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Glen Low
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 Posted: Mon Sep 15, 2003 9:55 am Post subject: Fun with ptr_fun |
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How do you select one of a number of overloaded functions (short of
using typeof?) for std::ptr_fun?
E.g.
int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
}
It's not exactly a std::ptr_fun_t I'm after, since I want really want
to do is, have a (function) template that is partially specialized on
various forms of function, but if I can't select a particular one of a
number of overloaded functions, how could it work?
Cheers,
Glen Low, Pixelglow Software
www.pixelglow.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ivan Vecerina
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| Posted: Mon Sep 15, 2003 3:45 pm Post subject: Re: Fun with ptr_fun |
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Hi Glen,
"Glen Low" <glenlow (AT) pixelglow (DOT) com> wrote
| Quote: | How do you select one of a number of overloaded functions (short of
using typeof?) for std::ptr_fun?
E.g.
int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
}
It's not exactly a std::ptr_fun_t I'm after, since I want really want
to do is, have a (function) template that is partially specialized on
various forms of function, but if I can't select a particular one of a
number of overloaded functions, how could it work?
|
Immediately casting the function name is how you can specify
a specific overload:
std::ptr_fun( ( int(*)(int) ) func );
The cast is used at compile time to resolve the name 'func'...
hth
--
http://ivan.vecerina.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ulrich Eckhardt
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| Posted: Mon Sep 15, 2003 3:46 pm Post subject: Re: Fun with ptr_fun |
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Glen Low wrote:
| Quote: | How do you select one of a number of overloaded functions (short of
using typeof?) for std::ptr_fun?
int func (int i);
float func (float f);
std::ptr_fun (func); // ambigious, which func is meant?
|
// disclaimer: uncompiled code ahead
typedef float fffunc (float f);
std::ptr_fun((fffunc*)&func);
Not sure how that is related to ptr_fun, but taking the address of a
particular function of a set of overloads is done by a simple cast. Yes,
its really ugly but it seems to be the only way there is. :(
Uli
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Gabriel Dos Reis
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| Posted: Mon Sep 15, 2003 3:47 pm Post subject: Re: Fun with ptr_fun |
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[email]glenlow (AT) pixelglow (DOT) com[/email] (Glen Low) writes:
| Quote: | How do you select one of a number of overloaded functions (short of
using typeof?) for std::ptr_fun?
|
by explicit cast-notation, either with old-style cast or with
static_cast<> -- yes, this is a hack, but a standard hack.
| Quote: |
E.g.
int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
|
std::ptr_fun ((int (*)(int)) &func);
std::ptr_fun(static_cast<float (*)(float)>(&fun));
| Quote: | }
It's not exactly a std::ptr_fun_t I'm after, since I want really want
to do is, have a (function) template that is partially specialized on
various forms of function, but if I can't select a particular one of a
number of overloaded functions, how could it work?
|
hardly. Unless you want overload sets to be first class values.
--
Gabriel Dos Reis <gdr (AT) integrable-solutions (DOT) net>
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Glen Low
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| Posted: Mon Sep 15, 2003 9:53 pm Post subject: Re: Fun with ptr_fun |
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After whacking around the bit of code, I think I've answered my own
question:
| Quote: | int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
}
|
To select the first:
std::ptr_fun (static_cast <int (*) (int)> (func));
To select the second:
std::ptr_fun (static_cast <float (*) (float)> (func));
My requirement in its bare essentials is to devise a compile-time
expression that uniquely selects one of the funcs. It appears that:
static_cast <int (*) (int)> (func)
and
static_cast <float (*) (float)> (func)
are so.
Cheers,
Glen Low, Pixelglow Software
www.pixelglow.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Ben Hutchings
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| Posted: Mon Sep 15, 2003 10:02 pm Post subject: Re: Fun with ptr_fun |
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In article <9215d7ac.0309142005.4251f874 (AT) posting (DOT) google.com>,
Glen Low wrote:
| Quote: | How do you select one of a number of overloaded functions
(short of using typeof?) for std::ptr_fun?
E.g.
int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
}
snip |
Use a static cast:
typedef float (*func_type)(float);
std::ptr_fun(func_type(func));
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Siemel Naran
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| Posted: Tue Sep 16, 2003 7:03 am Post subject: Re: Fun with ptr_fun |
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"Glen Low" <glenlow (AT) pixelglow (DOT) com> wrote in message
| Quote: | int func (int i);
float func (float f);
|
In addition to static_cast you can also declare a variable of the right type
int (*const ptr)(int) = &func;
--
+++++++++++
Siemel Naran
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Shay
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| Posted: Wed Sep 17, 2003 9:40 am Post subject: Re: Fun with ptr_fun |
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In article <9215d7ac.0309150642.66fc6ee5 (AT) posting (DOT) google.com>,
[email]glenlow (AT) pixelglow (DOT) com[/email] (Glen Low) wrote:
| Quote: | After whacking around the bit of code, I think I've answered my own
question:
int func (int i);
float func (float f);
int main ()
{
std::ptr_fun (func); // ambigious, which func is meant?
}
To select the first:
std::ptr_fun (static_cast <int (*) (int)> (func));
To select the second:
std::ptr_fun (static_cast <float (*) (float)> (func));
[snip] |
I noticed that about half the replies to your original post contained
suggestions to use C-style casts, leading to a hidden reinterpret_cast<>
(and undefined behavior if invoked) if the types differed:
int func( int );
(float (*)( int )) func // oops, but no compile error
static_cast<float (*)( int )> (func) // compile error
--
Shay
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Wil Evers
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| Posted: Wed Sep 17, 2003 8:28 pm Post subject: Re: Fun with ptr_fun |
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In article <postmaster-1609031240220001 (AT) aus-as2-003 (DOT) io.com>, Shay wrote:
| Quote: | I noticed that about half the replies to your original post contained
suggestions to use C-style casts, leading to a hidden reinterpret_cast
(and undefined behavior if invoked) if the types differed:
int func( int );
(float (*)( int )) func // oops, but no compile error
static_cast<float (*)( int )> (func) // compile error
|
Sure, a static_cast is better than a C-style cast. But then, no cast is
better than a static_cast:
int func(int);
float func(float);
int main()
{
int (*pf)(int) = func;
std::ptr_fun(pf);
return 0;
}
- Wil
--
Wil Evers, DOOSYS R&D BV, Utrecht, Holland
[Wil underscore Evers at doosys dot com]
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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