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Howard Gardner
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 Posted: Sat Jul 15, 2006 1:57 am Post subject: error: no instance of function template "test_c" matches the |
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/*
Sections a and b compile fine. There is a broken line in
section c (two broken versions of the broken line
are there, with diagnostics)
Other than the name changes, the the three sections differ
only in the default value of the size_t argument to the
temp_ struct.
The size_t argument to temp_a:
does not depend on the type argument
does use faux_ptr
The size_t argument to temp_b:
does depend on the type argument
does not use faux_ptr.
The size_t argument to temp_c:
depends ont the type argument
uses faux_ptr.
It's the form in section c that I'm trying to use,
naturally. I'm utterly baffled. If someone can see why this
doesn't work, please tell me!
*/
#include <ostream>
#include <cstddef>
// Promises to return a ptr to x
template< typename x >
x * faux_ptr();
// section a -----------------------------------------------
template< typename x, size_t = sizeof( faux_ptr< int >() ) >
struct temp_a{};
template< typename xSubject >
bool test_a( temp_a< xSubject > );
temp_a< int > instance_a;
size_t size_a = sizeof( test_a( instance_a ) );
// section b -----------------------------------------------
template< typename x, size_t = sizeof( x ) >
struct temp_b{};
template< typename xSubject >
bool test_b( temp_b< xSubject > );
temp_b< int > instance_b;
size_t size_b = sizeof( test_b( instance_b ) );
// section c -----------------------------------------------
template< typename x, size_t = sizeof( faux_ptr< x >() ) >
struct temp_c{};
template< typename xSubject >
bool test_c( temp_c< xSubject > );
temp_c< int > instance_c;
//size_t size_c = sizeof( test_c( instance_c ) );
/*
error: no instance of function template "test_c"
matches the argument list
argument types are: (temp_c<int, 4U>)
*/
//size_t size_c = sizeof( test_c< int >( instance_c ) );
/*
error: no instance of function template "test_c"
matches the argument list
argument types are: (temp_c<int, 4U>)
*/
int main()
{
using namespace std;
cout << typeid( instance_c ).name() << endl;
// outputs "temp_c<int, 4U>"
}
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Howard Gardner
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| Posted: Tue Jul 18, 2006 7:39 am Post subject: Re: error: no instance of function template "test_c" matches |
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Greg Herlihy wrote:
| Quote: |
template< typename x, size_t = sizeof( faux_ptr< x >() )
struct temp_c{};
Note that there are two parameterized type arguments for the temp_c
class template.
|
There aren't! x is a type parameter, but size_t is a non-type parameter.
I've got cleaner code to look at now. The forms of tpt that I've
commented out work fine. The one that I've left in doesn't.
I still don't know if it's legal code. In any event, it won't work on
Comeu, or gcc. It will work on VC++ 2005.
#include <ostream>
#include <cstddef>
template< typename x >
x * faux_ptr();
//template< typename x, size_t = sizeof( int * ) >
// struct tpt{};
//template< typename x, size_t = sizeof( faux_ptr< int >() ) >
// struct tpt{};
//struct has{ int memvar; };
//template< typename x, size_t = sizeof( faux_ptr< has >()->memvar ) >
// struct tpt{};
template< typename x, size_t = sizeof( faux_ptr< x >() ) >
struct tpt{};
template< typename x >
bool fnc( tpt< x > );
tpt< int > inst;
size_t size = sizeof( fnc( inst ) );
int main()
{
using namespace std;
cout << size << endl;
}
This code is the next step in the chain. It doesn't work right on Comeau
or VC++, noone has told me if it works on gcc.
I think it fails on Comeau for the same reason that the code above
fails. I don't know why it fails on VC++.
#include <ostream>
#include <cstddef>
struct has{ int memvar; };
template< typename x >
x * faux_ptr();
template< typename x, size_t = sizeof( faux_ptr< x >()->memvar ) >
struct tpt{};
typedef char no;
struct yes{ no val[2]; };
template< typename x >
no fnc( ... );
template< typename x >
yes fnc( tpt< x > * );
int main()
{
using namespace std;
cout << sizeof( fnc< has >( 0 ) ) << endl;
}
On my compiler, sizeof( no ) is 1, and sizeof( yes ) is 2.
The program selects the "no" version of fnc and prints:
1
I think that it should select the "yes" version of fnc and print:
2
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