why is this method hidden?

santana · 11-07-2008, 02:32 PM

Any one explain why the following class hierarchy overrides the method with const argument? Shouldn't it be an overload, allowing both methods to exist side by side?

#include <iostream>
using namespace std;

class B
{
public:
virtual void SetBlah(const double d[3])
{ cerr << "B::SetBlah()" << endl; }
};

class D : public B
{
public:
void SetBlah(double d[3])
{ cerr << "D::SetBlah()" << endl; }
};

int main()
{
double p[3]={0.0};
const double q[3]={1.0};

D d;
d.SetBlah(p);
d.SetBlah(q); // exists in the base class, why hidden here?
d.B::SetBlah(q);

return 0;
}

Sergei Steshenko · 11-07-2008, 02:43 PM

Quote:

Originally Posted by santana

Any one explain why the following class hierarchy overrides the method with const argument? Shouldn't it be an overload, allowing both methods to exist side by side?

#include <iostream>
using namespace std;

class B
{
public:
virtual void SetBlah(const double d[3])
{ cerr << "B::SetBlah()" << endl; }
};

class D : public B
{
public:
void SetBlah(double d[3])
{ cerr << "D::SetBlah()" << endl; }
};

int main()
{
double p[3]={0.0};
const double q[3]={1.0};

D d;
d.SetBlah(p);
d.SetBlah(q); // exists in the base class, why hidden here?
d.B::SetBlah(q);

return 0;
}

I am not a C++ guy, but I think you still have access to base class methods through 'upper'.

dmail · 11-07-2008, 03:00 PM

For questions like this you really should do you own leg work.

Quote:

13.2 Declaration matching [over.dcl]
1 Two function declarations of the same name refer to the same function if they are in the same scope and
have equivalent parameter declarations (13.1). A function member of a derived class is not in the same
scope as a function member of the same name in a base class. [Example:
class B {
public:
int f(int);
};
class D : public B {
public:
int f(char*);
};
Here D::f(char*) hides B::f(int) rather than overloading it.
void h(D* pd)
{
pd->f(1); //error:
// D::f(char*) hides B::f(int)
pd->B::f(1); //OK
pd->f("Ben"); //OK, calls D::f
}
—end example]

To fix your example add "using B::SetBlah" in D to bring it into scope.

santana · 11-07-2008, 03:33 PM

Quote:

Originally Posted by dmail

For questions like this you really should do you own leg work.

So what are you quoting? If I had that reference I could do my own leg work

dmail · 11-07-2008, 03:52 PM

I quoted from the C++ standard which is not free, yet there are draft versions available for free such as this

santana · 11-07-2008, 04:05 PM

You're right I really should be more familiar with the standard. Thanks for the link.

sharky · 11-07-2008, 04:31 PM

Quote:

Originally Posted by dmail

I quoted from the C++ standard which is not free, yet there are draft versions available for free such as this

How much leg work did it take to pinpoint the answer in this particular case?

dmail · 11-07-2008, 05:41 PM

Quote:

Originally Posted by sharky

How much leg work did it take to pinpoint the answer in this particular case?

For me none, I knew where the explanation was in the standard. The problem I have generally is that some people ask first and if they get no answer look later (this is not totally directed at the OP seeing as he didn't have a copy of the standard), where as I and many other hackers are more inclined to look first and ask if you can not find a solution to a question. For example have a look at my profile and see how many questions I have asked. If the OP had the standard then a quick search in the pdf would reveal the answer with an example which is so close to the OP's it is uncanny.