CODEWITHSUNDEEP
c++constantsc++20
yapkm01
yapkm01

Reputation: 3775

C++ : Is class type const always a top-level const?

Here are the following code:

#include <iostream>
using namespace std;

class A {
public:
   int *x;
};

int main() {
   int b=100;
   A a;
   a.x = &b;
   const A &m = a; // clause 1 - Is this top-level const? 
   int *r = m.x; // *r has no const yet allowed. Is it due to reference m being top level const?
}

Any help appreciated.
Thanks

Upvotes: 0

Views: 141

Answers (1)

bolov
bolov

Reputation: 75697

int const ca = 24;
int a = ca;

You would expect this to compile right? And indeed it does. Here I initialized a integer with the value 24.

You code is the same situation, except that instead of integer you have pointer to integer:

m is const so m.x is const. The x is const, i.e. x cannot be modified (via m). In stricter terms m.x is of type int * const i.e. constant pointer to integer.

int *r = m.x

Here you just initialize the pointer r with the value of the pointer m.x. The fact that m.x is const is not an issue. Both types without their top level cv are identical: pointer to (mutable) integer.

const A &m = a; // clause 1 - Is this top-level const?

Yes Talking about top level const on references is strange indeed. A reference cannot be rebound. However if we are talking about the referenced type, i.e. const A then yes, it is top level. But then again talking about top level const is useful only on pointer types. E.g. int *const cp vs const int *p.

Upvotes: 2

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