Hi folks,
I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T >
struct stupid {
T data;
void swap ( stupid & other ) { // line 12
swap( this->data, other.data ); // line 13
}
};
}
int main ( void ) {
xxx::stupid< xxx::empty > a, b;
a.swap(b); // line 22
}
The compiler complains:
In member function 'void xxx::stupid<T>::swap(xxx::stupid<T>&)
[with T = xxx::empty]':
file.cc:22: instantiated from here
file.cc:13: error: no matching function for call to
'xxx::stupid<xxx::empty>:
:swap(xxx::empty&, xxx::empty&)'
file.cc:12: note: candidates are: void xxx::stupid<T>::swap(xxx::stupid<T>&)
[with T = xxx::empty]
Obviously, the presence of the local swap-method in stupid<T> prevents the
compiler from looking outside for other possible swaps. I was under the
impression that the namespace where T (in this case xxx::empty) is defined
would be searched for a match. But that apparently does not happen. Why is
that?
Best
Kai-Uwe Bux 7 1704
Kai-Uwe Bux wrote: I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { // line 12 swap( this->data, other.data ); // line 13 }
};
}
int main ( void ) { xxx::stupid< xxx::empty > a, b; a.swap(b); // line 22 }
The compiler complains:
In member function 'void xxx::stupid<T>::swap(xxx::stupid<T>&) [with T = xxx::empty]': file.cc:22: instantiated from here file.cc:13: error: no matching function for call to 'xxx::stupid<xxx::empty>: swap(xxx::empty&, xxx::empty&)' file.cc:12: note: candidates are: void xxx::stupid<T>::swap(xxx::stupid<T>&) [with T = xxx::empty]
Obviously, the presence of the local swap-method in stupid<T> prevents the compiler from looking outside for other possible swaps.
No, it does not prevent it from looking. It prevents the compiler
from seeing it.
I was under the impression that the namespace where T (in this case xxx::empty) is defined would be searched for a match. But that apparently does not happen. Why is that?
It's called "name hiding", I believe. 'swap' name in the struct
'stupid' scope _hides_ the one in the namespace scope, so while
you're in 'xxx::stupid::swap', the other one is simply invisible.
V
Victor Bazarov wrote: Kai-Uwe Bux wrote: I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { // line 12 swap( this->data, other.data ); // line 13 }
};
}
[snip] I was under the impression that the namespace where T (in this case xxx::empty) is defined would be searched for a match. But that apparently does not happen. Why is that?
It's called "name hiding", I believe. 'swap' name in the struct 'stupid' scope _hides_ the one in the namespace scope, so while you're in 'xxx::stupid::swap', the other one is simply invisible.
Thanks a lot for the explanation. However, that leaves me with a problem:
how to tell the template stupid<T> which swap to use. I considered:
template < typename T >
struct stupid {
T data;
void swap ( stupid & other ) {
std::swap( this->data, other.data );
}
};
This, I think, will not find specialized/overloaded version of swap for
types not declared in namespace std; and I cannot dump my stuff in there.
Thus, I did:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T >
void global_swap ( T & a, T & b ) {
swap( a, b );
}
template < typename T >
struct stupid {
T data;
void swap ( stupid & other ) {
global_swap( this->data, other.data );
}
};
}
This will find the appropriate swap by looking at the argument type. But
introducing the forwarding function "global_swap" seems clumsy. I would
appreciate suggestions for how to improve upon this.
Thanks again
Kai-Uwe Bux
Kai-Uwe Bux wrote: [..] Thanks a lot for the explanation. However, that leaves me with a problem: how to tell the template stupid<T> which swap to use. I considered:
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { std::swap( this->data, other.data ); }
};
This, I think, will not find specialized/overloaded version of swap for types not declared in namespace std; and I cannot dump my stuff in there.
Yes, you can.
"17.4.3.1 Reserved names
[...]. A program may add template specializations for any
standard library template to namespace std. [...]"
So, you're totally allowed to specialise 'swap' for your class 'empty'
and put it in 'std' namespace.
[...]
V
"Kai-Uwe Bux" <jk********@gmx.net> wrote in message
news:di**********@murdoch.acc.Virginia.EDU... Hi folks,
I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { // line 12 swap( this->data, other.data ); // line 13 }
how bout xxx::swap instead?
Jon
Jon Slaughter wrote: "Kai-Uwe Bux" <jk********@gmx.net> wrote in message news:di**********@murdoch.acc.Virginia.EDU... Hi folks,
I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { // line 12 swap( this->data, other.data ); // line 13 }
how bout xxx::swap instead?
That would not allow using 'std::swap' for T other than 'empty'.
V
"Victor Bazarov" <v.********@comAcast.net> wrote in message
news:af********************@comcast.com... Jon Slaughter wrote: "Kai-Uwe Bux" <jk********@gmx.net> wrote in message news:di**********@murdoch.acc.Virginia.EDU... Hi folks,
I am still struggling with the rules for name lookup.
Please consider:
namespace xxx {
struct empty {};
void swap ( empty & a, empty & b ) {}
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { // line 12 swap( this->data, other.data ); // line 13 }
how bout xxx::swap instead?
That would not allow using 'std::swap' for T other than 'empty'.
V
Then he needs to specialize stupid::swap if T is of empty type to use
xxx::swap?
basicaly
if T is of type empty then use xxx::swap else use std::swap
or whatever
Victor Bazarov wrote: Kai-Uwe Bux wrote: [..] Thanks a lot for the explanation. However, that leaves me with a problem: how to tell the template stupid<T> which swap to use. I considered:
template < typename T > struct stupid {
T data;
void swap ( stupid & other ) { std::swap( this->data, other.data ); }
};
This, I think, will not find specialized/overloaded version of swap for types not declared in namespace std; and I cannot dump my stuff in there.
Yes, you can.
"17.4.3.1 Reserved names [...]. A program may add template specializations for any standard library template to namespace std. [...]"
So, you're totally allowed to specialise 'swap' for your class 'empty' and put it in 'std' namespace.
Great! Thanks for that crucial piece of information.
Best
Kai-Uwe Bux This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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