While answering this SO question (better read this "duplicate"), I came up with the following solution to dependent name resolution of an operator:
[temp.dep.res]/1:
In resolving dependent names, names from the following sources are considered:
- Declarations that are visible at the point of definition of the template.
- Declarations from namespaces associated with the types of the function arguments both from the instantiation context (14.6.4.1) and from the definition context.
#include <iostream>
#include <utility>
// this operator should be called from inside `istream_iterator`
std::istream& operator>>(std::istream& s, std::pair<int,int>& p)
{
s >> p.first >> p.second;
return s;
}
// include definition of `istream_iterator` only after declaring the operator
// -> temp.dep.res/1 bullet 1 applies??
#include <iterator>
#include <map>
#include <fstream>
int main()
{
std::ifstream in("file.in");
std::map<int, int> pp;
pp.insert( std::istream_iterator<std::pair<int, int>>{in},
std::istream_iterator<std::pair<int, int>>{} );
}
But clang++ 3.2 and g++ 4.8 don't find this operator (name resolution).
Doesn't the inclusion of <iterator> define the "point of definition of the template" istream_iterator?
Edit: As Andy Prowl points out, this has nothing to do with the Standard Library, but rather with name lookup (can be proven by mimicking the Standard Library with multiple operator>>, at least one in the namespace of the fake istream).
Edit2: A workaround, using [basic.lookup.argdep]/2 bullet 2
#include <iostream>
#include <utility>
// can include <iterator> already here,
// as the definition of a class template member function
// is only instantiated when the function is called (or explicit instantiation)
// (make sure there are no relevant instantiations before the definition
// of the operator>> below)
#include <iterator>
struct my_int
{
int m;
my_int() : m() {}
my_int(int p) : m(p) {}
operator int() const { return m; }
};
// this operator should be called from inside `istream_iterator`
std::istream& operator>>(std::istream& s, std::pair<my_int,my_int>& p)
{
s >> p.first.m >> p.second.m;
return s;
}
#include <map>
#include <fstream>
int main()
{
std::ifstream in("file.in");
std::map<int, int> pp;
pp.insert( std::istream_iterator<std::pair<my_int, my_int>>{in},
std::istream_iterator<std::pair<my_int, my_int>>{} );
}
Of course, you can also use your own pair type, as long as the workaround introduces an associated class in the namespace of the custom operator>>.