Re: discards qualifiers

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Wed, 17 Nov 2010 14:47:28 -0500

Message-ID:

<ic1bgh$83c$1@news.eternal-september.org>

On 11/17/2010 2:22 PM, Chameleon wrote:

#include <vector>

using namespace std;

struct Vector
{
double x,y,z;
double &operator[](int i) { return (&x)[i]; }
};

void here_it_comes(const vector<Vector> &vertex)

'vertex' refers to a constant vector. That means that the operator[] of
it returns a reference to a *constant* Vector. Your Vector::operator[]
is non-const (since it is apparently designed to allow putting it on the
left side of the assignment operator). Declare another operator[] in
Vector, like so:

    struct Vector
    {
       ...
       double operator[](int i) const { return (&x)[i]; }

and it's going to be OK.

{
double a = vertex[0][0]; // compile error
double b = const_cast<Vector&>(vertex[0])[0]; // no problem, but why the
need not to be const? I change it nowhere.
}

int main() { return 0; };
/*
1.cpp: In function 'void here_it_comes(const std::vector<Vector,
std::allocator<Vector> >&)':
1.cpp:14: error: passing 'const Vector' as 'this' argument of 'double&
Vector::operator[](int)' discards qualifiers
*/

V
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