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This is from the <iostream>:
namespace std
{
extern istream cin; ///< Linked to standard input
extern ostream cout;
...
It seems by using extern the data types defined in other namespaces will just be available?
externis used to refer to a variable defined in a different compilation unit (for now, you can think of a compilation unit as a .cpp file). The statements in your example declare rather than definecinandcout. It is telling the compiler that the definition of these objects is found in another compilation unit (where they are not declared asextern).