Passing an array of pointers as a const

Question

What is the difference between a an array being passed as a constant versus the array values being constants?

When passing an array of pointers to a function when every value is a constant:

`void display(Fraction* const ar[], int size);`

everything works fine but when the array is a constant

`void display(const  Fraction* ar[], int size);`

the compiler gives the following error when calling the function:

`error C2664: 'display' : cannot convert parameter 1 from 'Fraction *[3]' to 'const Fraction *[]'`

main:

int main()
{
    Fraction* fArray[3];
    Fraction* fOne = new Fraction();
    Fraction* fTwo = new Fraction();
    Fraction* fThree = new Fraction();
    fOne->num = 8;
    fOne->den = 9;
    fTwo->num = 3;
    fTwo->den = 2;
    fThree->num = 1;
    fThree->den = 3;
    fArray[0] = fOne;
    fArray[1] = fTwo;
    fArray[2] = fThree;
    display(fArray, 3);

    system("pause");
    return 0;
}

I assume you do not intend to modify ar inside display, so you could just add const everywhere, i.e. Fraction const * const ar[] which is then a legal conversion. — Henri Menke
– Henri Menke, Commented Jul 18, 2018 at 6:02

Cheers and hth. - Alf · Accepted Answer · 2018-07-18 05:54:21Z

3

This is a FAQ.

Note that const T* a[] means T const* a[], i.e. it's not the array itself that you have declared const; instead you have declared an array of pointers to const items.

Essentially, if the language provided an implicit conversion T** → T const**, then you could inadvertently attempt to change something that was originally declared const:

int const     v = 666;
int*          p;
int**         pp = &p;
int const**   ppHack = pp;    //! Happily not allowed!

*ppHack = &v;    // Now p points to the const v...

edited Jul 18, 2018 at 5:54

answered Nov 21, 2012 at 18:55

Cheers and hth. - Alf

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2 Comments

Dávid Tóth Over a year ago

May I ask for an update on the link, please? I am googling in paralell, but this would help.

Cheers and hth. - Alf Over a year ago

@DavidTóth: Done. The FAQ is now at isocpp.org.

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