Argument-Dependent Lookup

[siddhu]

Dear experts,

As far as I understood the ADL concept things should work in the
following way.

I have a global ostream operator defined as

//ostreamtest.cpp
#include<iostream>
using namespace std;

ostream& operator<<(ostream& out,const string& str)
{
out<<"in ostream"<<endl;
out<<str.c_str()<<endl;

return out;
}

int main()
{
string s("abcd");
cout<<s<<endl;
}

The output of above program is

in ostream
abcd

That means It is calling global ostream operator. But I expected from
this program to call ostream operator defined in std namespace.
Suggestions would be of great help.

Regards,
Siddharth

 

[Victor Bazarov]

Dear experts,

As far as I understood the ADL concept things should work in the
following way.

I have a global ostream operator defined as

//ostreamtest.cpp
#include<iostream>
using namespace std;

ostream& operator<<(ostream& out,const string& str)
{
out<<"in ostream"<<endl;
out<<str.c_str()<<endl;

return out;
}

int main()
{
string s("abcd");
cout<<s<<endl;
}

The output of above program is

in ostream
abcd

That means It is calling global ostream operator. But I expected from
this program to call ostream operator defined in std namespace.
Suggestions would be of great help.

Try including <string>. I don't think that without it your program is
well-defined (or at least portable).

V

 

[siddhu]

Try including <string>. I don't think that without it your program is
well-defined (or at least portable).

Even then it produces the same result. I am using g++ compiler.

 

[Victor Bazarov]

Even then it produces the same result. I am using g++ compiler.

ADL does not prohibit using the function from the global NS or dictates
the way in which overloaded functions are resolved. It only says that
if you didn't have the global operator<< defined, the one from 'std'
would be found.

V

 

[James Kanze]

As far as I understood the ADL concept things should work in the
following way.

I have a global ostream operator defined as

//ostreamtest.cpp
#include<iostream>
using namespace std;

ostream& operator<<(ostream& out,const string& str)
{
out<<"in ostream"<<endl;
out<<str.c_str()<<endl;
return out;
}

int main()
{
string s("abcd");
cout<<s<<endl;
}

The output of above program is

in ostream
abcd

That means It is calling global ostream operator. But I expected from
this program to call ostream operator defined in std namespace.

ADL only kicks in after normal unqualified name lookup, or for
dependent names in templates. Since the compiler finds the
global function during normal unqualified name lookup, it
doesn't pull in the corresponding function during ADL.
(Actually, I'm not too sure about this. The standard says that
the set of functions is the union of those found with
unqualified lookup and those found with ADL. Which sounds to me
as if both functions should be present, and the call should be
ambiguous. All of the compilers I have access to resolve it to
the global function, however.)