CODEWITHSUNDEEP
c++reference
Alex24
Alex24

Reputation: 610

Why I can declare a const reference using type-alias?

I have a simple question: As I know I can declare a const pointer to some datatype or a pointer to constant datatype but I can only declare a reference to a constant datatype only and no constant reference to a datatype; the fact that a reference is already constant because it cannot be rebound to another object.

So when I try to create a const ref to someDataType I get compile-time error. But the thing that matters me is when used with type alias using typedef or using. e.g:

#include <iostream>

int main() {

    int i{ 10 };
    //  int& const r1{ i }; // error: ‘const’ qualifiers cannot be applied to ‘int&’. Ok here.
    using rInt = int&; // or typedef int& rInt;
    const rInt r2{ i }; // why const is allowed here?
    ++r2; // this proves that the const is applied to the reference not to the object referred to.

    std::cout << r2 << std::endl; // 11

}

As you can see above I can add const to the reference which I think is Redundant in that context. But why C++ allows this with type-aliases but not directly?

Upvotes: 9

Views: 1506

Answers (2)

NathanOliver
NathanOliver

Reputation: 180845

This is a case where common sense comes into play. Since references cannot be reassigned they act as if they were const. Adding const to a reference declaration doesn't add anything and as such T & const is forbidder per [dcl.ref]/1

[...] Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name ([dcl.typedef], [temp.param]) or decltype-specifier ([dcl.type.simple]), in which case the cv-qualifiers are ignored.

You'll notice though that it is allowed then the reference is a typedef-name or decltype-specifier. So if T is T& then the const is ignored. If it wasn't it would make generic programming harder.

Upvotes: 5

eerorika
eerorika

Reputation: 238401

Because the standard say so:

[dcl.ref] ... Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name ([dcl.typedef], [temp.param]) or decltype-specifier ([dcl.type.simple]), in which case the cv-qualifiers are ignored

This is similar to how you cannot declare a reference reference, while it is possible through a typedef (where the references collapse into one):

int i;
int& iref = i;
//int& & irefref = iref; // not OK
using Iref = int&;
Iref& iretypedef = iref; // OK; collapses into int&

The CV-collapsing rules, just like reference collapsing rules are essential to make templates and type deductions usable.

Upvotes: 14

Related Questions