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Templates, copy ctor and type-conversion ctor

P: n/a
NVH
I know that this question may have been asked before but I can't
find it here so:
If there is a class:
class Foo
{
...
Foo (const Foo &num); // Copy constructor

template <typename T>
Foo (const T &num); // Conversion constructor
...
}

Then what's wrong with it and how can I fix it so that it may act
like this?

Thank you for any response to this.

Jun 24 '06 #1
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8 Replies


P: n/a
NVH wrote:
I know that this question may have been asked before but I can't
find it here so:
If there is a class:
class Foo
{
...
Foo (const Foo &num); // Copy constructor

template <typename T>
Foo (const T &num); // Conversion constructor
...
}

Then what's wrong with it and how can I fix it so that it may act
like this?
What exactly are you trying (but failing) to do?
Thank you for any response to this.


Oh, you're welcome.

Cheers! --M

Jun 25 '06 #2

P: n/a
"NVH" <da***********@gmail.com> wrote in message
news:11********************@r2g2000cwb.googlegroup s.com...
I know that this question may have been asked before but I can't
find it here so:
If there is a class:
class Foo
{
...
Foo (const Foo &num); // Copy constructor

template <typename T>
Foo (const T &num); // Conversion constructor
...
}

Then what's wrong with it and how can I fix it so that it may act
like this?
I don't know, what's wrong with it? What's the problem? Does it not
compile? Does it not work the way you expect it?

And act like what? You gotta tell us what the problem is.
Thank you for any response to this.

Jun 25 '06 #3

P: n/a
NVH
Yeah, sorry about that.
Here's a more clear one:

===================================
#include <iostream>
using namespace std;

class Foo
{
public:

Foo (const Foo &num)
{
number = num.number;
cout << 1 << endl;
} // Copy constructor

template <typename T>
Foo (const T &num)
{
number = num;
cout << 2 << endl;
} // Conversion constructor

void print (void)
{
cout << number << endl;
}
private:
auto int number; // Complier points to here for 1st
error
};

void main (void)
{
auto Foo G(5);
auto Foo H(G); // Compiler points to here for 2nd error

G.print(); // Display the content
H.print(); // Display the content
}
========================================

The compiler keeps on saying:
error C2071: 'number' : illegal storage class
error C2668: 'Foo::Foo' : ambiguous call to overloaded
function

Agian, thanks for any help is apreciated.

Jun 28 '06 #4

P: n/a

NVH wrote:
Yeah, sorry about that.
Here's a more clear one:

===================================
#include <iostream>
using namespace std;

class Foo
{
public:

Foo (const Foo &num)
{
number = num.number;
cout << 1 << endl;
} // Copy constructor

template <typename T>
Foo (const T &num)
{
number = num;
cout << 2 << endl;
} // Conversion constructor

void print (void)
{
cout << number << endl;
}
private:
auto int number; // Complier points to here for 1st
error
};

void main (void)
{
auto Foo G(5);
auto Foo H(G); // Compiler points to here for 2nd error

G.print(); // Display the content
H.print(); // Display the content
}
========================================

The compiler keeps on saying:
error C2071: 'number' : illegal storage class
error C2668: 'Foo::Foo' : ambiguous call to overloaded
function

Agian, thanks for any help is apreciated.


first error: you can't use auto here, in a struct/class - only in a
function, but you shouldn't even bother using it there either.
2nd: I suspect it is confused between the 2 constructors - either would
work. Although I would think it should choose the non-templated
version. What compiler are you using?

-Tony

Jun 28 '06 #5

P: n/a
NVH wrote:
Yeah, sorry about that.
Here's a more clear one:

===================================
#include <iostream>
using namespace std;

class Foo
{
public:

Foo (const Foo &num)
{
number = num.number;
cout << 1 << endl;
} // Copy constructor

template <typename T>
Foo (const T &num)
{
number = num;
cout << 2 << endl;
} // Conversion constructor

void print (void)
{
cout << number << endl;
}
private:
auto int number; // Complier points to here for 1st
error
Drop the auto. It's illegal here since you're not actually allocating
anything but rather just declaring a class that will be allocated
(either automatically or dynamically) later.
};

void main (void)
{
auto Foo G(5);
auto Foo H(G); // Compiler points to here for 2nd error
Again, drop the auto. It's legal here, but it's the default, and no one
uses it since it only adds clutter and verbosity.
G.print(); // Display the content
H.print(); // Display the content
}
========================================

The compiler keeps on saying:
error C2071: 'number' : illegal storage class
error C2668: 'Foo::Foo' : ambiguous call to overloaded
function


The problem is not with the code. It is legal, and various relatively
conformant compilers accept it fine (once the auto is deleted above,
that is). Thus, I will venture to guess that you are using VC++ 6,
which is non-conformant when it comes to templates. If that is the
case, you'll want to upgrade or search for a work-around that is
suitable for your situation. One might be to get rid of the copy
constructor Foo::Foo(const Foo&). For others, you'll want to ask in
Microsoft newsgroup since this is a compiler-specific issue. Several
such groups can be found here:

http://www.parashift.com/c++-faq-lit...t.html#faq-5.9

Cheers! --M

Jun 28 '06 #6

P: n/a
NVH
Sorry it takes so long for me to respond. I'm using Microsoft
Visual C++. I got rid of all the "auto" in the code (even the "auto"
next to "int") but the 2nd error still exist.
Thanks for responding to my problem.

Jul 6 '06 #7

P: n/a
NVH
:) I just got rid of the copy constructor and it work so Thanks
a lot to both you and Tony. But quick question: If I wanted to have
the copy constructor to do something else rather then the template,
what would I have to do?
For example I'd like something like this:
class Foo
{
public:

Foo (const Foo &num)
{
number = num;
cout << 3 << endl;
} // Copy constructor

template <typename T>
Foo (const T &num)
{
number = num;
cout << 2 << endl;
} // Conversion constructor

void print (void)
{
cout << number << endl;
}
private:
int number;
};

void main (void)
{
Foo G(5);
Foo H(G);

G.print(); // Display the content
H.print(); // Display the content
}

I'd like "2" to be printed out if the inputted type ("T") is not of
type Foo and "3" to be printed out if the type ("T") is of type Foo.

Thanks agian for your help.

Jul 6 '06 #8

P: n/a
NVH wrote:
:) I just got rid of the copy constructor and it work so Thanks
a lot to both you and Tony. But quick question: If I wanted to have
the copy constructor to do something else rather then the template,
what would I have to do?
For example I'd like something like this:
class Foo
{
public:

Foo (const Foo &num)
{
number = num;
cout << 3 << endl;
} // Copy constructor

template <typename T>
Foo (const T &num)
{
number = num;
cout << 2 << endl;
} // Conversion constructor

void print (void)
{
cout << number << endl;
}
private:
int number;
};

void main (void)
{
Foo G(5);
Foo H(G);

G.print(); // Display the content
H.print(); // Display the content
}

I'd like "2" to be printed out if the inputted type ("T") is not of
type Foo and "3" to be printed out if the type ("T") is of type Foo.

Thanks agian for your help.
The easiest way is to upgrade to a better compiler that won't barf on
valid code. You can download a free one from Microsoft.

Cheers! --M

Jul 6 '06 #9

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