Whats the difference between const int const& and const int& in C++?

Question

In c++ i can write const int const* for a constant pointer to an constant int. I can also write const int& for a const reference. So is const int const& a reference to a const int?

Thanks in advance!

In c++ i can write const int const* for a constant pointer to an int. No, thats a duplicate const for the int and none for the pointer. — tkausl
– tkausl, Commented Feb 27, 2019 at 16:39
Please separate your code from the text, it is not clear what types you try to create here — Slava
– Slava, Commented Feb 27, 2019 at 16:39
The difference is that you can write const int & but you can't write const int const &. — François Andrieux
– François Andrieux, Commented Feb 27, 2019 at 16:41

NathanOliver · Accepted Answer · 2019-02-27 16:51:38Z

9

const int const* is not a constant pointer to a constant int. To get that you need

const int * const.

In your case you apply both const to the int so you have a mutable pointer to a constant int.

The same thing happen to const int const&. To make a constant reference to a constant int you would need

const int & const

but that is illegal.

Do note that in both const int const* and const int const& it's not actually legal to apply const twice like that.

edited Feb 27, 2019 at 16:51

answered Feb 27, 2019 at 16:41

NathanOliver

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Hatted Rooster · Accepted Answer · 2019-02-27 16:41:56Z

5

So is const int const& a reference to a const int?

No, that's a compilation error.

Try compiling something like :

int z = 0;
const int const& a= z;

And you'll see the compiler will yell at you.

You can't duplicate const for the same type. Your premise that const int const* is a constant pointer to a constant int is wrong too.

answered Feb 27, 2019 at 16:41

Hatted Rooster

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4 Comments

François Andrieux Over a year ago

It seems that OP's compiler does not actually yell at them for that (though I'm pretty sure you're right, and it should).

xX_EASY HD LP MC AWP GOD_Xx Over a year ago

At Vs2017 and C++ 17 i didnt get an error :/ But ty

Hatted Rooster Over a year ago

Then instead of your compiler yelling at you, you should yell at your compiler.

François Andrieux Over a year ago

@xX_EASYHDLPMCAWPGOD_Xx If you disable language extensions with /Za you will get an error (example) though you should still be seeing a warning without it.

Naumann · Accepted Answer · 2019-02-27 18:13:11Z

5

It is always a good idea to read it backwards as suggested by the Clockwise/Spiral rule if you get stuck in understanding such declarations.

int const *    // pointer to const int
int * const    // const pointer to int
int const * const   // const pointer to const int
int * const * p3;   // pointer to const pointer to int
const int * const * p4;       // pointer to const pointer to const int

edited Feb 27, 2019 at 18:13

answered Feb 27, 2019 at 18:03

Naumann

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