Pointer to array with const qualifier in C & C++

Question

Consider following program:

int main()
{
    int array[9];
    const int (*p2)[9] = &array;
}

It compiles fine in C++ (See live demo here) but fails in compilation in C. By default GCC gives following warnings. (See live demo here).

prog.c: In function 'main':
prog.c:4:26: warning: initialization from incompatible pointer type [enabled by default]
     const int (*p2)[9] = &array;

But If I use -pedantic-errors option:

gcc -Os -s -Wall -std=c11 -pedantic-errors -o constptr constptr.c

it gives me following compiler error

constptr.c:4:26: error: pointers to arrays with different qualifiers are incompatible in ISO C [-Wpedantic]

Why it fails in compilation in C but not in C++? What C & C++ standard says about this?

If I use const qualifier in array declaration statement it compiles fine in C also. So, what is happening here in above program?

While I understand this is a good question, but simply asking What C & C++ standard says about this? is not good. Did you try to look into the standards? Which part you did not understand? Did you find any differences? Where is your research effort? Hope I'm understood. :) — Sourav Ghosh
– Sourav Ghosh, Commented Dec 28, 2015 at 6:21
For C see stackoverflow.com/questions/50164803 and stackoverflow.com/questions/28062095 — M.M
– M.M, Commented Nov 21, 2019 at 2:01

haccks · Accepted Answer · 2015-12-29 13:43:29Z

33

GCC-gnu

In GNU C, pointers to arrays with qualifiers work similar to pointers to other qualified types. For example, a value of type int (*)[5] can be used to initialize a variable of type const int (*)[5]. These types are incompatible in ISO C because the const qualifier is formally attached to the element type of the array and not the array itself.

C standard says that (section: §6.7.3/9):

If the specification of an array type includes any type qualifiers, the element type is so- qualified, not the array type.[...]

Now look at the C++ standard (section § 3.9.3/5):

[...] Cv-qualifiers applied to an array type attach to the underlying element type, so the notation “cv T,” where T is an array type, refers to an array whose elements are so-qualified. An array type whose elements are cv-qualified is also considered to have the same cv-qualifications as its elements. [ Example:
 typedef char CA[5];
 typedef const char CC;
 CC arr1[5] = { 0 };
 const CA arr2 = { 0 };
The type of both arr1 and arr2 is “array of 5 const char,” and the array type is considered to be const- qualified. —endexample]

Therefore, the initialization

const int (*p2)[9] = &array;

is assignment of type pointer to array[9] of int to pointer to array[9] of const int. This is not similar to assigning int * to a const int * where const is applied directly to the object type the pointer points to. This is not the case with const int(*)[9] where, in C, const is applied to the elements of the array object instead of the object the pointer points to. This makes the above initialization incompatible.

This rule is changed in C++. As const is applied to array object itself, the assignment is between same types pointer to const array[9] of int instead of type pointer to array[9] of int and pointer to array[9] of const int.

edited Dec 29, 2015 at 13:43

answered Dec 28, 2015 at 6:38

haccks

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17 Comments

Tom Karzes Over a year ago

I'd call this a bug in ISO C. It makes no sense. You should always be able to add const to a pointer target, at any level.

haccks Over a year ago

@TomKarzes; You should always be able to add const to a pointer target, at any level: No. Not in C.

M.M Over a year ago

@TomKarzes no, you shouldn't, e.g. the well-known incompatibility between int ** and const int ** (which are also incompatible in C++). If that implicit conversion existed , it'd be possible to write to a const object without using a cast.

M.M Over a year ago

@TomKarzes the harm is that you could write code that modifies a const object (causing undefined behaviour) and not get any compiler warnings or errors. See here for full description

Lightness Races in Orbit Over a year ago

@TomKarzes: Cars were very usable long before airbags even existed. Shall we remove them?

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