Consider the following program:
#include <iostream>
#include <iterator>
#include <vector>
#include <utility>
using namespace std; //just for convenience, illustration only
typedef pair<int, int> point; //this is my specialization of pair. I call it point
istream& operator >> (istream & in, point & p)
{
return in >> p.first >> p.second;
}
int main()
{
vector<point> v((istream_iterator<point>(cin)), istream_iterator<point>());
// ^^^ ^^^
//extra parentheses lest this should be mistaken for a function declaration
}
This fails to compile because as soon as ADL finds operator >> in namespace std it doesn't consider the global scope any more regardless of whether the operator found in std was a viable candidate or not. This is rather inconvenient. If I place the declaration of my operator >> into namespace std (which is technically illegal) the code compiles well as expected. Is there any way to resolve this issue other than make point
my own class rather than typedefing it as a specialization of a template in std namespace?
Thanks in advance
std::pair is a class template that provides a way to store two heterogeneous objects as a single unit. A pair is a specific case of a std::tuple with two elements. If neither T1 nor T2 is a possibly cv-qualified class type with non-trivial destructor, or array thereof, the destructor of pair is trivial.
Another way to initialize a pair is by using the make_pair() function. g2 = make_pair(1, 'a'); Another valid syntax to declare pair is: g2 = {1, 'a'};
A std::pair is a struct.
Adding an overload of operator>>
in namespace std
is forbidden, but adding a template specialization is sometimes allowed.
However, there are no user-defined types here, and the operators on standard types are not yours to redefine. Specializing operator>>(istream&, pair<mytype, int>)
would be reasonable.
section [namespace.std]
(section 17.6.4.2.1 of n3290) says
The behavior of a C++ program is undefined if it adds declarations or definitions to namespace
std
or to a namespace within namespacestd
unless otherwise specified. A program may add a template specialization for any standard library template to namespacestd
only if the declaration depends on a user-defined type and the specialization meets the standard library requirements for the original template and is not explicitly prohibited.
(emphasis mine)
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With