pointer to member function (gcc)

[Newsgroup - Ann]

I have the following code:

class nn_base {
public:
double phi(double v)
{
return v;
};
double ComputeY(int i, int j, double (nn_base::*activeFunc)(double))
{
return (this->*activeFunc)(1.0);
}
};

main()
{
nn_base nn1;
double x = nn1.ComputeY(1, 2, nn_base:hi);
return 0;
}

It can be compiled by Visual C++ both 6.0 and .NET but cannot by gcc 3.22,
which complains:


test7.cpp: In function `int main()':
test7.cpp:18: no matching function for call to `nn_base::ComputeY(int, int,
<unknown type>)'
test7.cpp:10: candidates are: double nn_base::ComputeY(int, int, double
(nn_base::*)(double))

what's wrong with that?

 

[Gianni Mariani]

I have the following code:

class nn_base {
public:
double phi(double v)
{
return v;
};
double ComputeY(int i, int j, double (nn_base::*activeFunc)(double))
{
return (this->*activeFunc)(1.0);
}
};

main()

should be

int main()

{
nn_base nn1;
double x = nn1.ComputeY(1, 2, nn_base:hi);

should be:

double x = nn1.ComputeY(1, 2, & nn_base:hi);

actually, I'm not sure about needing the "&" but it seems like it makes
more sense if it is needed.

VC++ likes the "&".

 

[Noah Roberts]

should be

int main()

In C any function that is declaired without a return type is defaulted
to return int. Therefore "main() {...}" is acceptable (though commonly
thought of as bad form) in C. Is this different in C++?

NR

 

[Jack Klein]

In C any function that is declaired without a return type is defaulted
to return int. Therefore "main() {...}" is acceptable (though commonly
thought of as bad form) in C. Is this different in C++?

NR

Implicit int is illegal in C++, and it has been illegal in C since the
1999 major update to the C language standard.

--
Jack Klein
Home: http://JK-Technology.Com
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[White Wolf]

In C any function that is declaired without a return type is defaulted
to return int. Therefore "main() {...}" is acceptable (though
commonly thought of as bad form) in C. Is this different in C++?

Yes. And as far as I know in C as well. C (today) is C99 and C99 does not
allow implicit int anymore. So main withhout a return type is illegal in
both C and C++.

 

[Kevin Goodsell]

should be:

double x = nn1.ComputeY(1, 2, & nn_base:hi);

actually, I'm not sure about needing the "&" but it seems like it makes
more sense if it is needed.

I believe it is needed in the case of a member function, but not in the
case of a non-member function.

-Kevin

 

[Newsgroup - Ann]

I believe it is needed in the case of a member function, but not in the
case of a non-member function.

-Kevin

Good, thank you all. Could anybody tell me why is the difference? Thanks.

-Ann

 

[Gianni Mariani]

In C any function that is declaired without a return type is defaulted
to return int. Therefore "main() {...}" is acceptable (though commonly
thought of as bad form) in C. Is this different in C++?

This is different in C++.

Only constructors and destructors may be declared without return types.

void is a valid return type indicating that nothing may be returned.