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const parameters to template class member functions

 
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Kenneth Massey
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PostPosted: Mon Jul 19, 2004 9:08 pm    Post subject: const parameters to template class member functions Reply with quote



I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

Kenneth


#include <stdio.h>

template<class T>
class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
};

int main() {
int k;
myclass<int*> mc(&k); // T = int*

const int* p = &k;

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));

return 0;
}


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Daniel Krügler (ne Spange
Guest
PostPosted: Tue Jul 20, 2004 6:12 pm    Post subject: Re: const parameters to template class member functions Reply with quote



Good morning Kenneth Massey,

Kenneth Massey schrieb:

Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

Kenneth


#include <stdio.h

template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
};

Please note that any template member functions are not automatically

instantiated. You have to
distinguish the point of definition of the template (non-standard
nomenclature) from the actual point of
instantiation (aka POI, just before main in your example).

Quote:
int main() {
int k;
myclass
const int* p = &k;

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));

return 0;
}

Since myclass<int*> does contain an int* member, you are effectivly

trying to
perform a code similar to:

const int* p = &k;
int* x = p;

in your call of ms.check(). This obviously leads to a violation of
constness contraints.

Hope that helps,

Daniel Krügler



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Jozef Saniga
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote



Hi,

Kenneth Massey wrote:
Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.
The compiler does not ignore the const. The const is there just not

where you expect it to be.

Quote:
Kenneth


#include <stdio.h

template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
When myclass is instantiated for int* check becomes:

int check (int* const y) { return x == y; }
since 'const T y' says that y is of type T and y is const. So y is of
type int* and it is const. Note that y is const not the int y points to.
Try to compile this piece of code that has a very similar problem:

int main ()
{
typedef int* T;
const T y;
y = 1;
return 0;
}

Quote:
};

int main() {
int k;
myclass
const int* p = &k;

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));

return 0;
}

Jozef


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Tokyo Tomy
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

[email]eigenmaster (AT) yahoo (DOT) com[/email] (Kenneth Massey) wrote in message news:<ac2b2aa1.0407190619.72a68f9d (AT) posting (DOT) google.com>...
Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

Kenneth


#include <stdio.h

template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
};

int main() {
int k;
myclass
const int* p = &k;

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));

return 0;
}


My MSVC++6 also issured the same error just as above. The
interpretation of the error seems to be easy as follows.

1. "const T y" in the check argument is converted to "T const y", just
like, for example,"const int i" is equivalent to "int const i".

2. T = int*, so the check argument becomes "int* const y" (const
pointer to int).

After I replaced "const int* p = &k;" with "int* const p = &k;", the
code compiled with my MSVC++6.

However, I just wander you can agree with this behavior.

How can you do if you want the check argument to be "const int*" to
accept the original p in the above code.

Following is one solution, but T x becomes pointer to const int as an
(probably) undesirable side effect.

#include <stdio.h>

template<class T>
class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(T y) { return x==y; }
};

int main() {
int k;
myclass<const int*> mc(&k); // T = const int*

const int* p = &k; // same as in the orginal code
printf("%dn",mc.check(p));

return 0;
}


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Gareth Stockwell
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

[email]eigenmaster (AT) yahoo (DOT) com[/email] (Kenneth Massey) wrote in message news:<ac2b2aa1.0407190619.72a68f9d (AT) posting (DOT) google.com>...
Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

template<class T
class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}
int check(const T y) { return x==y; }
};


Kenneth,

I think that what is happening here is that, given the signature
int check(const T);

with T=int*, upon instantiation the compiler is generating
int check(T* const);
rather than (as you want)
int check(const T*);

I can think of two solutions:

1. If you know that T will always be some kind of pointer, then
re-write the template such that T is the value type, rather than the
pointer type (so here, T=int). Then you can write the signature of
check directly:
int check(const T*);

2. More generally, I think you can get around the problem with a bit
of template metaprogramming. Basically you need a structure which
converts T to const T and gets the asterix in the correct place...

template struct MakeConst
{ typedef const Type result; };

template<typename Type>
struct MakeConst<Type*>
{ typedef const Type* result; };

Now you can write:

int check(typename MakeConst<T>::result);

Gareth


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Jeffrey Lau
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

The problem is the placement of the const in the declaration of 'p':
Quote:
int k;
myclass<int*> mc(&k); // T = int*

const int* p = &k;

If 'T' is 'int*', then 'const T' is 'int* const'; the constant version
of 'int*'. This is perhaps why most template code I have seen will use
'T const' instead of 'const T'. I have also adopted this declaration
style for all const variables in code:

int const constantInt(5);
char const* const constantPointerToConstantChar("Blah");

This allows you to read from right to left exactly the type of the
variable.


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vze2hb3b@verizon.net
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

In comp.lang.c++.moderated Kenneth Massey <eigenmaster (AT) yahoo (DOT) com> wrote:
Quote:

I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

Kenneth


#include <stdio.h

template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
};

int main() {
int k;
myclass
const int* p = &k;

Hi! The problem is that T is int *, so const T translates
to int * const (constant pointer), not to const int *
(pointer to constant integer). If you define p as

int * const p = &k;

then the compiler should be happy.

Best,

Alexander

Quote:

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));

return 0;
}

--
Aleksandr Morgulis
[email]aleksandr.morgulis (AT) verizon (DOT) net[/email]


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Ulrich Eckhardt
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

Kenneth Massey wrote:
Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

#include
template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }
^^^ *ouch*

Don't do that in examples, your real code might have valid reasons to do
so, but not examples.

Quote:
// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }

Yes, but that's something different: assuming T='int*', you have one 'int
const*' and one 'int * const'. If that makes no sense to you, just read
the FAQ, it's explained there.

Uli

--
Questions ?
see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first !


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Alberto Barbati
Guest
PostPosted: Wed Jul 21, 2004 3:26 pm    Post subject: Re: const parameters to template class member functions Reply with quote

Kenneth Massey wrote:
Quote:
I have run into a peculiar problem, in which the following sample code
does not compile (gcc 3.3). I have a template class with a member
function that should take a const version of the template parameter.
However, when I try to use the class with (T=int*), the compiler seems
to ignore the const and gives an error if I pass a const int*. Any
help would be much appreciated.

If you write

typedef int* T;

Then "const T" is "int* const" (a constant pointer to an int) which is
very different from "const int*" (a pointer to a constant int).

If you analyse your code you will realize that it is a more complex case
of this simple issue.

Alberto


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Christoph Schulz
Guest
PostPosted: Wed Jul 21, 2004 7:59 pm    Post subject: Re: const parameters to template class member functions Reply with quote

Hello!

Kenneth Massey wrote:
Quote:
I have run into a peculiar problem, in which the following sample
code does not compile (gcc 3.3). I have a template class with a
member function that should take a const version of the template
parameter. However, when I try to use the class with (T=int*), the
compiler seems to ignore the const and gives an error if I pass a
const int*. Any help would be much appreciated.

Kenneth

#include <stdio.h

template class myclass {
T x;
public:
myclass(T x0)Mad(x0) {}

int check(const T y) { return x==y; }

// compiles fine with this line: T is explicitly replaced by int*
// int check(const int* y) { return x==y; }
};

The problem is that "const T" is not "const int*" if the template is
instantiated with T=int*, but "int *const". That's similar to:
typedef int *T;
typedef const T CT;
Again, CT == "int *const" and *not* "const int*".

Quote:

int main() {
int k;
myclass
const int* p = &k;

// test.cpp: In function `int main()':
// test.cpp:25: error: invalid conversion from `const int*' to
`int*'
// test.cpp:25: error: initializing argument 1 of `int
myclass<T>::check(T)

printf("%dn",mc.check(p));


Here the compiler tries to use myclass<int*>::check(int *const) and
fails to perform the needed and disallowed conversion "const int*" ==>
"int *const". Of course, if you define a suitable check(const int *),
the compiler error goes away. But again, this has nothing to do with
ignoring const.

Regards,
Christoph Schulz


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Tokyo Tomy
Guest
PostPosted: Thu Jul 22, 2004 10:59 pm    Post subject: Re: const parameters to template class member functions Reply with quote

[email]hosoda (AT) jtec (DOT) or.jp[/email] (Tokyo Tomy) wrote in message news:<49c1da0b.0407200131.998a680 (AT) posting (DOT) google.com>...
Quote:
...
How can you do if you want the check argument to be "const int*" to
accept the original p in the above code.

Following is one solution, but T x becomes pointer to const int as an
(probably) undesirable side effect.

...

(continued from my previous post)
Another and better solution is shown below. The lesson herein may be:
Be careful for the combination of pointer and const. When I came
across the solution, I began to think that the behavior ("const T y"
is converted to "T const y") easily brings errors, but is acceptable.

#include <stdio.h>

template<class T>
class myclass {
T* x;
public:
myclass(T* const x0)Mad(x0) {}

//int check(T y) { return x==y; }
int check(const T* y) { return x==y; }
};

int main() {
int k;
myclass<int> mc(&k); // T = int

const int* p = &k; // same as in the orginal code

printf("%dn",mc.check(p));

return 0;
}


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