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, Feb 27 '19 at 16:39
Please separate your code from the text, it is not clear what types you try to create here — Slava, Feb 27 '19 at 16:39
The difference is that you can write `const int &` but you can't write `const int const &`. — François Andrieux, Feb 27 '19 at 16:41
@xX_EASYHDLPMCAWPGOD_Xx Most compilers will give you an error. Visual Studio gives you a warning, though it's known to allow a lot of things it shouldn't. What compiler are you using? — François Andrieux, Feb 27 '19 at 16:42
Possible duplicate of [Easy rule to read complicated const declarations?](https://stackoverflow.com/questions/7526152/easy-rule-to-read-complicated-const-declarations) — adrtam, Feb 27 '19 at 16:44
@xX_EASYHDLPMCAWPGOD_Xx Keep in mind that it's not because a compiler doesn't complain that code is valid or correct. The compiler is *not* required to identify every error in your code. — François Andrieux, Feb 27 '19 at 16:44

NathanOliver · Answer 1 · 2019-02-27T16:51:38.473

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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 '19 at 16:51

answered Feb 27 '19 at 16:41

NathanOliver

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ty too for ur answer! – xX_EASY HD LP MC AWP GOD_Xx Feb 27 '19 at 16:54

score 5 · Accepted Answer · answered Feb 27 '19 at 16:41

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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 '19 at 16:41

Hatted Rooster

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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). – François Andrieux Feb 27 '19 at 16:45
At Vs2017 and C++ 17 i didnt get an error :/ But ty – xX_EASY HD LP MC AWP GOD_Xx Feb 27 '19 at 16:46
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Then instead of your compiler yelling at you, you should yell at your compiler. – Hatted Rooster Feb 27 '19 at 16:46
@xX_EASYHDLPMCAWPGOD_Xx If you disable language extensions with `/Za` you will get an error ([example](https://godbolt.org/z/Q6iFr7)) though you should still be seeing a warning without it. – François Andrieux Feb 27 '19 at 16:48
thats a useful tip ty! – xX_EASY HD LP MC AWP GOD_Xx Feb 27 '19 at 16:52

Naumann · Answer 3 · 2019-02-27T18:13:11.363

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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 '19 at 18:13

answered Feb 27 '19 at 18:03

Naumann

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

3 Answers3