before-or-const-after — hubatish, Dec 21 '16 at 23:59

score 62 · Accepted Answer · edited Dec 07 '17 at 13:43

62

There is a huge difference between the two ways.

const bool isReady()

The code above will return a const bool, but it does not guarantee that the object will not change its logic state.

bool isReady() const

This will return a bool, and it guarantees that the logic state of your object will not change. In this case it is not necessary to write const in front of the return type. It makes no sense to return a const bool because it is a copy anyway. So making it const is useless. The second const is needed for const correctness, which is not used for speed reasons but to make your program more reliable and safe.

edited Dec 07 '17 at 13:43

psq

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answered Jan 31 '14 at 11:04

mkaes

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"You should really read about this" can you provide me some links? – nkint Jan 31 '14 at 11:37
3

@nkint: E.g. Effective C++ by Scott Meyers or Exceptional C++ by Herb Sutter. Here is the old one http://www.gotw.ca/gotw/006.htm and for C++11 this even got more important. http://herbsutter.com/2013/05/24/gotw-6a-const-correctness-part-1-3/ – mkaes Jan 31 '14 at 11:40
1

I don't think this is the best answer in term of explanation of the problem but the links are very good! – nkint Jan 31 '14 at 12:04

score 49 · Answer 2 · answered Jan 31 '14 at 11:03

They mean two differnt things:

const bool isReady() {
    return ready;
}

This returns a constant bool. Meaning a bool which cannot change value from the time it's been created.

bool getReady() const { 
    return ready;
}

This is a a constant function, meaning a function that will not alter any member variables of the class it belongs to. This is the style recommended to use for getters, since their only purpose is to retrieve data and should not modify anything in the process.

I think this is the most complete and succinct answer. – Frode Akselsen Jan 21 '21 at 21:01 — Frode Akselsen, Jan 21 '21 at 21:01

score 7 · Answer 3 · answered Jan 31 '14 at 11:06

const method informs compiler that you will not modify class instance on which this method is called:

class A {
public:
bool getReady() const {
    return ready;
}
};

so if you try to modify your object inside getReady() then compiler will issue error. Const methods are usefull where you have ie.: const A&, or const A*, then you can call only const methods on such objects.

as for:

const bool isReady() {
    return ready;
}

this const provides actually no real benefit, because bool is copied while isReady() returns. Such const whould make sense if returned type was a const char* or const A&, in such cases const makes your char string or A class instance immutable.

score 4 · Answer 4 · answered Jan 31 '14 at 11:02

A const getter has the signature

bool getReady() const

The other version isn't a const method, it just returns a const value (which is basically useless).

Having a const getter allows you to call it on const objects:

const Object obj;
obj.getReady();

This is only valid if getReady is marked as const.

score 3 · Answer 5 · answered Jan 31 '14 at 11:05

There is a difference between using the const keyword for the return type or for the method signature. In the first case the returned value will be a constant value. In the second case the method will be a so-called constant method, which cannot change the representation of the object. On constant objects, only the constant methods are callable.

C++ const in getter

5 Answers5

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