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Matt Taylor
Guest
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 Posted: Wed Sep 22, 2004 11:24 pm Post subject: Problems with void Template Type Parameters and Zero-Length |
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I am trying to use templates to create an optimal structure for a fixed-size
buffer that holds a homogenous array of elements. For brevity, this problem
is somewhat simplified from my implementation:
template<typename T1, size_t len>
struct buffer_t
{
// Array of elements
T1 ary[len / sizeof(T1)];
// Align upward to a multiple of sizeof(T1)
char pad[len - ((len / sizeof(T1)) * sizeof(T1))];
};
The problem is that, when len is a multiple of sizeof(T1), the pad variable
has length 0. For some reason the compiler finds this to be absurd. I need
to find some way to make this compile in that case. Is there any simple
resolution to this?
I am also wondering about the following:
template<typename T>
struct A
{
int a;
T b;
};
struct INGET
{
};
Why is A<INGET> legal but not A<void>? Semantically they would be the same,
right?
-Matt
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Rob Williscroft
Guest
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| Posted: Wed Sep 22, 2004 11:47 pm Post subject: Re: Problems with void Template Type Parameters and Zero-Len |
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Matt Taylor wrote in news:nQn4d.45958$uN5.12860 (AT) tornado (DOT) tampabay.rr.com
in comp.lang.c++:
| Quote: |
The problem is that, when len is a multiple of sizeof(T1), the pad
variable has length 0. For some reason the compiler finds this to be
absurd. I need to find some way to make this compile in that case. Is
there any simple resolution to this?
|
#include <iostream>
#include <cstddef>
using std::size_t;
template <typename T1, size_t len>
struct padding
{
static size_t const value = len - ((len / sizeof(T1)) * sizeof(T1));
};
template < typename T, size_t Count, size_t Pad >
struct base_buffer_t
{
T ary[ Count ];
char pad[ Pad ];
};
template < typename T, size_t Count >
struct base_buffer_t< T, Count, 0U >
{
T ary[ Count ];
typedef void pad; /* for using */
};
template <typename T1, size_t len>
struct buffer_t :
private base_buffer_t<
T1, len / sizeof(T1), len - ((len / sizeof(T1)) * sizeof(T1))
private:
typedef base_buffer_t<
T1, len / sizeof(T1), len - ((len / sizeof(T1)) * sizeof(T1))
;
public:
using base_t::ary;
using base_t::pad;
};
int main()
{
buffer_t< int, sizeof( int ) * 2 > int2;
int2.ary[1] = 2;
buffer_t< int, (sizeof( int ) * 1) + 1 > int1p1;
std::cout << sizeof( int2 ) << 'n';
std::cout << sizeof( int1p1 ) << 'n';
}
| Quote: | I am also wondering about the following:
template
struct A
{
int a;
|
This next line can't compile if the substitution T = void is done.
| Quote: | T b;
};
struct INGET
{
};
Why is A
same, right?
|
Its illegal because:
void b;
is illegal, i.e. you cannot declare objects of type void.
HTH.
Rob.
--
http://www.victim-prime.dsl.pipex.com/
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Matt Taylor
Guest
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| Posted: Fri Sep 24, 2004 9:23 pm Post subject: Re: Problems with void Template Type Parameters and Zero-Len |
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"Rob Williscroft" <rtw (AT) freenet (DOT) co.uk> wrote
| Quote: | Matt Taylor wrote in news:nQn4d.45958$uN5.12860 (AT) tornado (DOT) tampabay.rr.com
in comp.lang.c++:
[...]
Its illegal because:
void b;
is illegal, i.e. you cannot declare objects of type void.
|
Yeah, I know. I was mostly wondering the logic behind why one can create and
instantiate a structure with length 0 but not an object of type void since
they seem to me to be the same idea. This is very strange because you can
create a typedef which is just an alias for void, but you can't use void
itself.
Anyhow, thank you for your reply -- it has helped greatly.
-Matt
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Ron Natalie
Guest
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| Posted: Fri Sep 24, 2004 9:32 pm Post subject: Re: Problems with void Template Type Parameters and Zero-Len |
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"Matt Taylor" <com.sisecure@mtaylor> wrote
| Quote: |
Yeah, I know. I was mostly wondering the logic behind why one can create and
instantiate a structure with length 0 but not an object of type void since
they seem to me to be the same idea.
|
You can't create a structure of size 0. You can create a structure with no data
members in it, but it will still have size greater than zero.
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Old Wolf
Guest
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| Posted: Sun Sep 26, 2004 8:54 pm Post subject: Re: Problems with void Template Type Parameters and Zero-Len |
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"Matt Taylor" <com.sisecure@mtaylor> wrote:
| Quote: | I am trying to use templates to create an optimal structure for a fixed-size
buffer that holds a homogenous array of elements. For brevity, this problem
is somewhat simplified from my implementation:
template<typename T1, size_t len
struct buffer_t
{
// Array of elements
T1 ary[len / sizeof(T1)];
// Align upward to a multiple of sizeof(T1)
char pad[len - ((len / sizeof(T1)) * sizeof(T1))];
};
The problem is that, when len is a multiple of sizeof(T1), the
pad variable has length 0.
|
Do you need to be able to reference 'pad' ? If not then you
could try:
template
union buffer_t
{
T1 ary[len / sizeof(T1)];
char pad[len];
};
(If you need other data members in buffer_t then you would
have to make it a struct, and have a member union).
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Matt Taylor
Guest
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| Posted: Tue Sep 28, 2004 9:12 pm Post subject: Re: Problems with void Template Type Parameters and Zero-Len |
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"Old Wolf" <oldwolf (AT) inspire (DOT) net.nz> wrote
| Quote: | "Matt Taylor" <com.sisecure@mtaylor> wrote:
[...]
The problem is that, when len is a multiple of sizeof(T1), the
pad variable has length 0.
Do you need to be able to reference 'pad' ? If not then you
could try:
template
union buffer_t
{
T1 ary[len / sizeof(T1)];
char pad[len];
};
|
Ah, I had not thought of this. This is a very simple and intuitive way to
make it work.
My thanks and appreciation to both posters.
-Matt
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