discards qualifiers

[Chameleon]

#include <vector>

using namespace std;

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


void here_it_comes(const vector<Vector> &vertex)
{
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:perator[](int)' discards qualifiers
*/

 

[Victor Bazarov]

#include <vector>

using namespace std;

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


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:perator[]
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); }

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:perator[](int)' discards qualifiers
*/

V

 

[James Kanze]

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); }
};
void here_it_comes(const vector<Vector> &vertex)

[...]

Declare another operator[] in Vector,
like so:

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

and it's going to be OK.

But only if i is 0 (as it is in his test program). Otherwise,
he has undefined behavior.